Combustion and cracking

Hydrocarbon fuels release energy when they burn.

In plenty of oxygen, complete combustion oxidises the carbon to carbon dioxide and the hydrogen to water:

hydrocarbon + oxygen → carbon dioxide + water

Balance the symbol equation in a fixed order: carbon, then hydrogen, then oxygen.

1. Propane: C₃H₈ + O₂ → CO₂ + H₂O
2. 3 C atoms: 3 CO₂
3. 8 H atoms: 4 H₂O
4. O on the right: 3 × 2 + 4 × 1 = 10 atoms, so 5 O₂
5. C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

If the oxygen count comes out as a half (ethane needs 3½ O₂), double every number: 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O.

With too little oxygen, incomplete combustion also makes carbon monoxide (a toxic gas you cannot see or smell) and carbon (soot).

Never change a formula to balance an equation. Only change the big numbers in front.

Crude oil gives more long-chain hydrocarbons than anyone wants, and not enough petrol.

Cracking breaks long alkanes into shorter, more useful alkanes and alkenes. It is a thermal decomposition.

• Catalytic cracking: the alkane vapour is passed over a hot catalyst.
• Steam cracking: the vapour is mixed with steam and heated to a very high temperature.

Atoms are not made or lost, so you can find a missing product by counting.

1. C₁₂H₂₆ → C₈H₁₈ + X
2. Carbon: 12 − 8 = 4
3. Hydrogen: 26 − 18 = 8
4. X is C₄H₈: H = 2 × C, so it is an alkene (butene)

If a product has a number in front, multiply first. In C₁₀H₂₂ → X + 2 C₂H₄, the ethene accounts for 4 carbons and 8 hydrogens.

Alkenes have a C=C double bond, so they are unsaturated and more reactive than alkanes.

Shake a hydrocarbon with orange bromine water:

• alkene: orange to colourless (bromine adds across the double bond)
• alkane: no change, it stays orange

The alkenes from cracking are used to make polymers and other chemicals, and the short alkanes are used as fuels.