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Question 1 -- Homologous Series, Naming and Isomers
Total: 12 marks
A student in Pune uses a molecular model kit to build the first few members of several families of organic compounds. Her teacher explains that each family is called a homologous series.
(a)[4]
The student builds models of methane, ethane, propane and butane.
State four characteristics of the compounds in a homologous series such as the alkanes.
Model Answer -- 1(a)
All members contain the same functional group, so they undergo the same chemical reactions [1]
All members share the same general formula; for the alkanes this is CnH2n+2 [1]
Each member differs from the next by one CH₂ unit, so relative molecular masses differ by 14 [1]
Physical properties such as boiling point and viscosity change gradually as the chain length increases, giving a smooth trend down the series [1]
⚠ If you missed marks here: Saying members have "similar formulae" is too vague — the examiner wants the words same general formula. Likewise "similar properties" scores nothing on its own: chemical properties are the same because the functional group is the same, while physical properties show a gradual trend. Keep those two ideas separate and you collect two marks instead of none.
(b)[4]
The student is then given the structural formulae of some compounds.
(i) Name the compound CH₃CH₂CH₂CH₂OH. [1]
(ii) Name the compound CH₃CH=CH₂. [1]
(iii) A displayed formula shows more information than a structural formula. State what is shown in a displayed formula, and describe the displayed formula of ethane. [1]
(iv) Write the molecular formula of the alkene that contains four carbon atoms. [1]
Model Answer -- 1(b)
(i) Butan-1-ol — a four-carbon alcohol with the OH group on the end carbon [1]
(ii) Propene — a three-carbon alkene containing one C=C double bond [1]
(iii) A displayed formula shows every atom and every bond in the molecule; in ethane the two carbon atoms are joined by a single line, and each carbon carries three lines to three separate hydrogen atoms [1]
(iv) C₄H₈ — using the alkene general formula CnH2n with n = 4 [1]
⚠ If you missed marks here: "Butanol" without the number loses the mark at Extended level — the position of the OH group must be given, so write butan-1-ol. In (iii) the key phrase is all atoms and all bonds; answers that only mention atoms describe a molecular formula, not a displayed one.
(c)[4]
The student builds two different models, both using four carbon atoms and ten hydrogen atoms.
(i) State what is meant by the term structural isomers. [1]
(ii) Describe the structures of the two isomers of C₄H₁₀ and name each one. [2]
(iii) Explain why these two compounds are isomers of each other. [1]
Model Answer -- 1(c)
(i) Structural isomers are compounds with the same molecular formula but different structural formulae [1]
(ii) Butane has all four carbon atoms joined in a continuous unbranched chain [1]
(ii) 2-methylpropane has a chain of three carbon atoms with a CH₃ branch attached to the middle carbon [1]
(iii) Both molecules contain exactly four carbon and ten hydrogen atoms, so the molecular formula C₄H₁₀ is identical while the arrangement of the atoms is different [1]
⚠ If you missed marks here: The definition must contain both halves: same molecular formula AND different structural formula — one half alone scores zero. When describing 2-methylpropane, say the branch is on the middle carbon of a three-carbon chain; "a branched chain" with no detail does not earn the mark.
Question 2 -- Petroleum and Fractional Distillation
Total: 12 marks
The refinery complex at Jamnagar in Gujarat, India, is the largest in the world. Crude petroleum arrives by tanker and is separated into useful fractions in tall fractionating columns.
(a)[4]
Describe how fractional distillation separates petroleum into fractions. Your answer should refer to what petroleum is, how the column is operated and why different fractions leave the column at different heights.
Model Answer -- 2(a)
Petroleum is a mixture of many different hydrocarbons, so the components keep their own properties and can be separated physically [1]
The crude oil is heated so that it vaporises, and the hot vapour enters near the bottom of the column [1]
The column has a temperature gradient — hottest at the bottom and coolest at the top — and the vapours rise through it [1]
Each hydrocarbon condenses at the level where the column temperature falls to its boiling point, so fractions with different boiling point ranges are collected at different heights [1]
⚠ If you missed marks here: The word mixture is worth a whole mark — never call petroleum a compound. The most commonly missed point is the temperature gradient: you must state that the column is hot at the bottom and cool at the top, otherwise the statement "fractions condense at different levels" has no explanation behind it.
(b)[4]
Give one use for each of the following fractions.
(i) refinery gas [1]
(ii) gasoline [1]
(iii) naphtha [1]
(iv) bitumen [1]
Model Answer -- 2(b)
(i) Refinery gas is used as bottled gas for heating and cooking [1]
(ii) Gasoline (petrol) is used as fuel for cars [1]
(iii) Naphtha is used as a feedstock for making chemicals in the chemical industry [1]
(iv) Bitumen is used for surfacing roads (and for waterproofing roofs) [1]
⚠ If you missed marks here: Naphtha is the odd one out — it is not burned as a fuel but used as a chemical feedstock, the starting material for making other chemicals. Confusing kerosene (jet fuel) with gasoline (car fuel) is another classic slip; learn the column order and the uses come with it.
(c)[4]
Fuel oil is collected much lower in the column than refinery gas.
(i) State how the chain length, boiling point and viscosity of the hydrocarbons change going down the column. [3]
(ii) Explain, in terms of the attractive forces between molecules, why boiling point changes in this way. [1]
Model Answer -- 2(c)
(i) Going down the column the chain length of the hydrocarbon molecules increases [1]
(i) The boiling point range of the fractions increases down the column [1]
(i) The viscosity increases down the column (the liquids become thicker and flow less easily), while volatility decreases [1]
(ii) Longer molecules have stronger attractive forces between them, so more energy is needed to separate the molecules and the boiling point is higher [1]
⚠ If you missed marks here: Direction matters: the question asks about going down the column, so all three properties increase (and volatility falls). In (ii) never say "the covalent bonds are stronger" — boiling separates whole molecules from each other, so it is the intermolecular attractions, not the C–C bonds, that must be overcome.
Question 3 -- Alkanes: Combustion and Substitution
Total: 12 marks
A gas platform in the North Sea, off the coast of Scotland, produces natural gas together with small amounts of ethane and propane. These alkanes are separated and sold as fuels and chemical feedstocks.
(a)[3]
(i) Alkanes are described as saturated hydrocarbons. State what this means. [1]
(ii) Write the general formula of the alkanes. [1]
(iii) Apart from combustion and reaction with chlorine, alkanes take part in very few reactions. Suggest why alkanes are so unreactive. [1]
Model Answer -- 3(a)
(i) Saturated means the molecules contain only single bonds between carbon atoms — there are no C=C double bonds [1]
(ii) CnH2n+2 [1]
(iii) The C–C and C–H single bonds are strong and the molecules are non-polar, so there is no reactive functional group for other substances to attack [1]
⚠ If you missed marks here: "Saturated" must be defined in terms of single bonds only — answers about being "full of hydrogen" are not accepted without the bond statement. In (iii) any sensible reference to strong single bonds or to the lack of a functional group earns the mark; simply repeating "they are stable" restates the question.
(b)[4]
A sample of ethane is mixed with chlorine. Nothing happens in the dark, but when the mixture is placed in ultraviolet light a reaction occurs.
(i) Write a balanced symbol equation for the reaction of ethane with chlorine to form chloroethane. [1]
(ii) State the essential condition for this reaction and the term used to describe a reaction that needs this condition. [2]
(iii) Name the type of reaction taking place and explain why it is given this name. [1]
(ii) Ultraviolet light (or sunlight) is essential for the reaction to start [1]
(ii) A reaction that requires light energy is described as a photochemical reaction [1]
(iii) It is a substitution reaction, because one hydrogen atom in the ethane molecule is replaced by one chlorine atom [1]
⚠ If you missed marks here: Forgetting the HCl on the right-hand side is the classic equation error — in substitution the displaced hydrogen leaves as hydrogen chloride, never as H₂. Both words are needed in (ii): the condition is ultraviolet light and the describing term is photochemical; one without the other scores one mark, not two.
(c)[5]
Propane from the platform is sold as a fuel. A camping stove burns 8.8 g of propane, C₃H₈, in a plentiful supply of air.
(i) Write a balanced symbol equation for the complete combustion of propane. [1]
(ii) Calculate the number of moles of propane burned. (Ar: C = 12, H = 1) [1]
(iii) Calculate the mass of carbon dioxide produced. (Ar: O = 16) [2]
(iv) Calculate the percentage by mass of carbon in propane. [1]
⚠ If you missed marks here: It is a coincidence that Mr(propane) and Mr(CO₂) are both 44 — do not let that confuse the working. The 3 : 1 mole ratio from the equation is where most marks are lost: always write the ratio down before multiplying. In (iv) the numerator is the mass of carbon in one mole (36 g), not the number of carbon atoms.
Question 4 -- Cracking and Alkenes
Total: 12 marks
On Jurong Island in Singapore, one of the world's great petrochemical hubs, long-chain hydrocarbons from the fuel oil fraction are cracked to make smaller, more useful molecules.
(a)[4]
(i) State the conditions used to crack hydrocarbons in industry. [2]
(ii) Explain two reasons why oil companies crack some of their long-chain fractions. [2]
Model Answer -- 4(a)
(i) A high temperature of about 600 °C is used [1]
(i) The vapour is passed over a hot catalyst such as silica or alumina [1]
(ii) The demand for short-chain fractions such as gasoline is greater than the amount in crude oil, while long-chain fractions are in surplus, so cracking matches supply to demand [1]
(ii) Cracking also produces alkenes, which are needed as the starting materials for making polymers and ethanol [1]
⚠ If you missed marks here: "Heat it" is not enough for the conditions mark — quote a high temperature (around 600 °C) and name a catalyst. For the reasons, the examiner wants the supply-and-demand argument and the alkene argument; writing "to make smaller molecules" merely restates what cracking is and earns nothing.
(b)[4]
One cracking reaction at the plant is shown below.
C₁₆H₃₄ → C₈H₁₈ + 2X
(i) Deduce the molecular formula of X, showing your working. [2]
(ii) Name compound X and state the homologous series to which it belongs. [2]
Model Answer -- 4(b)
(i) Carbon atoms: 16 − 8 = 8 shared between two molecules of X, so each X has 4 carbon atoms; hydrogen atoms: 34 − 18 = 16, so each X has 8 hydrogen atoms [1]
(i) X is C₄H₈ [1]
(ii) X is butene [1]
(ii) It belongs to the alkene homologous series, general formula CnH2n [1]
⚠ If you missed marks here: The coefficient 2 in front of X is the trap — the leftover atoms must be divided between two molecules, so C₈H₁₆ is wrong. A quick check that your formula fits CnH2n confirms it is an alkene, which is exactly what cracking should produce alongside the shorter alkane.
(c)[4]
A technician is given two unlabelled test-tubes. One contains octane, C₈H₁₈, and the other contains butene, C₄H₈.
Describe a test the technician could use to identify which tube contains butene. State the result for each tube, and name the type of reaction and the organic product formed by butene in this test.
Model Answer -- 4(c)
Add aqueous bromine (bromine water) to each tube and shake [1]
With octane the bromine water stays orange — there is no reaction because octane is saturated [1]
With butene the bromine water is decolourised, turning from orange to colourless, because butene is unsaturated [1]
This is an addition reaction across the C=C double bond, and the product is dibromobutane (1,2-dibromobutane) [1]
⚠ If you missed marks here: Write colourless, never "clear" — bromine water is already clear (transparent) but it is orange. Both results are needed: an answer that only describes the positive result loses the octane mark. And the reaction is addition, not substitution — both bromine atoms join the molecule and no HBr is made.
Question 5 -- Two Routes to Ethanol
Total: 10 marks
In Maharashtra, sugar mills ferment molasses from sugar cane to make ethanol. In other countries, ethanol is manufactured from ethene obtained by cracking petroleum fractions.
(a)[3]
Describe how ethanol is produced by fermentation. Include a balanced symbol equation, the conditions used and the role of the yeast.
Model Answer -- 5(a)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ — aqueous glucose converted to ethanol and carbon dioxide [1]
The mixture is kept warm, between 25 °C and 35 °C, in the absence of oxygen (anaerobic conditions) [1]
The yeast supplies enzymes that catalyse the conversion of glucose to ethanol [1]
⚠ If you missed marks here: The equation must balance — both the 2 in front of the ethanol and the 2 in front of the carbon dioxide are needed. "Warm" alone is not a condition: quote the 25–35 °C range and state that air is excluded. The yeast mark needs the word enzymes (or catalyst); "yeast makes it ferment" is circular.
(b)[3]
Describe how ethanol is manufactured from ethene. Include a balanced symbol equation and the conditions of temperature, pressure and catalyst.
Model Answer -- 5(b)
C₂H₄ + H₂O → C₂H₅OH — ethene reacts with steam in an addition reaction [1]
A temperature of 300 °C and a pressure of 60 atm are used [1]
The catalyst is phosphoric acid [1]
⚠ If you missed marks here: The trio 300 °C / 60 atm / phosphoric acid must be learned as a set — sulfuric acid is the esterification catalyst, not the hydration catalyst, and mixing them up is the single most common error on this question.
(c)[4]
A company must choose between the two routes for a new ethanol plant.
Compare the advantages and disadvantages of fermentation and of the reaction of ethene with steam. In your answer refer to the raw materials, the rate of production, and the purity of the product.
Model Answer -- 5(c)
Fermentation uses sugars from plants such as sugar cane, a renewable resource, whereas ethene comes from petroleum, which is finite and will eventually run out [1]
Fermentation is slow and runs as a batch process that must be stopped, emptied and restarted, whereas the ethene route is fast and continuous [1]
Fermentation gives a dilute, impure solution of ethanol that must be concentrated by fractional distillation, whereas the ethene route gives essentially pure ethanol [1]
Valid overall comparison, e.g. the ethene route needs more energy for the high temperature and pressure, while fermentation uses gentler conditions but more land and labour [1]
⚠ If you missed marks here: This is a compare question, so every sentence should mention both routes — a list of facts about fermentation alone caps your score. The three comparison pairs to memorise are renewable/finite, slow batch/fast continuous, and impure dilute/pure product.
Question 6 -- Ethanoic Acid and Esters
Total: 12 marks
Malt vinegar, sprinkled over fish and chips across Britain, is a dilute solution of ethanoic acid made by oxidising the ethanol in fermented barley.
(a)[4]
Ethanol can be oxidised to ethanoic acid in different ways.
(i) Name the oxidising agent used in the laboratory, state how the reaction is carried out and describe the colour change observed. [3]
(ii) Explain how ethanol in beer or wine is converted to ethanoic acid when it is left open to the air. [1]
Model Answer -- 6(a)
(i) The oxidising agent is acidified potassium manganate(VII) [1]
(i) The ethanol is warmed (heated) with the acidified potassium manganate(VII) [1]
(i) The colour changes from purple to colourless as the manganate(VII) is reduced and ethanoic acid forms [1]
(ii) Bacteria oxidise the ethanol using oxygen from the air, converting it slowly to ethanoic acid (bacterial oxidation) [1]
⚠ If you missed marks here: The reagent's full name matters: acidified potassium manganate(VII). Writing "potassium permanganate turns clear" loses a mark twice over — the colour change is purple to colourless. In (ii) the two essential words are bacteria and oxygen; "the air turns it sour" names neither.
(b)[4]
Ethanoic acid is a weak acid.
(i) State what is meant by a weak acid. [1]
(ii) A spatula of sodium carbonate is added to dilute ethanoic acid. State the observation and how you would test the gas produced. [2]
(iii) Write a balanced symbol equation for the reaction of ethanoic acid with sodium carbonate. [1]
Model Answer -- 6(b)
(i) A weak acid is only partially dissociated (ionised) into its ions in aqueous solution [1]
(ii) Effervescence is seen as bubbles of gas are produced and the solid disappears [1]
(ii) Bubble the gas through limewater: carbon dioxide turns limewater milky (cloudy white) [1]
(iii) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ — sodium ethanoate, water and carbon dioxide [1]
⚠ If you missed marks here: "Partially dissociated in aqueous solution" is the exact phrase to learn — "not very strong" or "low pH" score nothing. In the equation the 2 in front of CH₃COOH (and of CH₃COONa) is where the balancing mark is usually lost, because the carbonate ion needs two hydrogen ions.
(c)[4]
Ethanoic acid reacts with alcohols to form sweet-smelling esters used in perfumes and food flavourings.
(i) Name the catalyst and the ester formed when ethanoic acid is warmed with ethanol, and state one feature of this reaction shown by the symbol ⇌. [3]
(ii) Name the ester formed from butan-1-ol and ethanoic acid. [1]
Model Answer -- 6(c)
(i) The catalyst is concentrated sulfuric acid (an acid catalyst) [1]
(i) The ester is ethyl ethanoate, formed together with water: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O [1]
(i) The ⇌ symbol shows the reaction is reversible and does not go to completion [1]
(ii) Butyl ethanoate — the alcohol gives the "butyl" part and the acid gives the "ethanoate" part [1]
⚠ If you missed marks here: Ester names always put the alcohol part first and the acid part second — "ethanoyl butanoate" and "butanoate ethyl" both fail. The catalyst here is concentrated sulfuric acid; phosphoric acid belongs to the hydration of ethene, and swapping them is the standard mix-up.
Question 7 -- Polymers
Total: 10 marks
India banned many single-use plastic items in 2022. A materials scientist in Bengaluru explains to a school group how different polymers are made and why plastic waste causes problems.
(a)[3]
Poly(ethene) is made from ethene by addition polymerisation.
(i) Describe what happens to the ethene molecules during addition polymerisation. [2]
(ii) Describe the repeat unit of poly(ethene). [1]
Model Answer -- 7(a)
(i) Many small monomer molecules (ethene) join together to form one very long chain molecule, and the polymer is the only product [1]
(i) The C=C double bond in each monomer opens up (becomes a single bond) so that each carbon can bond to the next monomer in the chain [1]
(ii) The repeat unit is –CH₂–CH₂– : two carbon atoms joined by a single bond, each carrying two hydrogen atoms, with continuation bonds extending from both ends of the unit [1]
⚠ If you missed marks here: Two ideas earn the two marks in (i): many monomers join into one long chain and the double bond opens. When describing a repeat unit always mention the continuation bonds at each end — a repeat unit drawn or described as a closed molecule is wrong.
(b)[4]
Nylon and PET are condensation polymers.
(i) Name the two types of monomer used to make nylon and the linkage formed between them. [2]
(ii) Name the linkage in PET and state why this type of polymerisation is called condensation polymerisation. [1]
(iii) Proteins are natural polymers. State the type of polymer to which proteins belong. [1]
Model Answer -- 7(b)
(i) Nylon is made from a dicarboxylic acid and a diamine [1]
(i) The monomers are joined by amide linkages (–CONH–), so nylon is a polyamide [1]
(ii) PET contains ester linkages (it is a polyester, made from a dicarboxylic acid and a diol); the process is called condensation polymerisation because a small molecule, water, is eliminated each time a linkage forms [1]
(iii) Proteins are natural polyamides — their amino acid units are joined by amide links [1]
⚠ If you missed marks here: The prefix di- is essential — each monomer must carry a functional group at both ends, otherwise the chain could not grow in both directions. And the defining feature of condensation polymerisation is the loss of water at every link; without that word the definition mark goes.
(c)[3]
Describe two environmental problems caused by plastic waste, and one way the problems can be reduced.
Model Answer -- 7(c)
Most plastics are non-biodegradable, so they persist for decades in landfill sites and break down into microplastics that harm animals in rivers and oceans [1]
Burning plastic waste releases toxic gases, for example carbon monoxide (and acidic gases from some polymers) [1]
Reduce, reuse and recycle — for example PET bottles can be collected, melted and re-formed into new products such as fleece clothing [1]
⚠ If you missed marks here: "Plastics cause pollution" is too vague for any mark — name the mechanism: non-biodegradable in landfill, microplastics in water, or toxic combustion gases. The solution mark needs a concrete action such as recycling PET, not just "dispose of it properly".
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