IGCSE Chemistry: Petroleum, Alkanes and Alkenes — 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 available free from your board.

Related: Petroleum, Alkanes and Alkenes study guide for the full explanation of fractional distillation, cracking and addition reactions.

Tier note: questions and parts marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus: the substitution reaction of alkanes with chlorine in detail — ultraviolet light and the products (11.4 outcomes 3–4) — and addition reactions of alkenes with bromine, hydrogen and steam, with their products (11.5 outcomes 5–6). Everything else is 0620 Core, including fuels and fractions (11.3), that alkanes are unreactive except in combustion and substitution by chlorine (11.4 outcome 2), cracking (11.5 outcomes 2–3), the bromine water test (11.5 outcome 4), and making ethanol from ethene and steam with an acid catalyst (11.6 outcome 1). 5070 has no tiers, so O Level candidates need all of it.

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Questions

1. State the property that fractional distillation of petroleum relies on to separate hydrocarbons into fractions. [1]

2. State two uses of the gasoline/petrol fraction and the bitumen fraction. [2]

3. Explain why alkanes are described as saturated, and why this makes them generally unreactive. [2]

4. Methane reacts with chlorine in the presence of ultraviolet light.

(a) Name this type of reaction. [1] (b) (0620 Extended, 5070 required) Write a balanced equation for the reaction of methane with chlorine to form monochloromethane. [2] (c) (0620 Extended, 5070 required) State the condition needed for this reaction to occur. [1]

5. Explain what cracking is and why it is carried out. [3]

6. Describe a chemical test, including the result, that would distinguish hex-1-ene (an alkene) from hexane (an alkane). [2]

7. Ethene reacts separately with (i) bromine and (ii) steam.

(a) (0620 Extended, 5070 required) Name the type of reaction occurring in both cases. [1] (b) Name the organic product formed with steam, and state the catalyst required. [2]

8. (0620 Extended, 5070 required) A student claims that both alkanes and alkenes can undergo addition reactions. Explain why this claim is incorrect for alkanes. [2]

9. (0620 Extended, 5070 required) Ethene is reacted with hydrogen in the presence of a catalyst.

(a) Name this type of reaction. [1] (b) Name the catalyst used, and the organic product formed. [2]

10. Name the fraction, other than gasoline/petrol or bitumen, used as (a) jet fuel and (b) chemical feedstock. [2]

11. Describe how the chain length, volatility, boiling point and viscosity of the fractions change as you move up a fractionating column, from bitumen at the bottom to refinery gas at the top. [3]

12. Write a balanced symbol equation for the reaction between ethene and bromine, and state what would be observed. (The equation is 0620 Extended, 5070 required; the observation is 0620 Core, 11.5 outcome 4.) [3]

13. Write a balanced equation for the complete combustion of octane, C₈H₁₈. [2]

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Answers

1. Boiling point (hydrocarbons of different chain length have different boiling points) [1].

2. Gasoline/petrol: fuel for cars [1]. Bitumen: making roads [1].

3. Alkanes contain only single covalent bonds between carbon atoms [1]; with no C=C double bond to react across, they have little to react with except in combustion and chlorine substitution [1].

4. (a) Substitution [1]. (b) CH₄ + Cl₂ → CH₃Cl + HCl [2] (1 mark for correct formulae, 1 mark for balancing). (c) Ultraviolet light (a photochemical reaction) [1].

5. Cracking breaks larger, less useful alkane molecules into smaller alkanes and alkenes, using heat and a catalyst [1] [1]. It is carried out because the supply of shorter-chain fractions (e.g. petrol) from fractional distillation does not match demand, so cracking converts excess long-chain fractions into the shorter, more useful, more reactive molecules that are wanted [1].

6. Shake each with bromine water [1]. Hex-1-ene decolourises the orange bromine water (addition reaction across the C=C bond); hexane leaves it orange (no reaction, as it has no double bond) [1].

7. (a) Addition [1]. (b) Ethanol [1]; catalyst: an acid catalyst [1].

8. Alkanes have no C=C double bond for another atom or group to add across [1]; addition reactions specifically require the double bond alkenes have, which is why alkanes react by substitution instead [1].

9. (a) Addition (hydrogenation) [1]. (b) Catalyst: nickel [1]; organic product: ethane — the C=C double bond is fully saturated by the addition of hydrogen across it [1].

10. (a) Kerosene/paraffin [1]. (b) Naphtha [1].

11. Moving up the column, chain length decreases [1], volatility increases and boiling point decreases [1], and viscosity decreases — so bitumen at the bottom is a long-chain, high-boiling-point, highly viscous fraction, and refinery gas at the top is short-chain, low-boiling-point and freely flowing [1].

12. C₂H₄ + Br₂ → C₂H₄Br₂ [2] (1 mark for correct formulae, 1 mark for balancing). The orange bromine water decolourises, confirming the addition reaction across the C=C double bond has taken place [1].

13. 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. Correct products [1]; balanced [1]. (C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O is also correct.)

Tip: the mark scheme accepts the equation with a fraction in front of O₂. Check that carbon, hydrogen and oxygen all balance. Try the real question next: Cambridge O Level Chemistry 5070, June 2025, Paper 22, Question 5.

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Where marks are usually lost

• Naming the alkane–chlorine reaction “addition” instead of substitution.
• Forgetting cracking needs both heat and a catalyst, not just one.
• Getting the bromine water colour change the wrong way round (it decolourises with alkenes, stays orange with alkanes).
• Writing the hydration of ethene as if it were combustion, rather than addition of steam across the double bond.
• Treating fractional distillation, cracking and polymerisation as one vague “processing crude oil” idea — they are three distinct processes: distillation separates the existing hydrocarbons by boiling point, cracking breaks long-chain molecules into shorter ones, and polymerisation joins alkene monomers together.
• (0620 Extended, 5070 required) Confusing hydrogenation (ethene + hydrogen, nickel catalyst, gives an alkane) with hydration (ethene + steam, acid catalyst, gives an alcohol) — same reaction type, completely different reagent and product.
• Naming the wrong fraction for a use — remember the order runs bitumen, lubricating oil, fuel oil, diesel/gas oil, kerosene/paraffin, naphtha, gasoline/petrol, refinery gas from bottom to top, with chain length and boiling point falling all the way up.
• (0620 Extended, 5070 required) Writing an addition equation with mismatched atom counts either side — check every atom balances, not just the carbons.
• Calling petroleum a single named fossil fuel — petroleum, coal and natural gas (mainly methane) are three separate fossil fuels, not different names for the same thing.