Combustion and cracking
Burning hydrocarbons
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.
- Propane: C₃H₈ + O₂ → CO₂ + H₂O
- 3 C atoms: 3 CO₂
- 8 H atoms: 4 H₂O
- O on the right: 3 × 2 + 4 × 1 = 10 atoms, so 5 O₂
- 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.
Cracking long alkanes
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.
- C₁₂H₂₆ → C₈H₁₈ + X
- Carbon: 12 − 8 = 4
- Hydrogen: 26 − 18 = 8
- 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.
Testing for alkenes
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.
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