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GCSE Chemistry Revision
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GCSE Chemistry revision
Cracking and alkenes
Carbon compounds as fuels and feedstock
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AQA student objectives
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Revision summary
Key knowledge
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Why Cracking is Needed
- Crude oil contains a mixture of hydrocarbons separated by fractional distillation, but longerchain hydrocarbons are thick, viscous, and less useful as fuels.
- Shorter-chain hydrocarbons are more flammable and therefore more valuable, so cracking converts the less useful long-chain hydrocarbons into more useful shorter ones.
What is Cracking?
- Cracking is a thermal decomposition reaction, meaning long-chain hydrocarbon molecules are broken down into shorter ones by heating.
- There are two methods of cracking you need to know: catalytic cracking and steam cracking.
Catalytic Cracking
- In catalytic cracking, long-chain hydrocarbons are first heated and vaporised (turned into a gas).
- The hydrocarbon vapour is then passed over hot powdered aluminium oxide, which acts as a catalyst to split the long chains into shorter hydrocarbon molecules.
Steam Cracking
- In steam cracking, the vaporised hydrocarbons are mixed with steam and heated to a very high temperature.
- This high temperature causes the long hydrocarbon chains to break apart into shorter hydrocarbon molecules.
Products of Cracking
- Cracking a long-chain alkane produces a shorter alkane and an alkene, for example: decane heptane + propene.
- The total number of carbon and hydrogen atoms must be conserved on both sides of the equation, just like any balanced chemical equation.
- Because there are insufficient hydrogen atoms for both products to be fully saturated, one product must contain a carbon–carbon double bond, making it an alkene.
Writing Balanced Equations for Cracking
- To find an unknown product, subtract the number of carbons and hydrogens in the known product from those in the reactant alkane.
- For example, decane (C₁₀H₂₂) cracked to give ethene (C₂H₄) leaves an unknown alkane with the formula C₈H₁₈.
- The balanced equation for this reaction is: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.