11.3 - Reactions of Alkanes

Combustion of a hydrocarbon

• Alkanes are generally unreactive because their C-C and C-H bonds are strong single covalent bonds, so they take part in very few reactions.
• Burning is the main reaction they undergo, which is why they matter as fuels.

Complete combustion

• When a hydrocarbon burns in a plentiful supply of oxygen, the reaction is called complete combustion.
• Every carbon atom is oxidised to carbon dioxide and every hydrogen atom to water:
  • hydrocarbon + oxygen → carbon dioxide + water

For methane, the simplest hydrocarbon:

CH₄ + 2O₂ → CO₂ + 2H₂O

Complete combustion releases a large amount of energy, which is why hydrocarbons are used as fuels.

Finding the products

If the gases leaving the burning fuel are collected:

• Cooling them condenses a colourless liquid - the water.
• Bubbling the remaining gas through limewater (calcium hydroxide solution) turns it from colourless to milky, identifying carbon dioxide.

Writing a combustion equation

Burning a hydrocarbon fuel in plenty of oxygen always gives carbon dioxide and water:

fuel + O₂ → CO₂ + H₂O

To balance the equation, put balancing numbers in front of the formulae. Never change the formulae themselves.

The method

1. Write the unbalanced equation. Put the fuel and oxygen (O₂) on the left, and carbon dioxide and water on the right.
2. Balance the carbon atoms. Count the carbon atoms in one molecule of fuel and write that number in front of CO₂.
3. Balance the hydrogen atoms. Count the hydrogen atoms in one molecule of fuel and halve it (each H₂O holds two). Write the result in front of H₂O.
4. Balance the oxygen atoms last. Count the oxygen atoms on the right: two in every CO₂ plus one in every H₂O. If the fuel contains oxygen, subtract it. Halve what's left (each O₂ holds two) and write the result in front of O₂.
5. Clear any half. If the number in front of O₂ ends in a half, double every balancing number so they are all whole numbers.
6. Check. Count each element on both sides to confirm the totals match.