AQA A-Level Chemistry: Alkanes (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.3.2 Alkanes of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.2.1 Fractional distillation of crude oil, 3.3.2.2 Modification of alkanes by cracking, 3.3.2.3 Combustion of alkanes and 3.3.2.4 Chlorination of alkanes. All of it is AS content, so nothing here is A-level only. The topic is examined on AS Paper 2 and A-level Paper 2, and A-level Paper 3 can test any content.

Learn the ideas first in the alkanes study guide and the alkanes revision notes. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. The mass of calcium oxide needed for flue gases is practised in the Group 2 practice questions, so it is not repeated here. Questions 6, 7 and 8 need a calculator. Use Ar: C = 12.0, H = 1.0, O = 16.0, S = 32.1, and R = 8.31 J K⁻¹ mol⁻¹.

Questions

1. Which statement about catalytic cracking is correct?

• A It takes place at high pressure without a catalyst.
• B It produces mainly alkenes.
• C It uses a zeolite catalyst and produces motor fuels and aromatic hydrocarbons.
• D It separates crude oil into fractions by boiling point.

[1]

2. Explain why the fractions with the longest carbon chains are collected near the bottom of a fractionating column. [3]

3. Hexadecane, C₁₆H₃₄, is cracked.

(a) One molecule of hexadecane forms one molecule of octane and butene only. Write an equation for this reaction. [1] (b) State the conditions used for thermal cracking. [2]

4. Explain the economic reasons for cracking the long-chain fractions obtained from crude oil. [3]

5. Heptane, C₇H₁₆, is a component of petrol.

(a) Write an equation for the complete combustion of heptane. [1] (b) Write an equation for the incomplete combustion of heptane to form carbon monoxide and water. [1] (c) State why carbon monoxide is a dangerous pollutant. [1]

6. A 2.15 g sample of an alkane burns completely in oxygen to form 6.60 g of carbon dioxide and 3.15 g of water. Use these data to find the molecular formula of the alkane. [4]

7. 1.50 g of butane, C₄H₁₀, burns completely. Calculate the volume of carbon dioxide formed at 101 kPa and 293 K. Give your answer in dm³ to 3 significant figures. [3]

8. A fuel oil contains 2.0% sulfur by mass. A boiler burns 500 kg of this fuel.

(a) Write an equation for the formation of sulfur dioxide from sulfur. [1] (b) Calculate the mass of sulfur dioxide formed, in kg, to 3 significant figures. Assume all the sulfur is converted to SO₂. [3] (c) Explain why calcium oxide can be used to remove sulfur dioxide from the flue gases. [2]

9. Car engines release oxides of nitrogen and other pollutants.

(a) Explain how nitrogen monoxide forms in a petrol engine. [2] (b) Write an equation for the reaction of carbon monoxide with nitrogen monoxide in a catalytic converter. [1] (c) Unburned pentane, C₅H₁₂, reacts with nitrogen monoxide in a catalytic converter to form carbon dioxide, water and nitrogen only. Write a balanced equation for this reaction. [1] (d) Name one other pollutant from the internal combustion engine and state how it forms. [1]

10. Methane reacts with chlorine in ultraviolet light to form chloromethane.

(a) Name the type of mechanism. [1] (b) Write an equation for the initiation step, and state the role of the ultraviolet light. [2] (c) Write the two propagation steps that form chloromethane. [2] (d) Write a termination step that forms ethane. [1] (e) Write the two propagation steps that convert chloromethane into dichloromethane. [2]

11. 20 cm³ of propane is mixed with 150 cm³ of oxygen and burned completely. All volumes are measured at the same room temperature and pressure, at which water is a liquid.

(a) Write an equation for the complete combustion of propane. [1] (b) Calculate the total volume of gas left after the reaction. [3] (c) Explain why the water formed does not count towards this volume. [1]

12. A refinery separates crude oil by fractional distillation. The table shows, for this refinery, the percentage of its output in selected fractions and the percentage of its sales that customers want from each of them. The data are invented for this question.

Fraction	Supply from distillation / %	Demand / %
Gases	3	5
Petrol	15	30
Kerosene	12	10
Fuel oil and heavier	45	25

Describe how fractional distillation separates crude oil, and use the data to explain why the refinery cracks some of its output. [6]

Answers

1. C [1] [1] Examiner insight: A and B describe thermal cracking and D describes distillation, so each wrong option tests a specific confusion; read all four before choosing.

2. Longer chains have more electrons and more surface contact between molecules [1], so van der Waals forces between molecules are stronger [1]. Their boiling points are higher, so they condense in the hotter region at the bottom of the column [1]. [3] Examiner insight: An answer that says “bonds break” when the alkane boils contradicts the idea of intermolecular forces and loses the force mark.

3. (a) C₁₆H₃₄ → C₈H₁₈ + 2C₄H₈ [1] (b) High temperature [1]; high pressure [1]. [3] Examiner insight: “Slight pressure” or “zeolite catalyst” belongs to catalytic cracking, so writing it here loses that mark.

4. Fractional distillation gives a surplus of long-chain fractions and too little of the short-chain fractions [1]. Short-chain fractions such as petrol are in higher demand, so they are more valuable [1]. Cracking also makes alkenes, which are valuable starting materials for polymers and other chemicals [1]. [3] Examiner insight: “Explain” needs the link between supply and demand; a bare list of products made by cracking scores at most one mark.

5. (a) C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O [1] (b) C₇H₁₆ + 7½O₂ → 7CO + 8H₂O [1] (c) It is toxic [1]. [3] Examiner insight: Multiples such as 2C₇H₁₆ + 15O₂ → 14CO + 16H₂O are acceptable; a half-balanced equation scores nothing.

6. n(C) = n(CO₂) = 6.60 ÷ 44.0 = 0.150 mol [1]. n(H) = 2 × n(H₂O) = 2 × (3.15 ÷ 18.0) = 0.350 mol [1]. Ratio C : H = 0.150 : 0.350 = 3 : 7, so the empirical formula is C₃H₇ [1]. An alkane must fit CₙH₂ₙ₊₂; C₃H₇ does not, but C₆H₁₄ does, so the molecular formula is C₆H₁₄ [1]. [4] Examiner insight: Forgetting to double the moles of water gives C : H = 0.150 : 0.175 and loses the second mark and an empirical formula (C₆H₇) that cannot be an alkane.

7. n(C₄H₁₀) = 1.50 ÷ 58.0 = 0.02586 mol [1]. n(CO₂) = 4 × 0.02586 = 0.1034 mol [1]. V = nRT ÷ p = 0.1034 × 8.31 × 293 ÷ 101 000 = 2.494 × 10⁻³ m³ = 2.49 dm³ [1]. [3] Examiner insight: Using p in kPa without adjusting gives an answer 1000 times too big; the unit conversion is part of the final mark.

8. (a) S + O₂ → SO₂ [1] (b) Mass of S = 0.020 × 500 = 10.0 kg = 1.00 × 10⁴ g [1]. n(S) = n(SO₂) = 1.00 × 10⁴ ÷ 32.1 = 311.5 mol [1]. Mass of SO₂ = 311.5 × 64.1 = 1.997 × 10⁴ g = 20.0 kg [1]. (c) SO₂ is an acidic oxide [1]; CaO is a basic oxide, so it reacts with (neutralises) SO₂, forming calcium sulfite, CaSO₃ [1]. [6] Examiner insight: In (c), “CaO absorbs SO₂” alone does not score; the acid–base idea is what the marks are for.

9. (a) Nitrogen and oxygen from the air [1] react at the high temperature inside the engine: N₂ + O₂ → 2NO [1]. (b) 2CO + 2NO → 2CO₂ + N₂ [1] (c) C₅H₁₂ + 16NO → 5CO₂ + 6H₂O + 8N₂ [1] (d) Any one: carbon monoxide from incomplete combustion; carbon (soot) from incomplete combustion; unburned hydrocarbons from fuel that does not burn [1]. [5] Examiner insight: Saying the nitrogen comes from the fuel does not earn the first mark in (a); alkanes contain no nitrogen.

10. (a) Free-radical substitution [1] (b) Cl₂ → 2Cl• [1]; UV light provides the energy to break the Cl–Cl bond [1]. (c) Cl• + CH₄ → •CH₃ + HCl [1] •CH₃ + Cl₂ → CH₃Cl + Cl• [1] (d) •CH₃ + •CH₃ → C₂H₆ [1] (e) Cl• + CH₃Cl → •CH₂Cl + HCl [1] •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl• [1] [8] Examiner insight: A propagation step that makes H• instead of HCl, or a dot placed on the wrong atom, does not earn the mark even if the atoms balance.

11. (a) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [1] (b) Oxygen used = 5 × 20 = 100 cm³, so oxygen left = 150 − 100 = 50 cm³ [1]. Carbon dioxide formed = 3 × 20 = 60 cm³ [1]. Total gas = 50 + 60 = 110 cm³ [1]. (c) Water is a liquid at room temperature, so it takes up a negligible volume compared with the gases [1]. [5] Examiner insight: Forgetting the unreacted oxygen gives 60 cm³; the excess reactant is a separate mark.

12. Award one mark for each point, up to 6.

• Crude oil is heated so that it vaporises, and the vapour enters the column [1].
• The column is hottest at the bottom and coolest at the top [1].
• Each hydrocarbon condenses where the temperature falls below its boiling point, so fractions with different boiling points (chain lengths) collect at different heights [1].
• The data show a surplus of long-chain fractions: fuel oil and heavier is 45% of supply but only 25% of demand [1].
• There is a shortage of short-chain fractions: petrol is 15% of supply but 30% of demand (gases 3% vs 5%) [1].
• Cracking breaks C–C bonds to turn the surplus, lower-value long chains into more valuable shorter products such as petrol and alkenes [1].

[6] Examiner insight: An answer that describes distillation well but never quotes the data cannot reach the top marks; use figures from the table to support both the surplus and the shortage.

Where marks are usually lost

• Saying covalent bonds break during distillation or boiling.
• Mixing up thermal and catalytic cracking conditions.
• Cracking equations that do not balance for hydrogen.
• Combustion equations with the wrong number of O₂ molecules.
• Forgetting to double the moles of water when finding moles of hydrogen.
• Leaving pressure in kPa in pV = nRT.
• Saying the nitrogen in NOₓ comes from the fuel.
• Describing CaO as “absorbing” SO₂ without the acid–base reason.
• Missing UV light in initiation, or propagation steps that do not regenerate a radical.
• Ignoring unreacted oxygen in gas-volume calculations.

Next steps

• Alkanes revision notes
• Alkanes study guide
• Halogenoalkanes practice questions
• Mole practice questions
• Energetics practice questions
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.2 Alkanes (3.3.2.1 Fractional distillation of crude oil, 3.3.2.2 Modification of alkanes by cracking, 3.3.2.3 Combustion of alkanes and 3.3.2.4 Chlorination of alkanes).