abbie559
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i know everything for the chem exam tomorrow but i cannot get my head round cracking !! i now how we get the long hydrocarbons we need to crack but whats a catalyst??? etc???
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boriapple
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Large hydrocarbon molecules are heated and vaporised. These vapours will pass a catalyst that helps break them down.

Catalyst - A substance that increases the rate of reaction by allowing for the reaction to occur at a lower energy.

They basically speed up how fast the hydrocarbons break down rather than actually break down the hydrocarbons themselves. Think of it like stomach acid. Food will naturally degrade over a long period of time but your stomach acid will quicken that process.
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yayforgcses
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Cracking is where you can turn long chain hydrocarbons into long-chain Hydrocarbons to make them more useful.Cracking involves heating heavy fractions that have been separated from the crude oil and vaporised hydrocarbons are then passed over a hot catalyst (a substance that speeds up a chemical reaction) and mixed with steam. This thermally decomposes the alkanes to form molecules called alkenes that follow the general formula CnH(2n) . You can check for the presence of alkenes by using bromine water. If an alkene is present it turns the bromine water from orange to colourless.
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Kallisto
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(Original post by yayforgcses)
Cracking is where you can turn long chain hydrocarbons into long-chain Hydrocarbons to make them more useful.Cracking involves heating heavy fractions that have been separated from the crude oil and vaporised hydrocarbons are then passed over a hot catalyst (a substance that speeds up a chemical reaction) and mixed with steam. This thermally decomposes the alkanes to form molecules called alkenes that follow the general formula CnH(2n) . You can check for the presence of alkenes by using bromine water. If an alkene is present it turns the bromine water from orange to colourless.
Did you mean long-chain of hydrocarbons to short-chain ones? just a typo, don't sweat it! your explanation is good nonetheless.
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Lonesome_Penguin
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(Original post by abbie559)
i know everything for the chem exam tomorrow but i cannot get my head round cracking !! i now how we get the long hydrocarbons we need to crack but whats a catalyst??? etc???
Cracking is where you use heat (around 650 degress Celsius) and a catalyst to break up longer chained alkanes into shorter chained alkanes and alkenes. (Since there is a greater demand for shorter-chained hydrocarbons as they are more useful for polymers, fuel etc)

The catalyst in this reaction effectively provides a surface for the reaction to take place, and is normally an aluminosilicate or zeolite.
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username3016746
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Surely you can google this?


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