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Cracking of Petroleum

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Once we've decided that the fractions that are coming off the petroleum distillation process aren't quite what we want and that we're going to crack them, there are two ways to crack a hydrocarbon mixture:

Thermal Cracking

The hydrocarbons are heated to 750-900 °C in the absence of oxygen. They won't catch fire, but they will tend to break up into smaller fragments - this is a reaction driven by entropy (the tendency for things to become more disodrrde). The fragments formed during the reaction are uncharged free radicals and can react in numerous ways.

Catalytic Cracking

The hydrocarbons are heated to the comparatively low temperature of 500-600 °C in the presence of a catalyst. The hydrocarbons break up to form positively charged fragements, and since the temperature is not as high, they can recombine as well as break up.

These two methods involve different chemical reactions and give different distributions of products.