Cracking involves breaking C–C bonds in alkanes.

The idea

Fractional distillation delivers more long chain alkane than anyone wants and less petrol than everyone wants, so the long chains are broken up. Cracking snaps C–C bonds in a large alkane to give smaller molecules, and because the fragments have to share out the hydrogen that was there, at least one product is normally unsaturated. Equations must balance exactly: C₁₄H₃₀ → C₈H₁₈ + C₆H₁₂ is one route, and C₁₄H₃₀ → C₈H₁₈ + 2C₃H₆ is another that a mark scheme accepts for the same starting material. The short alkanes go into motor fuel, while the alkenes are the raw material for poly(ethene), poly(propene) and much of the rest of the organic chemical industry.

What earns the marks

• A balanced equation in which carbon and hydrogen atoms tally on both sides.
• At least one unsaturated product where the hydrogen count requires it.
• Identifying C–C bonds as the bonds that break.
• An economic reason, such as converting a fraction in low demand into fuels and alkenes in high demand.

Common mistakes and how to put them right

• Writing an equation that does not balance, most often by dropping hydrogen atoms.

• Producing only alkanes from an alkane, which cannot balance unless hydrogen is also a product.

• Saying C–H bonds are broken in cracking rather than C–C bonds.

• Confusing cracking with combustion because both involve high temperatures.

Exam tips

• Balance carbon first, then hydrogen, and let the leftover hydrogen tell you whether an alkene is needed.
• More than one correct equation usually exists, so any balanced answer with sensible products will score.
• Say which product goes to fuel and which to polymers if the question asks about usefulness.