Hey Tara! Welcome to Topic 11 — Organic Chemistry, the biggest topic on the syllabus and, once you see the pattern, one of the most learnable. Here is the secret: organic chemistry is not hundreds of separate facts, it is four families of compounds (alkanes, alkenes, alcohols, carboxylic acids) connected by a small map of reactions. Every family is a homologous series — same functional group, same general formula, same chemistry — so learning one member means you have learned them all. The whole topic flows in one story: crude oil is separated into fractions, long chains are cracked into short ones plus alkenes, alkenes add things on (including water, to make alcohols), alcohols are oxidised to acids, acids and alcohols make esters, and alkenes or acid/alcohol pairs are stitched into polymers. Four sections, eighty practice questions. Let's build the map!
11.1 Formulae, Naming and Fuels
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Three Ways to Write the Same Molecule
Organic chemistry has its own language, and the first job is to read and write it fluently. Every organic molecule can be described at three levels of detail, and exam questions tell you exactly which one they want — so you must know the difference.
Molecular → Structural → Displayed: each shows more detail than the last
Molecular formula — just the count of each atom: ethanol is C₂H₆O. It tells you nothing about how the atoms are joined.
Structural formula — an unambiguous description of how the atoms are arranged, written on one line: ethanol is CH₃CH₂OH, ethene is CH₂=CH₂.
Displayed formula — a full drawing showing every atom and every bond as lines. If a question says "draw the displayed formula", every single H and every bond line must appear — including the O–H bond in an alcohol.
General formula — the family recipe: alkanes are CₙH₂ₙ₊₂, so any member's formula can be generated by choosing n.
Displayed formulae of methane, ethene, ethanol and ethanoic acid. Every atom shown, every bond drawn as a line. Check each carbon has exactly four bonds — a double bond counts as two.
Homologous Series: The Family Idea That Runs the Whole Topic
Organic compounds come in families called homologous series. The alkanes are one family; the alkenes, alcohols and carboxylic acids are three more. A homologous series is defined by five features, and Cambridge loves asking for any three of them:
A homologous series is a family of compounds with the same functional group and the same general formula
1. Same functional group — the atom or group of atoms responsible for the characteristic reactions (C=C for alkenes, –OH for alcohols, –COOH for carboxylic acids).
2. Same general formula — alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH.
3. Consecutive members differ by CH₂ — each member is one –CH₂– unit (and 14 units of Mᵣ) longer than the last.
4. Similar chemical properties — because the functional group is the same, every member reacts the same way.
5. A trend in physical properties — boiling point, viscosity and density rise gradually as the chain gets longer.
Exam Tip
Notice the split: chemical properties are similar (same functional group), but physical properties show a trend (chains get longer). Candidates who write "members of a homologous series have the same properties" lose the mark for being vague — say which kind of property behaves which way.
Naming: Two Halves of Every Name
Every name up to C4 is built from a stem (how many carbons?) plus an ending (which family?). Four stems, four endings — sixteen names for the price of eight.
| Carbons | Stem | Alkane (-ane) | Alkene (-ene) | Alcohol (-ol) | Carboxylic acid (-oic acid) |
|---|---|---|---|---|---|
| 1 | meth- | methane, CH₄ | — (needs C=C, so at least 2 C) | methanol, CH₃OH | methanoic acid, HCOOH |
| 2 | eth- | ethane, C₂H₆ | ethene, C₂H₄ | ethanol, C₂H₅OH | ethanoic acid, CH₃COOH |
| 3 | prop- | propane, C₃H₈ | propene, C₃H₆ | propan-1-ol, C₃H₇OH | propanoic acid, C₂H₅COOH |
| 4 | but- | butane, C₄H₁₀ | but-1-ene, C₄H₈ | butan-1-ol, C₄H₉OH | butanoic acid, C₃H₇COOH |
Memory Trick
"Monkeys Eat Purple Bananas" — Meth (1), Eth (2), Prop (3), But (4). Then the ending does the chemistry: -ane = all single bonds, -ene = one C=C, -ol = –OH, -oic acid = –COOH. "Propanol" decodes instantly: 3 carbons, alcohol family.
Supplement
Structural Isomers: Same Formula, Different Skeleton
Structural isomers are compounds with the same molecular formula but different structural formulae. C₄H₁₀ can be built two ways: a straight chain of four carbons (butane, CH₃CH₂CH₂CH₃) or a three-carbon chain with a CH₃ branch on the middle carbon (2-methylpropane, (CH₃)₃CH). Same atoms, different arrangement — so they are different compounds with slightly different boiling points, though they burn the same way because both are alkanes.
Position matters too: but-1-ene (CH₂=CHCH₂CH₃) has the double bond starting at carbon 1, but-2-ene (CH₃CH=CHCH₃) at carbon 2. Both are C₄H₈, so they are structural isomers of each other. The number in a name always tells you where the functional group sits — count from the end of the chain that gives the lowest number.
Careful with the trap in reverse: butane and but-1-ene are NOT isomers — C₄H₁₀ and C₄H₈ are different molecular formulae. Isomers must match formula-for-formula, atom-for-atom.
Fossil Fuels: Where the Carbon Comes From
The syllabus names three fossil fuels: coal, natural gas and petroleum (crude oil). All three formed from the remains of living things buried and compressed over millions of years — which is why they are finite and non-renewable. Natural gas is mainly methane, CH₄ — a one-line fact that appears on papers again and again. Petroleum itself is not a single substance but a mixture of hundreds of hydrocarbons — compounds containing hydrogen and carbon only — and a mixture is useless until it is separated.
Fractional Distillation of Petroleum
Crude oil is separated into fractions — groups of hydrocarbons with similar boiling points (not identical: a fraction is still a mixture). The separation works because different chain lengths have different boiling points. The oil is heated until most of it vaporises, the vapour enters a fractionating column that is hot at the bottom and cool at the top, and each hydrocarbon rises until it reaches the level at which it condenses. Long chains condense low down; short chains rise high, and the very shortest never condense at all — they leave the top as refinery gas.
The fractionating column at a refinery such as Jamnagar in Gujarat — the largest in the world. Learn the fractions in order from the top: refinery gas, gasoline, naphtha, kerosene, diesel, fuel oil, lubricating oil, bitumen — and one use for each.
Memory Trick
"Real Geologists Never Kid: Drilling Finds Loads (of) Bitumen." Refinery gas, Gasoline, Naphtha, Kerosene, Diesel, Fuel oil, Lubricating oil, Bitumen — top to bottom, coolest to hottest. Then anchor two uses that are never asked politely: naphtha = chemical feedstock and kerosene = jet fuel.
The Four Trends Down the Column
Every property of a fraction follows from one variable: chain length. Longer chains have stronger attractive forces between molecules, so more energy is needed to separate them.
| Property | Top of column (e.g. refinery gas) | Bottom of column (e.g. bitumen) |
|---|---|---|
| Number of carbon atoms (chain length) | Small (1–4) | Large (70+) |
| Boiling point | Low (still gases at 25 °C) | High (barely vaporises at 350 °C) |
| Volatility (how easily it vaporises) | High | Low |
| Viscosity (how thick / slow-flowing) | Low — runny | High — treacle-thick |
| Colour and flame | Pale, burns with a clean flame | Dark, burns with a smoky flame |
Exam Tip
Volatility trips everyone once: high volatility = low boiling point. They are the same trend stated in opposite words. If a question says "fraction X is more volatile than fraction Y", translate it instantly: X has the shorter chains, the lower boiling point and the lower viscosity, and it condenses higher up the column.
Worked Example 1
An alkane has 18 carbon atoms. (a) State its molecular formula. (b) A student says: "this alkane and propane have similar chemical properties." Justify the statement, and state two ways the compounds differ physically. [5]
Step 1: Use the general formula
Alkanes are CₙH₂ₙ₊₂. With n = 18: hydrogens = 2(18) + 2 = 38. Formula: C₁₈H₃₈. This is the whole point of a general formula — you never memorise big alkanes, you generate them.
Step 2: Same series → similar chemistry
Both are alkanes, so they belong to the same homologous series: same general formula, same functional group (only C–C and C–H single bonds). Members of a homologous series have similar chemical properties — both burn in oxygen, both are unreactive with most reagents, both undergo substitution with chlorine in UV light.
Step 3: Different chain length → different physical properties
C₁₈H₃₈ has a much longer chain, so it has a higher boiling point (it is a waxy solid/liquid while propane is a gas), a higher viscosity and a lower volatility. Any two of these earn the marks.
(a) C₁₈H₃₈ [1]. (b) Both are members of the same homologous series (alkanes) [1], so they have the same functional group / general formula and therefore similar chemical reactions, e.g. combustion [1]. Physically, C₁₈H₃₈ has a higher boiling point [1] and is more viscous / less volatile than propane [1].
Worked Example 2
Compound X has the structural formula CH₃CH₂CH₂OH. (a) Name X and the homologous series it belongs to. (b) Draw its displayed formula. (c) Supplement: draw and name a structural isomer of X. [5]
Step 1: Decode the name from stem + ending
Three carbons → stem prop-. The –OH group → family alcohols, ending -ol. The OH is on the end carbon (carbon 1), so the full name is propan-1-ol ("propanol" is usually accepted at core level).
Step 2: Draw every atom and every bond
Three carbons in a row, each with enough H to make four bonds: the first two carbons carry H above and below (plus one H on the left end), the third carbon carries two H and the O. Then the crucial detail: draw C–O as a line and O–H as a separate line. A drawn "–OH" blob with no bond shown between O and H loses the mark.
Step 3: Move the OH to make an isomer
Keep the molecular formula C₃H₈O but change the arrangement: put the –OH on the middle carbon. That gives CH₃CH(OH)CH₃, named propan-2-ol. Same molecular formula, different structural formula — the definition of structural isomers.
(a) Propan-1-ol, an alcohol [2]. (b) Displayed formula: H–C–C–C with all seven C–H bonds drawn, then C–O–H with both the C–O and O–H bonds shown as lines [2]. (c) Propan-2-ol: the –OH bonded to the middle carbon; same molecular formula C₃H₈O, different structure [1].
Worked Example 3
The Jamnagar refinery separates crude oil into fractions. (a) Name the process and explain how it works. [4] (b) Kerosene condenses higher up the column than diesel oil. Compare the two fractions in terms of chain length, boiling point and viscosity. [3]
Step 1: Name the process precisely
Fractional distillation. Not "distillation" alone, not "fractioning", not "cracking" — cracking is a chemical reaction that comes later; this is a physical separation that works because the hydrocarbons have different boiling points.
Step 2: Tell the story of the vapour
The crude oil is heated until it vaporises; the vapour enters a column that is hot at the bottom and cooler at the top. Vapours rise and condense at the level where the temperature falls below their boiling point. Short chains, with low boiling points, condense near the top; long chains condense near the bottom.
Step 3: Apply the trends to the pair
Kerosene is higher, so it has shorter chains (≈C10–C16 vs ≈C14–C20), a lower boiling point range and a lower viscosity than diesel oil. One variable — chain length — drives all three comparisons.
(a) Fractional distillation [1]. The oil is heated and vaporised [1]; the column is hot at the bottom, cool at the top [1]; hydrocarbons rise and condense where the temperature matches their boiling point, so different chain lengths collect at different heights [1]. (b) Kerosene has fewer carbon atoms / shorter chains [1], a lower boiling point [1] and a lower viscosity (flows more easily) than diesel [1].
Exam Tips for 11.1
1. Displayed formula = every atom AND every bond. The three classic losses: missing the O–H bond in alcohols and acids, giving a carbon five bonds, and drawing a C=C with one line. Count to four on every carbon before moving on.
2. Learn the homologous series definition as five bullet points. Same functional group, same general formula, differ by CH₂, similar chemical properties, trend in physical properties. A 3-mark question wants any three.
3. A fraction is still a mixture. Fractional distillation groups hydrocarbons with similar boiling points; it does not produce pure compounds. And "natural gas is mainly methane" is a one-mark gift — do not write "petrol".
4. Trends come as a package. Longer chain ⇒ higher boiling point ⇒ more viscous ⇒ less volatile ⇒ lower in the column. State the one the question asks for, in the direction it asks.
Apply It: Real-World Chemistry
From the world's biggest refinery on the Gujarat coast to a gas hob in Aberdeen, 11.1 is the section you can see out of any window.
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The Jamnagar refinery in Gujarat is the largest in the world, processing about 1.4 million barrels of crude oil every day. Tankers deliver crude from the Middle East; out of the far end come LPG cylinders, petrol, jet fuel for Mumbai airport, diesel for trucks, and bitumen for the national highways — all from the same black liquid.
How can one input liquid become so many different products, and why is each product taken from a different height of the column?
One Liquid, Hundreds of Compounds
Crude oil is a mixture of hundreds of different hydrocarbons with chain lengths from 1 carbon to over 70. A mixture can be separated by physical means, and the physical property that varies most usefully here is boiling point, which rises steadily with chain length.
Height = Temperature = Chain Length
The column is hot at the bottom (≈350 °C), cool at the top (≈25 °C). A rising vapour condenses at the level where the column temperature drops just below its boiling point. Long-chain bitumen molecules barely vaporise and drain from the bottom; C1–C4 refinery gases never condense and leave the top. Every outlet pipe in between collects one fraction — a batch of hydrocarbons with similar boiling points.
Chemistry Connection
Notice that nothing chemical has happened yet — not one bond has broken. Fractional distillation is pure physics: differences in boiling point. The chemistry (cracking, polymerising) starts in section 11.2, using these fractions as raw material.
2
A family in Aberdeen cooks on mains natural gas piped from fields under the North Sea. Their cousins on a Scottish island are off the gas grid, so they buy bottled LPG (propane in winter, butane in summer) — gases that were separated out as the refinery gas fraction.
What is mains natural gas chemically, and why do the islanders' two bottles contain different alkanes for different seasons?
Natural Gas = Mainly Methane
North Sea natural gas is mainly methane, CH₄ — the smallest alkane, a fossil fuel formed alongside petroleum. It is piped rather than bottled because its boiling point (−162 °C) is far too low to liquefy by pressure alone at room temperature.
Propane vs Butane: A Boiling-Point Decision
Propane (C₃H₈) boils at −42 °C; butane (C₄H₁₀) at −1 °C — the boiling point rises with chain length, exactly the homologous series trend. On a −5 °C winter morning, butane stays liquid in the bottle and no gas flows; propane still boils happily. In summer, cheaper butane is fine. The seasonal swap is the CₙH₂ₙ₊₂ trend in action.
Chemistry Connection
One homologous series, three lifestyles: methane piped (bp far too low to bottle), propane bottled for winter, butane bottled for summer. When an exam asks "state one piece of evidence that alkanes form a homologous series", the gradual trend in boiling point is exactly this.
3
At Delhi airport an Air India A350 is fuelled with aviation turbine fuel — essentially the kerosene fraction. The fuel tanker driver notices it flows almost like water, quite unlike the thick, dark fuel oil she once handled at a shipping port in Kochi.
Use the fraction trends to explain the difference between the two fuels, and why each suits its vehicle.
Two Fractions, Two Chain Lengths
Kerosene is roughly C10–C16; fuel oil is roughly C20–C50. Longer chains mean stronger attractions between molecules, so fuel oil has the higher boiling point, the higher viscosity (thick and slow-flowing) and the lower volatility and burns with a smokier flame.
Matching Fuel to Machine
A jet engine needs a fuel that vaporises readily at altitude, flows through fine nozzles at −40 °C, and burns cleanly — short-ish chains, low viscosity: kerosene. A ship's enormous slow diesel engine can pre-heat and burn cheap, viscous long-chain fuel oil. The refinery sells every fraction to the machine it fits.
Chemistry Connection
"Suggest why fraction X is suitable for use Y" questions are answered by naming a trend property (volatility, viscosity, boiling point) and linking it to the job. Never answer "because it is flammable" — every fraction is flammable; the trends are what distinguish them.
4
In May, road crews resurface a highway near Nagpur at 45 °C using bitumen — the fraction from the very bottom of the column. The bitumen arrives in heated tankers at about 150 °C, is poured and rolled, and by evening cars drive on it. In the UK the same material is called tarmac's binder and is laid at a gentler 20 °C.
Why does bitumen come from the bottom of the column, and why is it perfect for roads but useless as a fuel?
Bottom of the Column = Longest Chains
Bitumen molecules have more than 70 carbon atoms. Their boiling points are so high that they never vaporise in the column at 350 °C — they simply stay liquid and are drawn off at the base. Longest chains → highest boiling point → highest viscosity: at room temperature bitumen is a near-solid.
Why Roads and Not Fuel
A road binder must be solid at road temperature (even a 45 °C Nagpur afternoon), waterproof, and sticky when heated — exactly the properties of extreme chain length. As a fuel it fails: too involatile to vaporise and mix with air, and it burns with a filthy smoky flame. Every property that ruins it as a fuel makes it ideal as a surface.
Chemistry Connection
There is no "best" fraction — only best-for-a-job. The exam phrase "explain why bitumen is used for surfacing roads" wants high viscosity / solid at room temperature linked to very long chains, the same trend logic as every other fraction question, just at the far end of the scale.
5
A camping shop in Manali sells butane canisters for trekking stoves. A label warns: "Performance drops in cold conditions — use propane mix above 3000 m." A curious customer notices both gases smell the same when a valve leaks (a smelly additive) and both burn with the same blue flame.
Two different compounds, yet identical chemistry at the stove. What does this tell you about homologous series — and why does altitude favour propane?
Same Family, Same Chemistry
Propane C₃H₈ and butane C₄H₁₀ are consecutive alkanes — they differ by exactly one CH₂ unit. Same functional group, same general formula → similar chemical properties: both undergo complete combustion to CO₂ and H₂O with a blue flame. Chemically the stove cannot tell them apart.
Different Chain Length, Different Physics
The trend property, boiling point, is where they part company: butane stops vaporising near −1 °C, and a Himalayan campsite at dawn is far colder than that. Propane, boiling at −42 °C, keeps supplying gas. Similar chemistry, trending physics — the exact wording of the homologous series definition.
Chemistry Connection
This scenario is the 3-mark definition question in disguise: "compounds in the same homologous series have similar chemical properties but show a trend in physical properties." If you can explain a trekking stove, you can earn those marks.
Practice Questions: 11.1
20 multiple choice questions. Click an option to check your answer.
Your Score
0 / 20
Question 1
What is a homologous series?
B is the definition. A describes structural isomers. D is wrong because physical properties show a trend, not identity — only chemical properties are similar.
Question 2
Consecutive members of a homologous series differ by
Ethane C₂H₆ to propane C₃H₈ adds one C and two H — a CH₂ unit (relative mass 14). Saying "one carbon" alone misses the hydrogens.
Question 3
Which is the structural formula of ethanol?
A is the molecular formula — it counts atoms but does not show arrangement. B shows the atoms in order with the –OH on the end carbon. C (methoxymethane) has the same molecular formula but a different structure.
Question 4
The general formula of the alkanes is
Alkanes are CₙH₂ₙ₊₂ (check: methane, n = 1, CH₄ ✓). A is the alkene formula — the most common mix-up in the whole topic.
Question 5
An alkene has 6 carbon atoms. Its molecular formula is
Alkenes are CₙH₂ₙ: 2 × 6 = 12 hydrogens, so C₆H₁₂ (hexene). A is the alkane hexane.
Question 6
The functional group of the carboxylic acids is
–COOH (containing both a C=O and an O–H) is the carboxylic acid group. A is alcohols, C is alkenes, D is the ester linkage.
Question 7
The name "butene" tells you the molecule has
Stem but- = 4 carbons; ending -ene = alkene family with one C=C. Two halves, two pieces of information.
Question 8
In a displayed formula, every carbon atom must have
Carbon always forms four covalent bonds — but not necessarily all to hydrogen (A). A C=C double bond counts as two of the four. This is the quickest way to check your own drawing.
Question 9
Natural gas is mainly
Natural gas is mainly methane, CH₄ — a fact the syllabus states explicitly. The three fossil fuels are coal, natural gas and petroleum.
Question 10
A hydrocarbon is a compound containing
The word only is the mark. Ethanol (C, H and O) is an organic compound but NOT a hydrocarbon — a distinction examiners test deliberately.
Question 11
In a fractionating column, the temperature
Hot at the bottom (≈350 °C), cool at the top. Each vapour rises until the temperature falls below its boiling point, then condenses — that gradient IS the separation mechanism.
Question 12
Which fraction is collected at the TOP of the column?
Refinery gas (C1–C4) has the shortest chains and lowest boiling points, so it never condenses and leaves the top. Bitumen, longest chains, leaves the bottom.
Question 13
The main use of the kerosene fraction is
Kerosene (paraffin) is jet fuel. Cars use gasoline/petrol; roads use bitumen; the chemical feedstock fraction is naphtha.
Question 14
Which fraction is the main feedstock for making chemicals?
Naphtha is the odd one out among the fractions: not burned as a fuel but used as a feedstock for making chemicals (via cracking). Learn it as the exception.
Question 15
Compared with fuel oil, gasoline has
The four trends always travel together: short chains ⇒ low boiling point ⇒ runny (low viscosity) ⇒ evaporates easily (high volatility). Any answer that splits the package is wrong.
Question 16
Fractional distillation separates petroleum because the hydrocarbons differ in
The separation is physical and driven entirely by boiling point differences, which in turn come from chain length. Density and colour also vary but are not the separation mechanism.
Question 17
Supplement: Which pair are structural isomers?
Isomers need the SAME molecular formula. But-1-ene and but-2-ene are both C₄H₈ with the C=C in different positions. A differ by CH₂ (homologues); B differ by H₂; D are different families entirely.
Question 18
Supplement: 2-methylpropane is a structural isomer of
Count the atoms: 2-methylpropane has a 3-carbon chain plus a CH₃ branch = C₄H₁₀, the same molecular formula as butane. Different skeleton, same formula → isomers.
Question 19
Which statement about members of the same homologous series is correct?
Chemical: similar (same functional group). Physical: a trend (increasing chain length). D describes isomers, not homologues.
Question 20
The three fossil fuels named on the syllabus are
Coal, natural gas (mainly methane) and petroleum. Wood and ethanol are renewable (not fossil); hydrogen is not a fossil fuel at all.
11.2 Alkanes and Alkenes
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Alkanes: Saturated and (Mostly) Sleepy
Alkanes (general formula CₙH₂ₙ₊₂) are saturated hydrocarbons: every carbon–carbon bond is a single bond, so the molecule holds as many hydrogens as it possibly can. Be careful with the word — "saturated" is defined by single bonds only, not by a vague "full of hydrogen". Because the C–C and C–H bonds are strong and the molecule has no reactive functional group, alkanes are generally unreactive: they ignore acids, alkalis, metals and most other reagents.
They do exactly two things you need to know:
1. Combustion CH₄ + 2O₂ → CO₂ + 2H₂O
In plenty of oxygen, alkanes burn completely to carbon dioxide and water, releasing a lot of energy — the whole reason they are fuels.
In limited oxygen, combustion is incomplete: poisonous carbon monoxide (or sooty carbon) forms instead — link back to Topic 10 air quality.
2. Substitution by chlorine CH₄ + Cl₂ → CH₃Cl + HCl (in UV light)
Substitution = one atom is swapped for another: a Cl atom replaces one H atom, so the second product is HCl — the hydrogen does not vanish.
The reaction needs ultraviolet light to start — it is a photochemical reaction, one in which light supplies the energy (compare photosynthesis).
In the dark, nothing happens — a favourite exam observation.
Alkenes: The Double Bond Changes Everything
Alkenes (general formula CₙH₂ₙ) are unsaturated: they contain one C=C double bond, which is their functional group and the site of all their chemistry. The double bond can "open up" and bond to new atoms, so alkenes are far more reactive than alkanes — and industrially far more valuable.
The test to distinguish them: aqueous bromine (bromine water)
Alkene (unsaturated): the orange aqueous bromine is decolourised — it turns from orange to colourless — because the bromine adds across the C=C.
Alkane (saturated): the bromine water stays orange; no reaction (in the absence of UV light).
Say "decolourised", never "turns clear" — clear means transparent, and orange bromine water is already transparent. Colourless is a colour statement; clear is not.
Cracking: Big Molecules Into Small Ones (Plus Alkenes)
Here is the refinery's problem: fractional distillation gives too much of the long-chain fractions and not enough of the short ones. Customers queue up for petrol and for alkenes to make polymers; nobody wants a lake of fuel oil. The solution is cracking: breaking large, less useful alkane molecules into smaller, more useful alkanes AND alkenes (sometimes hydrogen too).
Cracking: large alkane → smaller alkane + alkene e.g. C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Conditions: high temperature and a catalyst (thermal decomposition — the molecule is broken by heat).
Check the balance: atoms are conserved: C: 10 = 8 + 2 ✓; H: 22 = 18 + 4 ✓. Cracking equations mark themselves.
Why an alkene must appear: the starting alkane has the maximum hydrogen (2n+2). Splitting it into two alkanes would need two extra hydrogens from nowhere — so at least one product must be hydrogen-poor: an alkene (or H₂ is released).
Cracking always goes big → small, and the products must include something unsaturated (an alkene) or hydrogen. Never write it backwards — joining small molecules into big ones is polymerisation, the opposite process.
Why cracking matters — two reasons the examiners expect: (1) it converts surplus long-chain fractions into short-chain fuels that are in higher demand (more petrol per barrel); (2) it is the main source of alkenes, the starting point for polymers, ethanol and most of the plastics industry. Cracking also produces hydrogen, used for making ammonia and margarine.
Addition Reactions of Alkenes: The Double Bond Opens
In an addition reaction, the C=C opens and a small molecule adds across it. Two molecules become one single product — nothing else is made. Compare substitution, where an atom is swapped and there are two products. Alkenes do three additions you must know:
| Reagent added | Conditions | Product | Example |
|---|---|---|---|
| Bromine (or aqueous bromine) | Room temperature — no catalyst needed | Dibromo compound (colourless) | C₂H₄ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane) |
| Hydrogen | Nickel catalyst, heat | The corresponding alkane | C₂H₄ + H₂ → C₂H₆ (ethane) |
| Steam (H₂O) | Acid catalyst (phosphoric acid), 300 °C, 60 atm | An alcohol | C₂H₄ + H₂O → C₂H₅OH (ethanol) |
Memory Trick
"Alkenes are ADDers; alkanes SUBstitute as SUBstitutes." The reactive family (alkenes) does addition — the C=C opens, one product, fast, room temperature for bromine. The sleepy family (alkanes) only manages substitution, and only when UV light forces it. If the organic reactant has a C=C, the answer is addition; if it is an alkane + Cl₂ + UV, it is substitution with HCl as the second product.
Supplement
Why "Saturated" and "Unsaturated" Are About Bonds, Not Vibes
A saturated compound has only single carbon–carbon bonds; an unsaturated compound has at least one C=C. The names come from hydrogen capacity: a saturated chain literally cannot fit another H atom, while an unsaturated one can (add H₂ across the C=C and you get the alkane). But in an exam, define by bonds: "butene is unsaturated because it contains a C=C double bond" scores; "butene is not full of hydrogen" does not. And remember the addition of hydrogen is exactly how margarine makers turn liquid unsaturated vegetable oils into spreadable saturated fats — the nickel catalyst at work.
Worked Example 1
Dodecane, C₁₂H₂₆, is cracked to produce propene and one other hydrocarbon. (a) Write the balanced equation. (b) State the conditions for cracking. (c) Explain TWO reasons why the petrochemical industry cracks large alkanes. [6]
Step 1: Count what is missing
Start: C₁₂H₂₆. Propene is C₃H₆. Missing carbons: 12 − 3 = 9. Missing hydrogens: 26 − 6 = 20. The other product is C₉H₂₀ — check against CₙH₂ₙ₊₂: 2(9) + 2 = 20 ✓, so it is the alkane nonane.
Step 2: Write and self-check the equation
C₁₂H₂₆ → C₉H₂₀ + C₃H₆. Atom audit: C 12 = 9 + 3 ✓; H 26 = 20 + 6 ✓. Every cracking equation balances by conservation — if yours does not, one formula is wrong.
Step 3: Conditions and reasons
Conditions: high temperature and a catalyst. Reasons: (1) short-chain fuels like petrol are in greater demand than the surplus long fractions, so cracking matches supply to demand; (2) it produces alkenes, which are the reactive feedstock for polymers and ethanol (and hydrogen as a bonus product).
(a) C₁₂H₂₆ → C₉H₂₀ + C₃H₆ [2]. (b) High temperature and a catalyst [1]. (c) Long-chain fractions are in surplus while short-chain fuels are in higher demand [1]; cracking supplies alkenes [1] needed to make polymers/ethanol, which cannot be obtained directly from distillation in useful amounts [1].
Worked Example 2
A student bubbles ethane and ethene separately through aqueous bromine, in the dark. (a) State the observation in each case. (b) Name the type of reaction for ethene and give the product's name and formula. (c) Explain why ethane DOES react with chlorine in sunlight and name both products. [6]
Step 1: The observations — precision wording
Ethene: the bromine water is decolourised — orange to colourless. Ethane: stays orange / no change. Do not write "turns clear" and do not say the bromine "disappears" — describe the colour change.
Step 2: Classify and name the ethene product
The C=C opens and Br adds to each carbon: an addition reaction. C₂H₄ + Br₂ → CH₂BrCH₂Br, 1,2-dibromoethane — one single product, which is why the colour vanishes into a colourless compound.
Step 3: The alkane needs light and swaps, not adds
Ethane has no C=C, so it cannot do addition. In UV light a photochemical substitution occurs: a chlorine atom replaces a hydrogen atom. C₂H₆ + Cl₂ → C₂H₅Cl (chloroethane) + HCl. Two products — the swapped-out hydrogen leaves as hydrogen chloride.
(a) Ethene: bromine water decolourised (orange → colourless) [1]; ethane: remains orange [1]. (b) Addition; product 1,2-dibromoethane, CH₂BrCH₂Br [2]. (c) In sunlight/UV a photochemical substitution takes place [1], giving chloroethane (C₂H₅Cl) and hydrogen chloride (HCl) [1].
Worked Example 3
Ethene is converted to (a) ethane and (b) ethanol by two different addition reactions. For each, name the reagent and the catalyst/conditions, and write the equation. [6]
Step 1: To ethane — add hydrogen
Reagent: hydrogen, H₂. Conditions: nickel catalyst with heat. C₂H₄ + H₂ → C₂H₆. The double bond opens, each carbon gains one H, and the unsaturated alkene becomes the saturated alkane — hydrogenation.
Step 2: To ethanol — add steam
Reagent: steam, H₂O. Conditions: phosphoric acid catalyst, 300 °C, 60 atm. C₂H₄ + H₂O → C₂H₅OH. The water splits across the double bond: H to one carbon, OH to the other. This is the industrial "catalytic addition of steam" route to ethanol that returns in 11.3.
Step 3: Spot the family resemblance
Both reactions are addition: C=C opens, small molecule adds, one product only. Only the reagent and catalyst change. Learn addition as one pattern with three costumes (Br₂, H₂, H₂O) rather than three separate reactions.
(a) Hydrogen with a nickel catalyst (heated): C₂H₄ + H₂ → C₂H₆ [3]. (b) Steam with a phosphoric acid catalyst at 300 °C and 60 atm: C₂H₄ + H₂O → C₂H₅OH [3].
Exam Tips for 11.2
1. "Decolourised", never "turns clear". Bromine water goes orange → colourless with an alkene. "Clear" describes transparency, not colour, and loses the mark every time.
2. Substitution keeps count. Alkane + Cl₂ in UV light: one H is replaced, so HCl is always the second product. Forgetting HCl (or writing it as H₂) is the classic slip. And name the reaction fully: photochemical substitution.
3. Cracking runs big → small, needs heat + catalyst, and must make an alkene (or H₂). Balance by counting atoms — the numbers do the work for you.
4. Addition = one product. If your addition equation has two products, it is wrong. Match each addition to its condition: Br₂ needs nothing, H₂ needs nickel, steam needs phosphoric acid at 300 °C/60 atm.
Apply It: Real-World Chemistry
Margarine tubs, cracker plants and a beaker of orange bromine water — the double bond earns its living everywhere.
1
Next to the Jamnagar refinery stands a cracker plant. It takes naphtha — a fraction few customers want to buy — heats it to around 800 °C over a catalyst, and ships out ethene and propene by the tonne to polymer factories, plus extra petrol-range alkanes.
Why does the refinery deliberately smash up molecules it just carefully separated, and why are the alkene products worth more than the alkane feed?
Supply vs Demand: The Refinery's Balancing Act
Crude oil's natural composition does not match what customers buy. There is a surplus of long-chain fractions (naphtha, fuel oil) and a shortage of short-chain petrol and of alkenes. Cracking — high temperature + catalyst — breaks the surplus into the shortage: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
Alkenes: Reactive = Valuable
Alkanes can really only burn. Alkenes have a C=C double bond that opens in addition reactions, so they can be turned into poly(ethene), poly(propene), ethanol and hundreds of other products. The reactivity that makes ethene dangerous to store is exactly what makes it the most important building block in the chemical industry.
Chemistry Connection
Exam questions ask "explain why cracking is important" and want BOTH halves: matching fuel supply to demand AND making alkenes for synthesis. One without the other is half marks.
2
A food factory in Rotterdam converts liquid sunflower oil into a firm spread. The oil is stirred with hydrogen gas over finely divided nickel at about 150 °C; the product is semi-solid, spreadable, and labelled "partially hydrogenated vegetable oil". In India the equivalent product, vanaspati, is made the same way.
What reaction converts the runny oil into a solid spread, and what happens to the C=C bonds along the way?
Addition of Hydrogen Across C=C
Vegetable oil molecules contain several C=C double bonds — they are unsaturated. With H₂ and a nickel catalyst, hydrogen adds across the double bonds (exactly like C₂H₄ + H₂ → C₂H₆), converting them to single bonds. Fewer double bonds → straighter molecules that pack together better → higher melting point → a solid spread.
Testing the Label's Claim
You could check "partially hydrogenated" with aqueous bromine: if some C=C remain, a sample still decolourises the orange solution. A fully saturated fat would leave it orange. The IGCSE bromine test scales all the way up to food science.
Chemistry Connection
This is the same nickel-catalysed hydrogenation as the syllabus reaction ethene → ethane, applied to bigger molecules. When a question says "margarine manufacture", write: hydrogen, nickel catalyst, addition across C=C, product saturated.
3
During a practical in a Cambridge school lab, two unlabelled gas jars are known to contain hexane vapour and hexene vapour. A student adds a few drops of aqueous bromine to each, stoppers them, and shakes — with the blinds down and the lights dimmed, on the teacher's instruction.
How does the test identify the gases, and why did the teacher dim the lights?
The Test Read Correctly
The jar containing hexene decolourises the bromine water — orange to colourless — because bromine adds across the C=C to form a colourless dibromo compound. The hexane jar stays orange: no double bond, no addition.
Why the Dark Matters
In bright light, alkanes are not perfectly innocent: UV can drive a slow photochemical substitution between hexane and bromine, fading the colour and muddying the result. In the dark, the alkane genuinely does nothing, so the test is clean. A subtle detail examiners love: the alkane/alkene distinction by bromine water strictly holds in the absence of UV light.
Chemistry Connection
One reagent, two reaction types: with alkenes bromine does fast addition (no light needed); with alkanes it manages only slow substitution (light required). Same bottle of orange liquid, completely different chemistry.
4
A kitchen in Glasgow: the gas hob burns North Sea methane with a crisp blue flame. The same week, a news story reports a family made ill by a blocked flue on an old gas heater — carbon monoxide poisoning. Same fuel, same house, very different outcomes.
Write the chemistry of both outcomes and explain what the blocked flue changed.
Complete Combustion: The Blue Flame
With a good air supply: CH₄ + 2O₂ → CO₂ + 2H₂O. Combustion is the alkane's one enthusiastic reaction — strongly exothermic, products fully oxidised, flame clean and blue. This is why alkanes, unreactive in almost every other way, run the world's kitchens.
Incomplete Combustion: The Silent Product
The blocked flue starved the flame of oxygen. In limited oxygen: 2CH₄ + 3O₂ → 2CO + 4H₂O — carbon monoxide, a colourless, odourless, toxic gas that binds to haemoglobin and stops the blood carrying oxygen (Topic 10 link). Yellow, sooty flames are the visible warning.
Chemistry Connection
"Alkanes are generally unreactive" has two exceptions on the syllabus — combustion and photochemical substitution — and combustion has two modes. The oxygen supply, not the fuel, decides whether the product is CO₂ or deadly CO.
5
An ethanol plant in Texas takes ethene piped from a cracker and mixes it with steam at 300 °C and 60 atmospheres over a phosphoric acid catalyst. Out flows ethanol — continuously, day and night, at 95%+ purity — destined for solvents, sanitisers and fuel.
Which reaction type is running, and why is this the industrial method of choice where petroleum is cheap?
Steam Addition Across the Double Bond
This is the catalytic addition of steam to ethene: C₂H₄ + H₂O → C₂H₅OH, with phosphoric acid catalyst at 300 °C and 60 atm. The water molecule splits across the C=C: H one side, OH the other. One reactant pair, one product — the addition signature.
Why Industry Likes It
The process is continuous (not batch), fast, and gives essentially pure ethanol without distillation from a dilute soup. Its weakness: ethene comes from cracking petroleum, a non-renewable resource, and the plant conditions are energy-hungry. That trade-off against fermentation is the centrepiece comparison of section 11.3.
Chemistry Connection
Keep the three condition sets filed separately: cracking (high temp + catalyst), hydrogenation (nickel), hydration (phosphoric acid, 300 °C, 60 atm). Exams award marks for attaching the right conditions to the right reaction — and deduct nothing so ruthlessly as a swap.
Practice Questions: 11.2
20 multiple choice questions. Click an option to check your answer.
Your Score
0 / 20
Question 1
An alkane is described as saturated because
Define saturation by BONDS: only single C–C bonds. A is the vague answer examiners refuse — "full of hydrogen" describes the consequence, not the definition.
Question 2
Which is the correct equation for the complete combustion of methane?
Complete combustion gives CO₂ and H₂O, balanced with 2O₂. C and D show incomplete combustion products (CO or soot), which need limited oxygen.
Question 3
Methane reacts with chlorine only when
The substitution is photochemical — UV light supplies the energy to start it. Nickel belongs to hydrogenation, phosphoric acid to steam hydration.
Question 4
The products of CH₄ + Cl₂ in UV light are
Substitution SWAPS one H for one Cl, so the displaced hydrogen leaves with the other chlorine as HCl. One organic product plus HCl — never H₂.
Question 5
Which observation shows a hydrocarbon is unsaturated?
Only a C=C reacts with aqueous bromine at room temperature, turning it orange → colourless. All hydrocarbons burn, so A distinguishes nothing.
Question 6
When ethene reacts with bromine water, the correct description is
"Decolourised" (orange → colourless) is the required wording. "Clear" means transparent — the solution was already clear — and scores nothing.
Question 7
Cracking is best described as
Big → small, by thermal decomposition. A is polymerisation (the reverse direction); C is fractional distillation, a physical process.
Question 8
The conditions for cracking are
High temperature + catalyst (thermal decomposition). UV belongs to substitution; nickel to hydrogenation; 60 atm to steam hydration.
Question 9
C₁₄H₃₀ is cracked to give C₈H₁₈ and one other product. The other product is
Count: C: 14 − 8 = 6; H: 30 − 18 = 12. C₆H₁₂ fits CₙH₂ₙ — hexene, an alkene, exactly what cracking should produce alongside the alkane.
Question 10
Why is cracking economically important? (i) short-chain fuels are in greater demand than long fractions; (ii) it produces reactive alkenes for making polymers; (iii) it turns alkenes into alkanes for petrol
(i) and (ii) are the two textbook reasons. (iii) is backwards — cracking makes alkenes FROM alkanes, not the reverse.
Question 11
Ethene reacts with hydrogen to form ethane. The catalyst and reaction type are
Hydrogenation: H₂ adds across the C=C over a nickel catalyst — an addition reaction with one product, ethane.
Question 12
The manufacture of ethanol from ethene uses
Catalytic addition of steam: phosphoric acid, 300 °C, 60 atm. C describes fermentation, which starts from glucose, not ethene.
Question 13
In an addition reaction, how many products are formed?
Addition = two molecules combine into ONE product. Substitution gives two (the organic product + HCl). Counting products is the fastest way to classify a reaction.
Question 14
The product of ethene + bromine is
BOTH bromine atoms add, one to each carbon of the opened C=C: CH₂BrCH₂Br. A (one Br) would be substitution's product; D invents two products for an addition.
Question 15
Why are alkanes generally unreactive?
No reactive functional group + strong single bonds = chemically boring (except combustion and photochemical substitution). C describes alkenes, the reactive family.
Question 16
A photochemical reaction is one that
Photochemical = driven by light (methane + chlorine in UV; photosynthesis). Light is a reactant's energy source, not a product.
Question 17
Which molecule will NOT decolourise aqueous bromine in the dark?
Propane is a saturated alkane — no C=C, no addition, bromine water stays orange. The -ene endings all signal a double bond.
Question 18
Cracking C₁₆H₃₄ produces two ethene molecules and one other hydrocarbon. Its formula is
Two C₂H₄ remove C₄H₈. Remainder: C 16−4 = 12; H 34−8 = 26 → C₁₂H₂₆, an alkane (2n+2 ✓). Always audit both elements.
Question 19
Margarine manufacture hardens vegetable oils by
Hydrogenation: H₂ + Ni catalyst adds across the double bonds, making the oil more saturated and solid — the ethene → ethane reaction at industrial scale.
Question 20
Supplement: But-1-ene reacts with steam over an acid catalyst. The type of product formed is
Steam adds across the C=C exactly as with ethene: H to one carbon, OH to the other → an alcohol, C₄H₉OH. The steam-addition pattern generalises across the whole alkene family — homologous series in action.
11.3 Alcohols and Carboxylic Acids
▼
Ethanol: One Molecule, Two Factories
Ethanol, C₂H₅OH, is the alcohol you must know inside out: solvent, fuel, and the "alcohol" of everyday speech. Industrially it is made by two completely different routes, and comparing them is one of the most frequently examined discussions on the paper.
Route 1 — Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Raw material: glucose (sugar) from plants — sugarcane, maize — a renewable resource.
Agent: yeast, whose enzymes catalyse the reaction.
Conditions: gentle warmth, 25–35 °C — hot enough for the enzymes to work quickly, cool enough not to denature them — and the absence of oxygen (anaerobic). With oxygen present, the ethanol would be oxidised onwards.
Output: a dilute, impure solution of ethanol that must be concentrated by fractional distillation; the process runs in batches and is slow.
Route 2 — Catalytic addition of steam: C₂H₄ + H₂O → C₂H₅OH
Raw material: ethene from cracking petroleum — non-renewable.
Conditions: 300 °C, 60 atm, phosphoric acid catalyst — learn all three.
Output: essentially pure ethanol, made fast in a continuous process.
| Fermentation | Catalytic addition of steam | |
|---|---|---|
| Raw material | Sugar / glucose — renewable | Ethene from petroleum — non-renewable |
| Conditions | 25–35 °C, yeast, no oxygen — cheap, low energy | 300 °C, 60 atm, phosphoric acid — energy-expensive equipment |
| Rate | Slow (days) | Fast |
| Process type | Batch — stop, empty, restart | Continuous — runs non-stop |
| Product purity | Dilute and impure; needs fractional distillation | Essentially pure |
| Best suited to | Countries with cheap crops and sunshine (Brazil, India) | Countries with cheap petroleum and infrastructure |
Exam Tip
Fermentation conditions are a three-part answer: yeast (source of enzymes), 25–35 °C, absence of oxygen. Each has a reason: enzymes catalyse; too cold = too slow, too hot = enzymes denatured (not "killed" — enzymes are not alive, though "yeast killed" is acceptable); oxygen would oxidise the ethanol (to ethanoic acid — wine turning to vinegar). Condition + reason = the full marks.
Ethanol as a Fuel
Ethanol burns cleanly and exothermically: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Because fermentation ethanol comes from plants that absorbed CO₂ while growing, crop-based ethanol is close to carbon-neutral and is blended into petrol worldwide (E10 in the UK, E20 in India, up to E100 in Brazil). Its other everyday job: an excellent solvent in perfumes, medicines and sanitisers.
From Ethanol to Ethanoic Acid: Oxidation
Ethanol's –OH can be oxidised to the carboxylic acid group –COOH, turning ethanol into ethanoic acid, CH₃COOH. The syllabus names three ways:
Ethanol −[O]→ Ethanoic acid
1. Combustion — burning is oxidation, though it goes all the way to CO₂ and H₂O rather than stopping at the acid.
2. Microbial (bacterial) oxidation — acetobacter bacteria + oxygen from the air slowly oxidise ethanol to ethanoic acid: how vinegar is made, and why opened wine sours.
3. Acidified potassium manganate(VII) — heating ethanol with this purple oxidising agent gives ethanoic acid; the purple colour is decolourised as MnO₄⁻ is reduced.
Carboxylic Acids: Weak but Genuine Acids
Ethanoic acid behaves like every acid you met in Topic 7 — it just does it gently, because it is a weak acid: only partially dissociated into ions in solution (CH₃COOH ⇌ CH₃COO⁻ + H⁺). Its salts are ethanoates.
| Reaction | Products | Example | Observation |
|---|---|---|---|
| + reactive metal | Salt + hydrogen | Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂ | Effervescence; gas pops with lighted splint |
| + base (alkali/oxide) | Salt + water | CH₃COOH + NaOH → CH₃COONa + H₂O | Neutralisation; warms slightly |
| + carbonate | Salt + water + carbon dioxide | 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ | Fizzing; gas turns limewater milky |
Memory Trick
Carboxylic acids follow the MASH rules you already know: Metal → salt + hydrogen, Alkali/base → salt + water, carbonate→ Salt + water + CO₂, and they turn litmus red — just Half-heartedly, because a weak acid is only partially dissociated. New family, old acid chemistry.
Supplement
Esterification: Acid + Alcohol ⇌ Ester + Water
Warm a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid (catalyst), and the two condense into an ester — sweet-smelling compounds responsible for fruit flavours and used as solvents:
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
ethanoic acid + ethanol ⇌ ethyl ethanoate + water
The naming rule: the alcohol provides the first word (ethanol → ethyl), the acid provides the second (ethanoic acid → ethanoate). So methanol + butanoic acid → methyl butanoate. The reaction is reversible (⇌) and the water comes from the acid's –OH plus the alcohol's –H.
The ester linkage: a C=O and a C–O–C bridging oxygen. The water eliminated during esterification is assembled from the acid's –OH and the alcohol's H. This same linkage returns in 11.4 as the repeat-joint of polyesters like PET.
Worked Example 1
Brazil makes most of its ethanol by fermenting sugarcane juice; Saudi Arabia makes ethanol from ethene and steam. (a) Give the equation for fermentation and state the three essential conditions. [4] (b) Suggest why each country chose its method. [2]
Step 1: Equation with the invisible catalyst
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ — glucose splits into two ethanol and two carbon dioxide. Yeast is written over the arrow, not in the equation: its enzymes are catalysts, not reactants.
Step 2: Conditions, each with its why
Yeast — supplies the enzymes that catalyse fermentation. 25–35 °C — warm enough for a good rate, cool enough that the enzymes are not denatured. Anaerobic (no oxygen) — oxygen would allow bacteria to oxidise the ethanol to ethanoic acid.
Step 3: Match method to country
Brazil: abundant cheap sugarcane (renewable feedstock), so slow batch fermentation is economic. Saudi Arabia: little farmland but abundant cheap petroleum, so ethene from cracking + steam addition (fast, continuous, pure) is the better fit. Neither method is universally "best" — feedstock decides.
(a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1]; conditions: yeast (enzymes as catalyst) [1], 25–35 °C [1], absence of oxygen [1]. (b) Brazil has cheap renewable sugar crops, favouring fermentation [1]; Saudi Arabia has cheap petroleum/ethene, favouring catalytic steam addition [1].
Worked Example 2
A bottle of wine left open for weeks tastes of vinegar, and dilute ethanoic acid fizzes when added to sodium carbonate. (a) Explain the change in the wine, naming the type of reaction. [2] (b) Write the equation for the reaction with sodium carbonate and name the salt. [3] (c) Supplement: explain why ethanoic acid solution has a higher pH than hydrochloric acid of the same concentration. [2]
Step 1: Wine to vinegar = microbial oxidation
With the cork off, bacteria (acetobacter) use oxygen from the air to oxidise ethanol to ethanoic acid — the same reaction that vinegar factories run on purpose. Type: oxidation (microbial/bacterial).
Step 2: An acid is an acid
Carboxylic acid + carbonate → salt + water + CO₂: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. The salt is sodium ethanoate. The fizzing is CO₂ — the standard carbonate test.
Step 3: Weak vs strong at equal concentration
Ethanoic acid is a weak acid: only partially dissociated into CH₃COO⁻ and H⁺; HCl is fully dissociated. Same concentration of acid, but a lower concentration of H⁺ in the ethanoic acid — so a higher pH (nearer 7) and slower fizzing.
(a) Ethanol is oxidised to ethanoic acid by bacteria with oxygen from the air (microbial oxidation) [2]. (b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ [2]; the salt is sodium ethanoate [1]. (c) Ethanoic acid is partially dissociated (weak) so its solution contains a lower H⁺ concentration than fully-dissociated HCl [2].
Worked Example 3
Supplement: Propanoic acid is warmed with methanol and a little concentrated sulfuric acid. (a) Name the organic product and state the catalyst's role. (b) Write the equation. (c) Explain how the ester's name is constructed. [5]
Step 1: Identify the two partners' jobs
The alcohol (methanol) contributes the first half of the name: methyl. The acid (propanoic acid) contributes the second: propanoate. Product: methyl propanoate. Concentrated sulfuric acid is the catalyst (and absorbs the water formed, pushing the equilibrium right).
Step 2: Equation with the equilibrium arrow
C₂H₅COOH + CH₃OH ⇌ C₂H₅COOCH₃ + H₂O. Esterification is reversible — use ⇌. The water molecule is built from the acid's –OH and the alcohol's –H.
Step 3: Beware the reversed name
Propyl methanoate would be a completely different ester (from propanol + methanoic acid). Ester names are read alcohol-first, acid-second — direction is everything, and this is precisely the trap set in multiple-choice questions.
(a) Methyl propanoate; concentrated sulfuric acid is the catalyst [2]. (b) C₂H₅COOH + CH₃OH ⇌ C₂H₅COOCH₃ + H₂O [2]. (c) The alcohol gives the first word (methanol → methyl); the acid gives the second (propanoic → propanoate) [1].
Exam Tips for 11.3
1. Fermentation vs hydration is a table question — answer in contrasts. Renewable vs non-renewable, slow batch vs fast continuous, impure vs pure, low-energy vs high-energy conditions. Pair each point both ways for full marks.
2. Do not swap the temperatures. Fermentation 25–35 °C (enzymes denature if hotter); steam addition 300 °C, 60 atm, phosphoric acid. Writing "yeast at 300 °C" is an instant zero for conditions.
3. Three oxidation routes to ethanoic acid: combustion, bacterial oxidation (vinegar), acidified potassium manganate(VII) — and with KMnO₄ the observation is purple decolourised.
4. Weak acid = partially dissociated. Not "less concentrated", not "dilute" — concentration and strength are different axes. Ethanoic acid still does all the MASH reactions, just more slowly than a strong acid of equal concentration.
Apply It: Real-World Chemistry
Sugarcane fields in Uttar Pradesh, malt vinegar on chips in Yorkshire, and a bottle of nail-varnish remover — welcome to the –OH and –COOH economy.
1
India blends 20% ethanol into petrol (E20), and most of that ethanol is fermented from sugarcane molasses in distilleries across Uttar Pradesh and Maharashtra. Vats of diluted molasses are seeded with yeast, sealed, and held at about 30 °C for a few days; the dilute brew is then concentrated by fractional distillation.
Explain each design choice: the yeast, the 30 °C, the sealed vats and the distillation step — and why India prefers this route over the ethene route.
The Fermentation Recipe, Justified
Yeast supplies the enzymes that catalyse C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. 30 °C sits in the enzymes' happy zone: colder is uneconomically slow, hotter denatures them and fermentation stops. Sealed vats keep oxygen out — with air, bacteria would oxidise the hard-won ethanol into ethanoic acid.
Why Ferment At All?
Fermentation gives a dilute, impure solution (the yeast dies at around 15% ethanol), hence the fractional distillation to fuel grade. India still chooses it because molasses is a cheap, renewable by-product of its giant sugar industry, while ethene would have to come from imported petroleum. Bonus: crop-based ethanol is roughly carbon-neutral, supporting E20's emissions case.
Chemistry Connection
This one distillery scene contains four markable syllabus items: the fermentation equation, all three conditions with reasons, the purification step, and the renewable-vs-non-renewable comparison. Exam questions about "country X chooses method Y" are really asking: what feedstock is cheap there?
2
A fish-and-chip shop in Leeds splashes malt vinegar over everything. The vinegar factory upstream makes it from malted barley: first a fermentation to alcohol, then the liquid is trickled through tanks packed with wood shavings while warm air is blown through — the exact opposite of the sealed fermentation vat.
Why does the first stage exclude air while the second stage blasts air through, and what chemistry happens in each tank?
Stage 1: Anaerobic — Protect the Ethanol
Yeast ferments the barley sugars to ethanol in the absence of oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Air is excluded precisely because oxygen would trigger the next reaction too early.
Stage 2: Aerobic — Oxidise It On Purpose
Now the goal is oxidation: acetobacter bacteria on the wood shavings use the oxygen in the blown air to oxidise ethanol to ethanoic acid — microbial oxidation. Vinegar is simply a dilute solution of ethanoic acid (plus flavour compounds). The wood shavings maximise the surface where bacteria, ethanol and air meet.
Chemistry Connection
One factory, both halves of the syllabus story: fermentation needs NO oxygen; vinegar-making needs oxygen. The same pair of facts answers "why did my opened wine go sour?" — because leaving the cork off runs stage 2 by accident.
3
During the pandemic, Indian sugar mills and Scottish gin distilleries alike switched to making hand sanitiser — typically 70–80% ethanol. The same bottles note "flammable: keep from open flame". Meanwhile Brazil's flex-fuel cars happily burn E100 — near-pure ethanol — at the pump.
Which two properties of ethanol do the sanitiser and the fuel tank each exploit, and what equation describes the fuel use?
Solvent and Fuel: The Two Star Roles
Sanitiser exploits ethanol as a solvent (dissolves oils, mixes fully with water) and disinfectant. The warning label exploits nothing — it fears the second property: ethanol burns readily. In the engine that is the whole point: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, a clean exothermic combustion.
Why a Crop Can Fuel a Car
The CO₂ released on burning was absorbed from the atmosphere by photosynthesis while the sugarcane grew, so bioethanol is close to carbon-neutral — unlike petrol, whose carbon has been locked away since the dinosaurs. That, plus renewability, is why governments mandate blends like E10 and E20.
Chemistry Connection
When asked "give two uses of ethanol", the safe pair is fuel and solvent. And know the combustion equation cold — it doubles as one of the three oxidation routes from ethanol.
4
A cook in Chennai pours vinegar onto a spoonful of baking soda while making a sponge, and the mixture erupts in froth. Her niece, revising for IGCSE, points out this is the same chemistry as her lab test for carbonates — and that the leftover liquid tastes salty rather than sour.
Write the chemistry of the froth, name the salt formed, and explain why ethanoic acid froths more gently than hydrochloric acid would.
The Froth Is CO₂
Acid + carbonate (or hydrogencarbonate) → salt + water + CO₂. With baking soda: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The salt is sodium ethanoate — the salty taste. The bubbles raise the sponge; the lab version turns limewater milky.
Gentle Fizz = Weak Acid
Ethanoic acid is weak — only partially dissociated, so at any moment the solution holds a lower concentration of H⁺ than an HCl solution of equal concentration. Fewer H⁺ collisions per second → slower fizzing. Same total gas eventually (same moles of acid), just delivered more politely.
Chemistry Connection
Carboxylic acids are not a new kind of acid — they are Topic 7 acids with a carbon tail. Metal → salt + H₂; base → salt + water; carbonate → salt + water + CO₂. The only new words are "weak", "partially dissociated" and the salt family name "ethanoates".
5
A flavour chemist in Geneva builds fruit aromas from small esters: one smelling of pear drops, another of pineapple, another of apple. Each is made the same way — warming a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid — and each smells nothing like its sharp, sour parents.
What reaction builds the aromas, why is the sulfuric acid there, and how would the chemist name the ester made from butanoic acid and ethanol?
Esterification: Condensation in Miniature
Acid + alcohol ⇌ ester + water, with concentrated sulfuric acid as catalyst. The acid's –OH and the alcohol's –H leave together as water; the remaining fragments join through the ester linkage –COO–. The reaction is reversible, so the ⇌ arrow matters.
Name It Alcohol-First
Ethanol gives ethyl; butanoic acid gives butanoate: the ester is ethyl butanoate (pineapple!). Reversing the ingredients — butan-1-ol + ethanoic acid — would give butyl ethanoate, a different molecule with a different smell. Direction is chemistry here, not just grammar.
Chemistry Connection
Hold onto the ester linkage –COO– formed with loss of water: in 11.4 the same trick, done at BOTH ends of every molecule, builds PET polyester bottles. A perfume-sized reaction becomes a polymer factory just by using difunctional ingredients.
Practice Questions: 11.3
20 multiple choice questions. Click an option to check your answer.
Your Score
0 / 20
Question 1
The equation for fermentation is
Glucose → 2 ethanol + 2 carbon dioxide, catalysed by yeast enzymes. A is the ethene route; C is respiration with oxygen — fermentation is anaerobic.
Question 2
The best temperature for fermentation is 25–35 °C because
The window balances rate (too cold = too slow) against enzyme survival (too hot = denatured). Ethanol boils at 78 °C, so A is nonsense.
Question 3
Fermentation must be carried out in the absence of oxygen because oxygen would
With air present, bacteria oxidise ethanol to ethanoic acid (vinegar) — ruining the product. That is also why opened wine sours.
Question 4
Which set of conditions is used to make ethanol from ethene?
Catalytic addition of steam: 300 °C, 60 atm, phosphoric acid. The other sets belong to fermentation, hydrogenation and photochemical substitution respectively.
Question 5
An advantage of fermentation over the catalytic addition of steam is that fermentation
Sugar is renewable; ethene from petroleum is not. Speed, purity and continuity are all advantages of the OTHER method — a favourite switcheroo.
Question 6
Two major uses of ethanol are
Fuel (burns cleanly, blends with petrol) and solvent (perfumes, medicines, sanitisers) are the syllabus pair.
Question 7
The complete combustion of ethanol produces
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Complete combustion of any C/H/O compound gives CO₂ and H₂O.
Question 8
Which is NOT a way of oxidising ethanol to ethanoic acid?
Steam addition MAKES ethanol from ethene — it is hydration, not oxidation. A and D are the same bacterial process; B is the lab oxidising agent.
Question 9
When ethanol is oxidised by acidified potassium manganate(VII), the colour change observed is
Potassium manganate(VII) is purple; as it oxidises the ethanol it is itself reduced and decolourised. (Orange-to-green is the OTHER oxidising agent, dichromate — not required here.)
Question 10
Vinegar is essentially a dilute solution of
Ethanoic acid, made by microbial oxidation of ethanol. The sour taste and pH below 7 are the weak acid at work.
Question 11
Ethanoic acid is a weak acid because it
Weak = partial dissociation (CH₃COOH ⇌ CH₃COO⁻ + H⁺). Strength and concentration are independent: you can have concentrated weak acid or dilute strong acid.
Question 12
Ethanoic acid reacts with magnesium. The products are
Acid + metal → salt + hydrogen — same as any acid. The salt of ethanoic acid is an ethanoate: (CH₃COO)₂Mg.
Question 13
Which observation occurs when ethanoic acid is added to sodium carbonate solution?
Acid + carbonate → salt + water + CO₂; the CO₂ fizzes and turns limewater milky. Sodium ethanoate is soluble, so no precipitate.
Question 14
Compared with hydrochloric acid of the same concentration, ethanoic acid has
Partial dissociation → fewer H⁺ ions → higher pH (closer to 7) and slower reactions, even at identical concentration.
Question 15
Supplement: The catalyst for esterification is
A few drops of concentrated sulfuric acid catalyse ester formation (and help remove the water). Nickel is for hydrogenation; yeast for fermentation.
Question 16
Supplement: Ethanoic acid + ethanol ⇌ ? + water. The missing product is
Alcohol first (ethyl), acid second (ethanoate): ethyl ethanoate, CH₃COOC₂H₅, the classic pear-drops ester and nail-varnish solvent.
Question 17
Supplement: The ester methyl butanoate is made from
Read the name backwards: methyl ← methanol; butanoate ← butanoic acid. Answer A would give butyl methanoate — the reversed ester and the reversed trap.
Question 18
Supplement: In esterification, the water molecule is formed from
The acid loses its whole –OH, the alcohol loses only the H of its –OH; the alcohol's oxygen stays in the ester as the bridging O of –COO–.
Question 19
Which statement about the two industrial routes to ethanol is correct?
Each method's strengths are the other's weaknesses: hydration is fast/continuous/pure but petroleum-based; fermentation is renewable but slow, batch and dilute.
Question 20
The salt formed when ethanoic acid is neutralised by potassium hydroxide is
Salts of ethanoic acid are ethanoates: CH₃COOK plus water. Acid + alkali → salt + water, as always.
11.4 Polymers
▼
Polymers: Very Long Molecules from Very Small Ones
A polymer is a large molecule built by joining together thousands of small repeating molecules called monomers. Plastics are synthetic polymers; proteins and DNA are natural ones. The syllabus wants two construction methods — addition and condensation polymerisation — and the ability to draw and decode repeat units for both.
Addition Polymerisation: The Double Bonds Do the Joining
Alkene monomers join when their C=C double bonds open and link the molecules into one continuous chain. Nothing is lost: the polymer is the only product, and it contains every atom of every monomer.
n C₂H₄ → −(CH₂–CH₂)ₙ− (ethene → poly(ethene))
Monomer: an alkene — it MUST have a C=C to open.
Repeat unit: the monomer with the double bond opened: two carbons, single-bonded, with bonds extending through the brackets on both sides, and n after the bracket.
Naming: poly(monomer): ethene → poly(ethene); chloroethene → poly(chloroethene) (PVC); propene → poly(propene).
The three marks of a correct repeat unit: (1) the C=C has become a single bond, (2) bonds extend through the brackets on both sides, (3) n is written after the bracket. Leaving the double bond in place is the single most common polymer error in exams.
Exam Tip
Going backwards — polymer to monomer: take one repeat unit (two backbone carbons with their attached groups), delete the extension bonds, and reinstate the C=C. If the repeat unit shows CHCl–CH₂, the monomer is CHCl=CH₂, chloroethene. Every atom on the backbone stays exactly where it was — only the bond order changes.
Supplement
Condensation Polymerisation: Join AND Eject
In condensation polymerisation, monomers join through their functional groups and a small molecule (usually water) is expelled at every link. The monomers do not need C=C; instead each must carry two functional groups, one at each end, so the chain can grow in both directions.
Polyamides (e.g. nylon): made from a dicarboxylic acid (HOOC–□–COOH) + a diamine (H₂N–□–NH₂). Each –COOH condenses with an –NH₂, losing H₂O and forming an amide linkage (–CO–NH–).
Polyesters (e.g. PET): made from a dicarboxylic acid + a diol (HO–□–OH). Each link is an ester linkage (–COO–) — the esterification of 11.3 repeated at both ends of every monomer. PET (polyethylene terephthalate) is the fizzy-drink-bottle polyester, and it is recyclable: it can be melted and re-formed, or broken back into its monomers and repolymerised.
Proteins are natural polyamides: amino acids each carry an –NH₂ and a –COOH, and they condense into chains linked by amide (peptide) bonds — the same linkage as nylon.
Block diagrams are exactly what the exam wants: boxes for the carbon skeletons, real atoms for the functional groups and linkages. Nylon's joins are amide (–CONH–); PET's are ester (–COO–); one water molecule leaves per linkage formed.
Addition vs Condensation: The Comparison Table
| Addition polymerisation | Condensation polymerisation (S) | |
|---|---|---|
| Monomer requirement | C=C double bond (alkenes) | Two functional groups per monomer (e.g. –COOH + –NH₂, or –COOH + –OH) |
| Number of products | One — the polymer only | Two — polymer + small molecule (water) at every link |
| Atoms in polymer | ALL atoms of the monomers | Monomer atoms MINUS the water lost |
| Linkage in chain | Plain C–C backbone | Amide (–CONH–) in polyamides; ester (–COO–) in polyesters |
| Examples | Poly(ethene), poly(propene), PVC | Nylon (polyamide), PET (polyester), proteins (natural polyamide) |
Plastics: The Problem With Success
The properties that make plastics useful — cheap, tough, unreactive — are exactly what makes them an environmental disaster. Poly(alkene) plastics are non-biodegradable: no bacteria can digest their inert C–C backbones, so they persist for centuries. The syllabus wants you to discuss:
The plastics problem: durable was the point — and is the problem
Landfill: non-biodegradable waste accumulates; sites fill; land and groundwater are affected.
Oceans and microplastics: plastic waste fragments into microplastics that are eaten by marine animals and enter food chains — including ours.
Burning: incineration releases toxic gases — carbon monoxide from incomplete combustion, and acidic HCl from burning PVC (chlorine-containing polymers).
Responses: reduce, reuse, recycle (PET is widely recycled — melted or depolymerised back to monomers), develop biodegradable polymers, and legislate (single-use plastic bans).
Worked Example 1
Chloroethene has the structure CH₂=CHCl. (a) Draw the repeat unit of the polymer it forms and name the polymer. (b) State the type of polymerisation and explain why this monomer can polymerise. (c) Explain why burning waste PVC is hazardous. [6]
Step 1: Open the double bond, keep everything else
Backbone: two carbons joined by a single bond. One carbon keeps its two H; the other keeps one H and the Cl. Draw extension bonds through the brackets on both sides and write n. The Cl stays exactly where it was — do not move it or lose it.
Step 2: Classify and justify
Addition polymerisation: the monomer contains a C=C double bond which opens, allowing the molecules to join with no other product. Name: poly(chloroethene) — PVC.
Step 3: The combustion hazard
PVC contains chlorine, so burning releases the toxic acidic gas hydrogen chloride (HCl), along with carbon monoxide if oxygen is limited. This is why incineration of mixed plastic waste needs gas scrubbing.
(a) Repeat unit: −(CH₂–CHCl)− with single C–C bond, bonds through the brackets, subscript n; polymer: poly(chloroethene) / PVC [3]. (b) Addition polymerisation; possible because the monomer has a C=C that opens [2]. (c) Burning releases toxic gases: HCl (from the chlorine) and CO in limited oxygen [1].
Worked Example 2
Supplement: Nylon is made from a dicarboxylic acid and a diamine. (a) Using block diagrams, show the structure of the polymer, labelling the linkage. (b) Name the other product and state how many of its molecules form per linkage. (c) Explain why nylon is called a polyamide and how proteins are related. [6]
Step 1: Set up the two monomers
Dicarboxylic acid: HOOC–□–COOH. Diamine: H₂N–□–NH₂. The boxes stand for unspecified carbon chains; the business ends are the four functional groups — two per monomer, which is what lets a chain (not just one ester-like pair) form.
Step 2: Condense: –COOH + H₂N– → –CONH– + H₂O
Each acid group loses –OH, each amine group loses one H; they join as the amide linkage –CO–NH– and the OH + H leave as one water molecule per linkage. The polymer alternates: □acid–CONH–□amine–CONH–…
Step 3: Name the family and the natural cousin
The chain is held together by many amide links → a polyamide. Proteins are natural polyamides: amino acid monomers (each with –NH₂ AND –COOH on one molecule) condense through the same –CONH– linkage.
(a) Alternating boxes joined by –CO–NH– units, linkage labelled amide [3]. (b) Water; one molecule per linkage formed [1]. (c) Its repeating join is the amide linkage, hence polyamide [1]; proteins are natural polyamides built from amino acids via the same linkage [1].
Worked Example 3
A council must choose between landfilling, incinerating, or recycling its waste PET bottles. Evaluate the three options, including the relevant chemistry. [6]
Step 1: Landfill — the do-nothing option
PET is non-biodegradable — microorganisms cannot break it down — so bottles persist for centuries, landfill space runs out, and fragments escape as microplastics into waterways and food chains. Cheap now, expensive forever.
Step 2: Incineration — energy but emissions
Burning recovers energy and shrinks volume, but releases CO₂ (greenhouse gas) and, with incomplete combustion, toxic carbon monoxide; mixed plastic streams containing PVC add acidic HCl. Requires costly flue-gas cleaning.
Step 3: Recycling — the chemistry-backed choice
PET is a polyester and is recyclable: bottles are sorted, shredded and melted for re-forming into fibres and new bottles, or chemically broken back into monomers and repolymerised to food-grade quality. Saves crude-oil feedstock and cuts both landfill and combustion emissions; the costs are collection and sorting.
Landfill: PET is non-biodegradable, persists, creates microplastics [2]. Incineration: energy recovered but toxic/greenhouse gases released [2]. Recycling: PET can be melted and re-formed or depolymerised to its monomers and remade, conserving petroleum feedstock — the best option despite sorting costs [2].
Exam Tips for 11.4
1. Repeat-unit checklist: single C–C backbone (the C=C has gone!), all side groups kept in place, bonds extending through the brackets, n outside. Four checks, four marks protected.
2. Monomer from polymer: isolate two backbone carbons, restore the C=C, delete the through-bonds. The side groups tell you which alkene it was.
3. Addition vs condensation in one line each: addition = C=C opens, polymer is the only product; condensation = two functional groups react, water eliminated at every link. Polyamide = –CONH– (nylon, proteins); polyester = –COO– (PET).
4. Environmental answers need chemistry words, not just "bad for the planet": non-biodegradable, microplastics, toxic combustion products (CO, HCl from PVC), and PET's recyclability. Name them and the marks follow.
Apply It: Real-World Chemistry
Bottles, sarees, parachutes and a problem the size of an ocean — polymers built the modern world and now we have to tidy up after them.
1
India recycles a higher share of its PET bottles than almost any other large country — informal collectors gather them by the tonne, and plants in Panipat spin the shredded flakes into polyester fibre for clothing. A bottle drunk in Delhi may return as a school uniform.
What kind of polymer is PET, and what chemical features make bottle-to-fibre recycling possible?
PET Is a Polyester (a Condensation Polymer)
PET is built from a dicarboxylic acid + a diol, joined by ester linkages (–COO–) with a water molecule lost at every link. Bottle plastic and clothing polyester are the same polymer in different shapes — which is precisely why one can become the other.
Two Recycling Routes
Thermal: PET softens when heated, so flakes can be melted and re-formed (spun into fibre, blown into new bottles). Chemical: the ester links can be broken to give back the monomers, which are purified and repolymerised — producing food-grade PET indistinguishable from new. Either way, less petroleum is cracked for fresh feedstock.
Chemistry Connection
The syllabus line "PET is recyclable" hides real chemistry: condensation polymers have linkages that can be reversed (add water back, get monomers), unlike the inert C–C backbone of poly(ethene). The bond type decides the afterlife.
2
Marine biologists sampling the Indian Ocean find microplastic fragments in plankton, in fish stomachs, and even in sea salt sold in markets. Much of it began as poly(ethene) bags and packaging that entered rivers years ago and has been fragmenting ever since — but never disappearing.
Why does poly(ethene) fragment but never biodegrade, and why does that matter to a food chain?
An Alkane in Disguise
Poly(ethene) is essentially a gigantic alkane: an inert backbone of strong C–C single bonds with no functional groups. Decomposer bacteria have no enzymes that can break it — it is non-biodegradable. Sunlight and abrasion only crack objects into ever-smaller pieces: microplastics, chemically unchanged.
Small Enough to Eat, Too Stable to Digest
Once fragments reach plankton size they are eaten by filter feeders, pass up the food chain, and accumulate — carrying adsorbed pollutants with them. The same unreactivity that makes a poly(ethene) bag food-safe makes it indigestible to every organism in the ocean.
Chemistry Connection
One idea explains both halves of the topic: alkanes are unreactive (11.2), and poly(ethene) is a giant alkane (11.4) — so plastic pollution is the reactivity chapter's conclusion wearing an environmental costume. Exam answers should say "non-biodegradable" and "microplastics", not "it litters".
3
A skydiving centre near Pune repacks parachutes of woven nylon — light, elastic and astonishingly strong for its weight. The same polymer, drawn into finer thread, is in toothbrush bristles, guitar strings and sarees blended for durability.
What class of polymer is nylon, what monomers build it, and where else does the same linkage appear — including inside your own body?
A Polyamide by Condensation
Nylon is a polyamide, made by condensation polymerisation of a dicarboxylic acid (HOOC–□–COOH) and a diamine (H₂N–□–NH₂). Every –COOH/–NH₂ pair condenses to an amide linkage –CONH–, expelling one water molecule. The long regular chains pack and attract strongly — hence fibres of exceptional strength.
The Linkage You Are Made Of
The identical –CONH– join holds together proteins — your hair, muscle, enzymes — built from amino acid monomers. Proteins are natural polyamides; nylon is the synthetic imitation, invented in 1935 as "artificial silk" (silk itself is a protein — the mimicry was deliberate).
Chemistry Connection
Exam shortcut: see "polyamide", write –CONH–; see "polyester", write –COO–; see "protein", write "natural polyamide". Three associations cover nearly every condensation-polymer mark on the paper.
4
India banned many single-use plastics in 2022; the UK taxes plastic packaging with less than 30% recycled content. Meanwhile a street vendor's stall shows the alternatives: paper straws, cloth bags, banana-leaf plates — and still plenty of poly(ethene) film, because nothing else is as cheap or as waterproof.
Why do governments target single-use plastic specifically, and what does "dealing with the problem" look like in chemistry terms?
Why Single-Use Is the Worst Case
A carrier bag is used for minutes but persists for centuries, because poly(ethene) is non-biodegradable. Multiply by billions of bags and the mismatch between useful life and chemical lifetime fills landfills, chokes drains (a real monsoon-flooding factor), and feeds the ocean's microplastic load.
The Chemist's Toolbox
Reduce and reuse (fewer bags, thicker reusable ones); recycle where the polymer allows (PET especially); incinerate with energy recovery only with scrubbing, since burning can release CO and, from PVC, HCl; and develop biodegradable polymers whose linkages microbes CAN hydrolyse. Legislation simply pushes users toward these options.
Chemistry Connection
A 6-mark "discuss the problems of plastics and how to deal with them" question wants BOTH lists: problems (non-biodegradable, landfill, microplastics, toxic combustion gases) and responses (reduce/reuse/recycle, energy recovery, biodegradable alternatives). Tick them off like a shopping list.
5
A materials lab in Cambridge tests two clear films: one is poly(ethene) from ethene; the other is a new compostable polyester made from plant-derived acid and alcohol monomers. Buried in warm compost, the polyester film vanishes in months; the poly(ethene) film emerges intact, just dirtier.
Explain the different fates in terms of how each polymer was built and the bonds holding it together.
Built Differently, Broken Differently
Poly(ethene) is an addition polymer: its backbone is pure C–C, formed by opening C=C bonds, with no weak points. The polyester is a condensation polymer: its chain is stitched with ester linkages, formed by expelling water — and what condensation made, hydrolysis can unmake.
Microbes Attack the Linkages
Compost microorganisms produce enzymes that hydrolyse ester links (add water back), snipping the chain into small fragments they can digest — so the film biodegrades. No organism has enzymes for the inert C–C backbone, so poly(ethene) survives untouched. Biodegradability is a property of the linkage, not of "plastic" in general.
Chemistry Connection
This closes the loop of the whole topic: esterification (11.3) run forward builds PET and compostable films; run backward (hydrolysis) it recycles or biodegrades them. Reactions, not materials, are what you really learned in Topic 11.
Practice Questions: 11.4
20 multiple choice questions. Click an option to check your answer.
Your Score
0 / 20
Question 1
A polymer is
Monomers (small) join to make the polymer (large) — A states it backwards. Decane has ten carbons but is no polymer; the repeating structure is what counts.
Question 2
Which molecule can undergo addition polymerisation?
Addition polymerisation requires a C=C double bond to open — only the alkene qualifies. The monomer test is always: does it have C=C?
Question 3
In addition polymerisation, the number of products formed is
The double bonds open and every atom of every monomer ends up in the chain — nothing is expelled. Water-per-link is the signature of CONDENSATION polymerisation.
Question 4
The polymer formed from ethene is called
Name = poly(monomer): poly(ethene), the plastic of bags and bottles caps. Polyester and polyamide are condensation families.
Question 5
Which is a correct feature of the repeat unit of poly(ethene)?
The double bond OPENS during polymerisation, becoming single, and the freed bonds extend to neighbouring units through the brackets. Keeping the C=C is the classic drawing error.
Question 6
A polymer chain contains the section …–CH₂–CHCl–CH₂–CHCl–… . Its monomer is
Take one repeat unit (CH₂–CHCl), restore the double bond: CH₂=CHCl, chloroethene. C would give Cl on every carbon; D is not even unsaturated.
Question 7
Supplement: Condensation polymerisation differs from addition polymerisation because condensation
Condensation = join + eject water; the monomers need two functional groups each, not C=C. Nylon and PET are made industrially, so D fails.
Question 8
Supplement: Nylon is made from
Acid (–COOH ends) + diamine (–NH₂ ends) → amide links → polyamide (nylon). Acid + diol gives a polyESTER like PET — option A is that trap.
Question 9
Supplement: The linkage in a polyamide is
Amide = –CO–NH–, formed from –COOH + H₂N– with loss of water. –COO– is the ester linkage of polyesters.
Question 10
Supplement: PET, used for drinks bottles, is
PET = polyester: acid + diol joined by ester links, water expelled. It is synthetic and recyclable — two facts the syllabus states directly.
Question 11
Supplement: Proteins are described as natural
Amino acid monomers condense through amide linkages — the same –CONH– as nylon — so proteins are natural polyamides.
Question 12
Plastics cause problems in landfill because they are
Microorganisms cannot decompose the inert polymer chains, so plastic waste persists for centuries and landfill sites fill up.
Question 13
Microplastics are a concern mainly because they
Tiny fragments are ingested by plankton and filter feeders and pass up food chains — plastic does not dissolve (that inertness again).
Question 14
Burning PVC (poly(chloroethene)) waste is hazardous because it can release
The chlorine in PVC leaves as acidic, toxic HCl, and incomplete combustion of any plastic gives CO. This is the standard argument against open burning of plastic waste.
Question 15
Which is NOT a sensible way of dealing with plastic waste?
Rivers deliver plastic straight to the ocean, where it fragments into microplastics. Reduce, reuse, recycle, and redesign are the accepted strategies.
Question 16
Poly(propene) is made from propene. During the polymerisation
Addition polymerisation: double bonds open, chain forms, nothing lost — the CH₃ side groups stay attached and dangle from the backbone.
Question 17
Supplement: In a condensation polymerisation forming nylon, for every amide linkage made, the number of water molecules released is
Each –COOH + H₂N– condensation expels exactly one H₂O (the OH from the acid, the H from the amine). One link, one water.
Question 18
Supplement: A polyester differs from a polyamide in that the polyester's monomers include
Both use a dicarboxylic acid; the partner decides the family. Diol → ester links → polyester (PET). Diamine → amide links → polyamide (nylon).
Question 19
Supplement: PET is described as recyclable because it can be
Thermal recycling (melt and re-form) and chemical recycling (hydrolyse ester links → monomers → new PET) both work because of PET's reversible ester linkages.
Question 20
Which statement correctly matches polymer to type?
The final sorting question: alkene monomer → addition (poly(ethene)); acid + diamine → polyamide (nylon); acid + diol → polyester (PET), both condensation.