why are large alkane molecules cracked to form smaller molecules to use it as a fuel?
Many of the fractions that we get by distilling crude oil are not very useful. They contain hydrocarbon molecules which are too long for us to use them as fuels. The hydrocarbons that contain very big molecules are thick liquids or solids with high boiling points. They are difficult to vapourise and do not burn easily, and hence are no good fuels. So, they are broken down into smaller useful alkanes by cracking. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.
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