Combustion Reactions (DP IB Chemistry): Revision Note

Combustion reactions

• Combustion is a rapid, exothermic reaction involving:

  • Fuel – e.g. metals, non-metals, hydrocarbons, or alcohols

  • Oxygen – combustion is a type of oxidation

  • Ignition source – spark, flame, or heat buildup (e.g. from volatile liquids or high oxygen levels)

• Commonly referred to as burning

• Produces heat and light, making it exothermic

• Two main types:

  • Complete combustion (with sufficient oxygen)

  • Incomplete combustion (limited oxygen)

Combustion of metals

• All metals can oxidise, but not all metals can combust

  • Some metals will only combust if they have a high surface area, e.g. they are finely divided as filings or a powder

Sparklers demonstrate the combustion of iron

Photo by Jez Timms on Unsplash

• All metals oxidise, but not all combust (burn with a flame).

  • Some only combust if they have a high surface area, e.g. as powders or filings.

Reactivity and combustion

• Less reactive metals (e.g. copper):

  • Do not combust (no flame)

  • They do oxidise

    • E.g. copper forms black copper oxide on heating

• More reactive metals (e.g. s-block metals):

  • Do combust in air

  • They form ionic oxides

    • These oxides are basic, with a pH > 7 when dissolved in water

• The standard example of a metal that combusts in air is magnesium, which burns with a bright white flame:

magnesium + oxygen →  magnesium oxide

2Mg (s) + O₂ (g) → 2MgO (s)

• So, the general word equation for the combustion of suitable metals is:

metal + oxygen →  metal oxide

Combustion of non-metals

• Many non-metals can form different oxides during combustion, depending on their oxidation states

• p-block non-metals typically form covalent oxides when they burn in oxygen:

  • These oxides are usually acidic and form solutions with pH < 7 when dissolved in water

• A common example of a non-metal that combusts in air is sulfur, which burns with a blue flame

sulfur + oxygen →  sulfur dioxide

S (s) + O₂ (g) → SO₂ (g) 

• So, the general word equation for the combustion of non-metals is:

non-metal + oxygen →  non-metal oxide

Complete combustion of organic compounds

• Many organic compounds are used as fuels because they release large amounts of energy when combusted.

• They don’t spontaneously combust because their reactions have a high activation energy:

  • This makes fuels easy and safe to transport and store

• The organic compounds that are commonly used as fuels include:

  • Hydrocarbons - particularly alkanes

  • Alcohols

What is complete combustion?

• When fuels such as hydrocarbons and alcohols are burnt in excess (plenty of) oxygen, complete combustion takes place

  • This means that all carbon and hydrogen will be oxidised

  • Therefore, the products of complete combustion are carbon dioxide and water 

• The word equation for complete combustion is:

fuel + oxygen → carbon dioxide + water

Combustion of hydrocarbons

• For example, the word and chemical equations for the complete combustion of methane are:

  • Complete combustion of methane word equation:

methane + oxygen → carbon dioxide + water

• Complete combustion of methane chemical equation:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

Combustion of Alcohols

• Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water

alcohol + oxygen → carbon dioxide + water

• For example, the word and chemical equations for the complete combustion of ethanol are:

  • Complete combustion of ethanol word equation:

ethanol + oxygen → carbon dioxide + water

• Complete combustion of ethanol chemical equation:

C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l)

• Lower alcohols, like ethanol, burn with an almost invisible flame, making them suitable fuels

• Ethanol can be produced sustainably through the fermentation of sugars

• Its energy density (energy released per kg) is lower than gasoline, so vehicles using ethanol may need:

  • A larger fuel tank, or

  • More frequent refuelling

• Blending ethanol with gasoline or diesel:

  • Increases energy density

  • Makes flames more visible, improving safety in fires

• Using farmland for fuel production may reduce land available for food crops, raising socio-economic and ethical issues