Combustion

What is combustion?

Combustion is a reaction where a substance (the fuel) reacts quickly with oxygen, releasing energy as heat and light. Burning is combustion.

Combustion is exothermic: it gives out energy. That is why we burn fuels for cooking, heating, cars and power stations.

The fire triangle

A fire needs three things. Remove any one and the fire goes out.

Hydrocarbons

Most fuels are hydrocarbons: compounds made of only hydrogen and carbon. Examples: methane $\mathrm{C}{\mathrm{H}}_4$ (natural gas), propane ${\mathrm{C}}_3{\mathrm{H}}_8$ (barbecue gas), butane ${\mathrm{C}}_4{\mathrm{H}}_{10}$ (lighter fuel), octane ${\mathrm{C}}_8{\mathrm{H}}_{18}$ (in petrol).

Complete combustion

With plenty of oxygen, a hydrocarbon burns completely to make carbon dioxide and water only.

$$\text{hydrocarbon}+\text{oxygen}\to \text{carbon dioxide}+\text{water}$$

$$\mathrm{C}{\mathrm{H}}_4(\mathrm{g})+2{\mathrm{O}}_2(\mathrm{g})\to \mathrm{C}{\mathrm{O}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$$

Signs: a clean blue flame, lots of heat, no soot.

Incomplete combustion

With not enough oxygen, there is not enough to turn all the carbon into $\mathrm{C}{\mathrm{O}}_2$. Instead it makes:

• Carbon monoxide ($\mathrm{CO}$): a colourless, odourless, poisonous gas. It stops blood carrying oxygen.
• Carbon (C): black soot.
• Water is still made.

$$2C{\mathrm{H}}_4+3{\mathrm{O}}_2\to 2CO+4{\mathrm{H}}_2\mathrm{O}$$ $$\mathrm{C}{\mathrm{H}}_4+{\mathrm{O}}_2\to \mathrm{C}+2{\mathrm{H}}_2\mathrm{O}$$

Signs: a smoky yellow flame, less heat, soot.

Testing the products

You can prove that burning a hydrocarbon makes water and carbon dioxide.

• Water: collect the liquid that forms on a cold surface. It turns blue cobalt chloride paper pink (or white anhydrous copper sulfate blue).
• Carbon dioxide: bubble the gases through limewater. It turns milky.

Burning metals and other elements

Combustion is not only for fuels. When an element burns in oxygen, it forms an oxide.

$$2Mg(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2MgO(\mathrm{s})$$

Magnesium burns with a dazzling white light, so bright you should not look at it directly. That is why it was used in old camera flashes and is still used in fireworks.

Combustion and the environment

• Burning fossil fuels releases carbon dioxide, a greenhouse gas that is causing climate change.
• Fuels containing sulfur release sulfur dioxide, which causes acid rain.
• Incomplete combustion releases soot particles that harm lungs.

Quick summary

• Combustion is a fast, exothermic reaction of a fuel with oxygen.
• Fire triangle: fuel, oxygen, heat. Remove one to stop a fire.
• Hydrocarbons contain only hydrogen and carbon.
• Complete combustion (plenty of oxygen): $\mathrm{C}{\mathrm{O}}_2$ + water, blue flame.
• Incomplete combustion (limited oxygen): poisonous carbon monoxide and/or soot + water, yellow flame.
• Tests: water turns cobalt chloride paper pink, carbon dioxide turns limewater milky.
• Elements burn in oxygen to form oxides. Magnesium burns with a bright white light.

Quiz