Cracking breaks large hydrocarbons into smaller, more useful molecules

1. Cracking breaks large hydrocarbon molecules into smaller, more useful ones by heating them.
2. Cracking is useful because the supply of long-chain hydrocarbons is greater than the demand for them.
3. There is a much higher demand for smaller hydrocarbons, especially those used as fuels.
4. One large molecule can split into several smaller molecules, so cracking gives a mixture of products.

Two cracking methods: heat with a catalyst, or heat with steam

1. Both methods start by heating a long-chain hydrocarbon until it vaporises.
2. Catalytic cracking
   1. The hydrocarbon vapour is passed over a hot catalyst.
   2. The large molecules split into smaller molecules on the catalyst surface.
3. Steam cracking
   1. The hydrocarbon vapour is mixed with steam.
   2. The mixture is heated to a very high temperature so the large molecules split.

Alkenes are more reactive and decolourise bromine water

1. The products of cracking include alkanes and a more reactive type of hydrocarbon called alkenes.
2. Alkenes are more reactive than alkanes.
3. Bromine water is used as the test for an alkene.
4. With an alkene, bromine water changes from orange to colourless.
5. With no alkene present, such as with an alkane, the bromine water stays orange.

Balancing a cracking equation conserves every carbon and hydrogen atom

1. A balanced cracking equation has the same number of carbon and hydrogen atoms on both sides.
2. Count the carbon atoms first, then the hydrogen atoms.
3. Change only the coefficients in front of the formulae, never the small numbers inside them.
4. For example, take $C_{14}{H}_{30}\to C_8{H}_{18}+2C_3{H}_6$.
   1. The left side has 14 carbons and 30 hydrogens.
   2. The right side has $8+(2\times 3)=14$ carbons and $18+(2\times 6)=30$ hydrogens.
   3. The coefficient 2 balances the equation without changing either product formula.

Hydrocarbons supply both the fuels and the raw materials modern life depends on

1. Some smaller alkanes from cracking are used as fuels, which are always in high demand.
2. Suitable smaller hydrocarbons can be blended into petrol.
3. Burning these fuels releases energy for transport and heating.
4. Alkenes are used as feedstock to make polymers and many other chemicals.
5. Products made from hydrocarbon feedstock include packaging, synthetic fibres, paints, detergents and medical equipment.
6. Modern transport, heating and manufacturing all depend on the energy and raw materials from hydrocarbons.