Cracking

Fractional distillation gives more of the long-chain fractions than are needed, and not enough of the short-chain fractions such as petrol. Cracking solves this. It breaks long-chain alkanes into shorter-chain alkanes and alkenes.Cracking breaks C-C bonds, which needs a lot of energy. The products are more useful: short-chain alkanes are used as fuels (petrol), and alkenes such as ethene and propene are used to make polymers and other chemicals.Thermal cracking uses high temperatures (up to about 900 °C) and high pressure. It gives a high proportion of alkenes.Catalytic cracking uses a zeolite catalyst at about 450 °C and a slight pressure. The catalyst lowers the temperature needed, saving energy, and gives a high proportion of branched alkanes and aromatic hydrocarbons, which are good for petrol.The molecules break at random positions, so cracking produces a mixture, which is separated by fractional distillation.Balancing cracking equations: the numbers of C and H atoms must be the same on both sides. At least one product must be an alkene, because there are not enough hydrogen atoms to make only alkanes.Worked example: dodecane, C₁₂H₂₆, is cracked to octane and ethene.
C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄
Check: carbon 12 = 8 + 4; hydrogen 26 = 18 + 8.Worked example: C₁₀H₂₂ → C₇H₁₆ + ?. The missing product has 10 - 7 = 3 C and 22 - 16 = 6 H, so it is C₃H₆ (propene).