Cracking and alkenes
The breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vaporising them and passing them over a hot catalyst. is a reaction in which larger saturated A compound that contains hydrogen and carbon only. A collection of two or more atoms held together by chemical bonds. are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:
- the original starting hydrocarbons are Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds.
- the products of cracking include alkanes and Unsaturated hydrocarbon with a double bond between the carbon atoms., members of a different A 'family' of organic compounds that have the same functional group and similar chemical properties.
For example, hexane can be cracked to form butane and ethene:
hexane 鈫 butane + ethene
C6H14 鈫 C4H10 + C2H4
The total number of carbon and hydrogen atoms in the A substance formed in a chemical reaction. is the same as in the starting alkane.
Question
C16H34 is an alkane which can be used as the starting chemical in cracking. One of the products of cracking this A substance formed by the chemical union of two or more elements. is an alkane which has 10 carbon atoms in it. Write a balanced symbol equation for this cracking reaction.
C16H34 鈫 C10H22 + C6H12
Various methods can be used for cracking, eg catalytic cracking and steam cracking:
- Catalytic cracking uses a temperature of approximately 550掳C and a A person or thing that causes an event or change. known as a zeolite which contains aluminium oxide and silicon oxide
- Steam cracking uses a higher temperature of approximately 550掳C and no catalyst
The slideshow describes the process of catalytic cracking.
Image caption, 1. The structure of hexane
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Reasons for cracking
Cracking is important for two main reasons:
- it helps to match the supply of In fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other. with the demand for them
- it produces alkenes, which are useful as A raw material used to provide reactants for an industrial reaction. for the Substance made from crude oil using chemical reactions. industry
Supply and demand
The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, In fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures. of Mixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms. produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.
Smaller hydrocarbons are more useful as Material that is used to produce heat, like coal, oil or gas. than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
Alkenes
Alkanes and alkenes both form homologous series of hydrocarbons, but:
- alkanes are A saturated hydrocarbon contains no carbon-to-carbon double bonds, only single bonds., their carbon atoms are only joined by C-C single bonds
- alkenes are An unsaturated compound contains at least one double or triple bond., they contain at least one C=C double bond
As a result, alkenes are more The tendency of a substance to undergo a chemical reaction. than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a A large molecule formed from many identical smaller molecules known as monomers..
Alkenes will react with bromine water and turn it from orange/brown to colourless. This is the way to test for a double C=C bond in a molecule.