Ethene is an alkene produced during the cracking of crude oil fractions. Explain why cracking is carried out in the petroleum industry and describe what happens to the bonds in ethene during an addition reaction with hydrogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cracking is an industrial process used to convert large hydrocarbon molecules into smaller, more useful ones. Ethene is one of the products of cracking and is an important raw material in the chemical industry.
Model answer (5 marks)
Cracking is carried out because there is greater demand for shorter‑chain or smaller hydrocarbons, such as fuels, than for longer‑chain ones. It also produces alkenes and other unsaturated molecules that are useful as monomers or starting materials for plastics.
Ethene contains a carbon–carbon double bond (C=C). During an addition reaction with hydrogen the double bond breaks or opens up. The two hydrogen atoms add across the two carbon atoms, giving ethane, which has a single C–C bond.
Ethene contains a carbon–carbon double bond (C=C). During an addition reaction with hydrogen the double bond breaks or opens up. The two hydrogen atoms add across the two carbon atoms, giving ethane, which has a single C–C bond.
Examiner tips
- Use the word ‘add’ to show the addition reaction; mention the double bond breaking and hydrogen atoms adding across the carbons.
- Show the final product (ethane) and note that the C–C bond remains single.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Failing to state that the double bond breaks during the addition reaction.
- Describing the reaction as a substitution rather than an addition.
- Not mentioning that the product is ethane with a single C–C bond.
Mark scheme (5 marks)
- Cracking is carried out because there is greater demand for shorter-chain / smaller hydrocarbons (such as fuels) than for longer-chain ones
- Cracking also produces alkenes / unsaturated molecules which are useful as monomers / starting materials for plastics
- Ethene contains a carbon–carbon double bond (C=C)
- During the addition reaction, the double bond breaks / opens up
- The hydrogen atoms bond to / add across the two carbon atoms, forming ethane (a single C–C bond remains)
Key terms in this question
cracking · addition reaction · alkene
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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