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Question 1 -- Fossil Fuels and the Fractionating Column
Total: 12 marks
The Grangemouth refinery in Scotland processes crude oil piped ashore from the North Sea. A visiting student is shown around the site and asks how the different fuels are obtained.
(a)[4]
(i) Name the three fossil fuels. [1]
(ii) Name the main constituent of natural gas and give its molecular formula. [1]
(iii) Petroleum is described as a mixture of hydrocarbons. State what is meant by the terms mixture and hydrocarbon. [2]
Model Answer -- 1(a)
(i) The three fossil fuels are coal, natural gas and petroleum (crude oil) [1]
(ii) Natural gas is mainly methane, CH₄ [1]
(iii) A mixture contains substances that are not chemically combined, so the components keep their own properties and can be separated by physical means [1]
(iii) A hydrocarbon is a compound containing hydrogen and carbon atoms only [1]
⚠ If you missed marks here: The word only in the hydrocarbon definition is worth the whole mark — "a compound containing hydrogen and carbon" also describes ethanol and loses the point. Do not list "petrol" as a fossil fuel: petrol (gasoline) is a fraction obtained from petroleum, not a fossil fuel itself.
(b)[4]
In the fractionating column, petroleum is separated into fractions. Give one use for each of the following fractions.
(i) kerosene [1]
(ii) diesel oil [1]
(iii) fuel oil [1]
(iv) lubricating oil [1]
Model Answer -- 1(b)
(i) Kerosene (paraffin) is used as fuel for jet aircraft [1]
(ii) Diesel oil (gas oil) is used as fuel for diesel engines in lorries, buses and trains [1]
(iii) Fuel oil is used as fuel for ships and for home heating systems [1]
(iv) Lubricating oil is used for lubricants, waxes and polishes [1]
⚠ If you missed marks here: Each fraction has its fixed syllabus use — kerosene means jet fuel, not "lamps" alone, and fuel oil pairs with ships and heating. Writing "fuel" without saying what it fuels is never enough; the vehicle or purpose is what earns the mark.
(c)[4]
Refinery gas leaves at the very top of the column; gasoline is collected just below it.
Explain why gasoline is a suitable fuel for car engines. In your answer refer to the size of its molecules, its boiling point range, its volatility and its viscosity, and explain the link between molecular size and these properties.
Model Answer -- 1(c)
Gasoline contains small, short-chain molecules, so it has a low boiling point range and is collected near the top of the column [1]
It is highly volatile — it evaporates easily, so it mixes with air in the engine and ignites readily [1]
It has a low viscosity, so it flows easily through the fuel pipes and injectors [1]
These properties follow because the attractive forces between small molecules are weak, so little energy is needed to separate them [1]
⚠ If you missed marks here: Volatility and viscosity are different properties — volatile means evaporates easily, viscous means thick and slow-flowing — and each carries its own mark. The final explanation must reach the idea of weak intermolecular attractions; stopping at "because the molecules are small" leaves the last mark on the table.
Question 2 -- Alcohols and Alkenes: Naming and Trends
Total: 12 marks
A student in Kochi, Kerala, is revising the alcohol and alkene homologous series. She lists the first few members of each series and looks for patterns in their formulae and boiling points.
(a)[4]
(i) Name the functional group present in all alcohols. [1]
(ii) Name the first three members of the alcohol homologous series. [2]
(iii) State by how much the relative molecular mass increases from one alcohol to the next, and explain why. [1]
Model Answer -- 2(a)
(i) The hydroxyl group, –OH [1]
(ii) Methanol and ethanol are the first two members [1]
(ii) Propan-1-ol (accept propanol) is the third member [1]
(iii) The relative molecular mass increases by 14, because consecutive members differ by one CH₂ unit (12 + 2 × 1) [1]
⚠ If you missed marks here: The functional group of an alcohol is –OH; writing "OH⁻" turns it into a hydroxide ion and loses the mark — alcohols are covalent molecules, not ionic compounds. The 14 in (iii) must be tied to the CH₂ unit to score.
(b)[4]
The student builds models of two alkenes. Both molecules contain four carbon atoms and eight hydrogen atoms. In the first, the C=C double bond is between the first and second carbon atoms of the chain; in the second, it is between the second and third carbon atoms.
(i) Name each of the two alkenes. [2]
(ii) State the term that describes the relationship between these two compounds and justify it. [1]
(iii) Describe the displayed formula of propene. [1]
Model Answer -- 2(b)
(i) The first alkene is but-1-ene — the double bond starts at carbon 1 [1]
(i) The second alkene is but-2-ene — the double bond starts at carbon 2 [1]
(ii) They are structural isomers: the same molecular formula, C₄H₈, but different structural formulae because the double bond is in a different position [1]
(iii) Propene shows three carbon atoms; a double line (C=C) joins the first two carbons, a single line joins the second and third, and every carbon carries enough single lines to hydrogen atoms to give each carbon four bonds in total (CH₂=CH–CH₃) [1]
⚠ If you missed marks here: The locant numbers matter: but-1-ene and but-2-ene are different compounds, and plain "butene" cannot separate them. In any displayed formula every carbon must show exactly four bonds — examiners routinely deduct for a five-bonded carbon next to the double bond.
(c)[4]
The student finds these boiling points: methanol 65 °C, ethanol 78 °C, propan-1-ol 97 °C.
(i) Describe the trend in boiling point and predict a value for butan-1-ol. [2]
(ii) Explain why boiling point changes in this way along the series. [1]
(iii) Explain why all these alcohols undergo the same chemical reactions. [1]
Model Answer -- 2(c)
(i) The boiling point increases steadily (by roughly 13–19 °C per carbon) as the chain gets longer [1]
(i) Butan-1-ol is predicted to boil at about 117 °C (accept any value from about 110 to 125 °C) [1]
(ii) Longer molecules have stronger attractive forces between them, so more energy is needed to separate the molecules [1]
(iii) All members contain the same –OH functional group, and the functional group determines the chemical reactions [1]
⚠ If you missed marks here: A prediction from a trend must continue the pattern — the gaps are 13 then 19, so an answer near 115–117 °C is expected; 100 °C would ignore the data. In (ii) the energy is used to overcome forces between molecules, never to break the C–O or O–H bonds.
Question 3 -- LPG: Combustion and Substitution
Total: 12 marks
Millions of Indian households cook with LPG cylinders. LPG is a mixture of the alkanes propane and butane, obtained from the refinery gas fraction of petroleum.
(a)[4]
(i) Write a balanced symbol equation for the complete combustion of butane, C₄H₁₀. [2]
(ii) A cooker burns LPG in a kitchen with blocked ventilation grilles. Name the toxic gas that may form and explain why it forms under these conditions. [2]
Model Answer -- 3(a)
(i) 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O — correct formulae of all species [1]
(i) Correctly balanced with 2 : 13 : 8 : 10 (accept halved values with 6½O₂) [1]
(ii) Carbon monoxide, CO, may form [1]
(ii) The blocked vents limit the supply of oxygen, so combustion is incomplete [1]
⚠ If you missed marks here: Butane needs the awkward 2 : 13 balance — check it atom by atom (8 C, 20 H, 26 O on each side). In (ii) write carbon monoxide, formula CO; "carbon dioxide is poisonous" is wrong on both counts, and the reason must mention the limited oxygen supply.
(b)[4]
A cylinder releases 5.8 g of butane into a burner where it burns completely.
(i) Calculate the number of moles of butane burned. (Ar: C = 12, H = 1) [1]
(ii) Calculate the mass of carbon dioxide produced. (Ar: O = 16) [2]
(iii) Calculate the percentage by mass of carbon in butane. [1]
⚠ If you missed marks here: The mole ratio comes from the carbon count — one butane contains four carbons, so it must give four CO₂ whatever the equation coefficients look like. In (iii) use masses (48 out of 58), not atom counts (4 out of 14): percentage by mass always divides mass by mass.
(c)[4]
Propane reacts with bromine vapour when the mixture is exposed to sunlight.
(i) Write a balanced symbol equation for the formation of bromopropane, C₃H₇Br. [1]
(ii) Name the type of reaction and the essential condition. [2]
(iii) Explain why no reaction occurs when the same mixture is kept in the dark. [1]
Model Answer -- 3(c)
(i) C₃H₈ + Br₂ → C₃H₇Br + HBr — correct formulae and balanced, with hydrogen bromide as the second product [1]
(ii) It is a substitution reaction — a hydrogen atom is replaced by a bromine atom [1]
(ii) Ultraviolet light (sunlight) is the essential condition, making this a photochemical reaction [1]
(iii) In the dark there is no light energy to start the reaction (to break the Br–Br bond), so the unreactive alkane does not react [1]
⚠ If you missed marks here: The by-product is HBr, not H₂ and not Br₂ left over — count the atoms. Remember the contrast that examiners love: alkenes decolourise bromine in the dark by addition, but alkanes need UV light and react by substitution. Mixing up those two behaviours costs marks in both halves of the topic.
Question 4 -- Making and Using Alkenes
Total: 12 marks
A petrochemical works on Teesside in north-east England cracks long-chain alkanes to supply ethene for the region's chemical industry.
(a)[4]
One reaction in the cracker is shown below.
C₂₀H₄₂ → 2C₂H₄ + C₄H₈ + X
(i) State what is meant by cracking and give the conditions used. [2]
(ii) Deduce the molecular formula of X, showing your working, and state whether X is an alkane or an alkene. [2]
Model Answer -- 4(a)
(i) Cracking is the breaking down of large alkane molecules into smaller, more useful alkanes and alkenes [1]
(i) It requires a high temperature (about 600 °C) and a catalyst such as silica or alumina [1]
(ii) Carbons: 20 − (4 + 4) = 12; hydrogens: 42 − (8 + 8) = 26; so X is C₁₂H₂₆ [1]
(ii) C₁₂H₂₆ fits CnH2n+2, so X is an alkane [1]
⚠ If you missed marks here: With two different small products you must subtract both before finding X — the 2 in front of C₂H₄ means eight hydrogens, not four. The final check against the general formulae (2n+2 for alkane, 2n for alkene) is what turns a formula into the second mark.
(b)[4]
Ethene from the cracker is converted into other products by addition reactions.
(i) Ethene reacts with hydrogen. Name the catalyst, the type of reaction and the product. [2]
(ii) Ethene reacts with steam. Name the product and state the catalyst used. [2]
Model Answer -- 4(b)
(i) Hydrogen adds across the double bond using a nickel catalyst [1]
(i) This is an addition (hydrogenation) reaction and the product is ethane, a saturated alkane [1]
(ii) Steam adds to ethene to give ethanol: C₂H₄ + H₂O → C₂H₅OH [1]
(ii) The catalyst is phosphoric acid (at 300 °C and 60 atm) [1]
⚠ If you missed marks here: Pair each addition with its catalyst: hydrogen goes with nickel, steam goes with phosphoric acid. Swapping them is the most common error. The hydrogen product must be named ethane — one letter away from ethene, but a different compound entirely.
(c)[4]
A quality-control chemist must confirm that a gas cylinder labelled "propene" does not contain propane.
(i) Describe the test and the result that would confirm the cylinder contains propene. [2]
(ii) Name the type of reaction occurring in the test and the organic product formed. [2]
Model Answer -- 4(c)
(i) Bubble the gas through aqueous bromine (bromine water) [1]
(i) Propene decolourises the bromine water from orange to colourless; propane would leave it orange [1]
(ii) It is an addition reaction across the C=C double bond [1]
(ii) The organic product is 1,2-dibromopropane (accept dibromopropane) [1]
⚠ If you missed marks here: A test answer needs both the reagent and the observation for the positive case — and here the negative control (propane leaves it orange) is what makes the test conclusive. The product contains two bromine atoms because the whole Br₂ molecule adds; "bromopropane" describes the substitution product and is wrong.
Question 5 -- Ethanol as a Fuel in Brazil
Total: 10 marks
In Brazil, most cars run on gasohol — a blend of petrol and ethanol. The ethanol is made by fermenting sugar-cane juice in enormous steel tanks.
(a)[3]
(i) Write the balanced symbol equation for the fermentation of glucose. [1]
(ii) The tanks are kept at about 30 °C. Explain what happens to the rate of ethanol production if the temperature rises to 60 °C, and why. [1]
(iii) Explain why air is kept out of the tanks. [1]
Model Answer -- 5(a)
(i) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1]
(ii) At 60 °C the enzymes in the yeast are denatured (destroyed), so fermentation stops and no more ethanol is made [1]
(iii) Fermentation must be anaerobic: if oxygen is present the ethanol is oxidised to ethanoic acid (and the yeast respires aerobically instead) [1]
⚠ If you missed marks here: At 60 °C the enzymes are denatured — "the yeast dies" is accepted by some markers but "the reaction slows down" is the opposite of the expected answer and scores nothing. Keeping air out is about preventing oxidation of the product, a link that also explains why open wine turns to vinegar.
(b)[3]
A restaurant keeps an opened bottle of wine for several weeks. The wine gradually tastes sour.
(i) Explain what has happened to the ethanol in the wine. [2]
(ii) In the laboratory the same overall change can be carried out in minutes. State the reagent used and the colour change seen. [1]
Model Answer -- 5(b)
(i) Bacteria (acetic acid bacteria) have oxidised the ethanol using oxygen from the air [1]
(i) The product is ethanoic acid, which gives the sour, vinegary taste [1]
(ii) Warm with acidified potassium manganate(VII); the colour changes from purple to colourless [1]
⚠ If you missed marks here: Two separate marks hide in (i): the agent (bacteria plus oxygen from the air) and the product (ethanoic acid). "The wine went off" earns neither. The laboratory reagent must be described as acidified potassium manganate(VII) with the purple-to-colourless change.
(c)[4]
Some people describe Brazilian ethanol as a "greener" fuel than petrol.
Evaluate this claim. Give two arguments that support it and two arguments that count against it.
Model Answer -- 5(c)
For: sugar cane is a renewable raw material — a new crop can be grown every year, whereas petroleum is finite [1]
For: the carbon dioxide released when the ethanol burns was absorbed from the atmosphere by the growing cane during photosynthesis, so the fuel is close to carbon-neutral [1]
Against: large areas of land are needed to grow the cane — land that could grow food or remain forest [1]
Against: energy (often from fossil fuels) is used in farming, transport and the fractional distillation of the ethanol, so the process is not truly carbon-neutral [1]
⚠ If you missed marks here: "It is renewable" and "it is carbon-neutral" are two different arguments — keep them separate to bank both marks. On the other side, the strongest counter-argument is the fossil-fuel energy consumed in distillation; an evaluation that only praises ethanol answers half the question.
Question 6 -- Comparing Acids and Making Esters
Total: 12 marks
A class in Birmingham compares dilute hydrochloric acid with dilute ethanoic acid of the same concentration, then uses the ethanoic acid to make a sweet-smelling ester.
(a)[4]
The two acids have the same concentration.
(i) Predict which solution has the higher pH and explain your answer in terms of dissociation. [3]
(ii) Identical magnesium ribbons are dropped into each acid. Compare what is observed. [1]
Model Answer -- 6(a)
(i) The ethanoic acid solution has the higher pH [1]
(i) Ethanoic acid is a weak acid — it is only partially dissociated into ions in aqueous solution [1]
(i) Hydrochloric acid is a strong acid and is completely dissociated, so it has the higher concentration of H⁺ ions and the lower pH [1]
(ii) Both give effervescence, but the bubbles form more slowly with ethanoic acid because fewer H⁺ ions are present at any moment [1]
⚠ If you missed marks here: Weak refers to partial dissociation, never to concentration — both solutions here have the same concentration, which is exactly the point of the experiment. Higher pH means less acidic; students who reverse the pH scale give the right reasoning and still lose the first mark.
(b)[4]
Ethanoic acid shows the typical reactions of an acid.
(i) Write a balanced symbol equation for the reaction of ethanoic acid with aqueous sodium hydroxide, and name the salt formed. [2]
(ii) Write a balanced symbol equation for the reaction of ethanoic acid with magnesium, and state the test for the gas produced. [2]
Model Answer -- 6(b)
(i) CH₃COOH + NaOH → CH₃COONa + H₂O [1]
(i) The salt is sodium ethanoate [1]
(ii) 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ — magnesium ethanoate and hydrogen [1]
(ii) Hold a lighted splint at the mouth of the tube; hydrogen burns with a squeaky pop [1]
⚠ If you missed marks here: The ethanoate ion is written CH₃COO⁻, so the magnesium salt needs the bracket: (CH₃COO)₂Mg. Writing MgCH₃COO ignores the charges. The salt name comes from the acid — ethanoic acid gives ethanoate salts, never "acetate" in this exam.
(c)[4]
The class warms ethanoic acid with propan-1-ol and a few drops of concentrated sulfuric acid. After a minute, a fruity smell is detected.
(i) Name the ester responsible for the smell and state the role of the concentrated sulfuric acid. [2]
(ii) Name the other product of the reaction and state why the mixture never converts completely to ester. [1]
(iii) Give one commercial use of esters. [1]
Model Answer -- 6(c)
(i) The ester is propyl ethanoate — propyl from the alcohol, ethanoate from the acid [1]
(i) The concentrated sulfuric acid is the catalyst for the esterification [1]
(ii) Water is also formed, and because the reaction is reversible an equilibrium mixture forms — it never goes to completion [1]
(iii) Esters are used in perfumes and in food flavourings [1]
⚠ If you missed marks here: Get the name order fixed: alcohol part then acid part, so propan-1-ol + ethanoic acid gives propyl ethanoate ("ethyl propanoate" is a different ester made from the opposite pair). The equilibrium mark needs the word reversible or the ⇌ idea, not just "some is left over".
Question 7 -- Polymers Around Us
Total: 10 marks
A design-technology teacher shows her class three materials: a PTFE-coated frying pan, a PET drinks bottle, and a nylon climbing rope.
(a)[3]
PTFE is made from the monomer tetrafluoroethene, CF₂=CF₂.
(i) Name the type of polymerisation used to make PTFE and explain how the monomer molecules join together. [2]
(ii) Describe the repeat unit of PTFE. [1]
Model Answer -- 7(a)
(i) Addition polymerisation — the polymer is the only product [1]
(i) The C=C double bond in each monomer opens, allowing thousands of monomers to link into one long chain [1]
(ii) The repeat unit is –CF₂–CF₂– : two carbon atoms joined by a single bond, each bonded to two fluorine atoms, with continuation bonds at both ends [1]
⚠ If you missed marks here: The repeat unit of an addition polymer contains a single C–C bond — the double bond of the monomer has opened, so drawing or describing C=C inside the chain is the error markers see most. Continuation bonds at both ends are also compulsory.
(b)[4]
The PET bottle and the nylon rope are both condensation polymers.
(i) Name the linkage in PET and the two types of monomer from which it is made. [2]
(ii) Name the linkage in nylon. [1]
(iii) Proteins and nylon contain the same linkage. State how proteins differ from nylon. [1]
Model Answer -- 7(b)
(i) PET is a polyester containing ester linkages (–COO–) [1]
(i) It is made from a dicarboxylic acid and a diol, with water eliminated at each linkage [1]
(ii) Nylon is a polyamide containing amide linkages (–CONH–) [1]
(iii) Proteins are natural polyamides built from amino acid monomers, whereas nylon is synthetic and made from a dicarboxylic acid and a diamine [1]
⚠ If you missed marks here: Ester linkage pairs with acid + diol; amide linkage pairs with acid + diamine. Swap the second monomer and both marks vanish. For (iii) the accepted contrast is natural versus synthetic, or amino-acid monomers versus two different monomers — either earns the mark.
(c)[3]
The empty PET bottle can be sent to landfill, incinerated, or recycled.
Give one problem with each of landfill and incineration, and explain why recycling is possible for PET.
Model Answer -- 7(c)
Landfill: PET is non-biodegradable, so the bottle persists for many years and landfill sites fill up (fragments may become microplastics) [1]
Incineration: burning plastics can release toxic gases such as carbon monoxide into the air [1]
Recycling works because PET can be collected, melted and re-formed into new bottles or fibres, saving crude oil and reducing waste [1]
⚠ If you missed marks here: Each disposal route needs its own specific problem — "bad for the environment" twice earns nothing. The recycling mark rests on the fact that PET melts and can be re-formed; that physical property is what separates recyclable thermoplastics from waste that must be dumped or burned.
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