Chemistry C1 5.1 Cracking hydrocarbons.

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How can hydrocarbon molecules be broken down?
By heating them with steam to a very high temperature or by passing their vapours over a hot catalyst.
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What does cracking produce?
Alkanes and alkenes.
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What are alkenes?
Unsaturated hydrocarbons.
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What is the test for alkenes?
Alkenes turn bromine water from orange to colourless.
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What is the process of breaking down large hydrocarbon molecules called?
Cracking.
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How can cracking be done?
By heating a mixture of hydrocarbon vapours and steam to a very high temperature or by passing hydrocarbon vapours over a hot catalyst.
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What happens during cracking?
Thermal decomposition reactions produce a mixture of smaller molecules.
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What are the smaller molecules that are produced by cracking larger hydrocarbons?
Some of the smaller molecules are alkanes and some are alkenes.
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What are alkanes and their general formula?
Saturated hydrocarbons with the general formula C(_n)H(2n+2) which are more useful as fuels.
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What are alkenes and their general formula?
Unsaturated hydrocarbons with the general formula C(_n)H(2n).
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How do alkane and alkene molecules differ?
Alkenes are unsaturated hydrocarbons which contain fewer hydrogen atoms than alkanes with the same number of carbon atoms.
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What kind of bond does an alkene have?
Alkenes have a double bond between two carbon atoms and this makes them more reactive than alkanes.
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How can cracking produce different mixtures of alkanes and alkenes?
Because different hydrocarbons can be used and the conditions for the reaction can be changed.
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Why might an oil company want to crack large hydrocarbons to make smaller alkanes?
To make fuels that are more useful or for which there is more demand, large hydrocarbons do not burn easily or are less in demand.
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In what ways are alkenes different from alkanes?
They are unsaturated, have a double bond, have a different general formula, have fewer hydrogen atoms than the corresponding alkane, are more reactive, react with or decolourise bromine water.
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**LOOK UP EXAMPLES OF A CRACKING REACTION**
**LOOK UP THE STRUCTURE OF AN ALKENE/ALKANE MOLECULE**
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Card 2

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What does cracking produce?

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Alkanes and alkenes.

Card 3

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What are alkenes?

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Card 4

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What is the test for alkenes?

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Card 5

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What is the process of breaking down large hydrocarbon molecules called?

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