IGCSE Chemistry: Petroleum, Alkanes and Alkenes — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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.

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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) Write a balanced equation for the reaction of methane with chlorine to form monochloromethane. [2] (c) 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) 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. A student claims that both alkanes and alkenes can undergo addition reactions. Explain why this claim is incorrect for alkanes. [2]

9. 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. [3]

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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].

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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.
• 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.
• 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.