Combustion Reactions (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

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

Brightly lit sparkler emitting vivid orange and white sparks against a dark black background, creating a festive and celebratory atmosphere.
Sparklers are coated with iIron powder which gives the characteristic sparks when it undergoes combustion

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

Examiner Tips and Tricks

  • Careful: When some metals combust, they do not form the typical oxides:

    • Sodium forms sodium peroxide, Na2O2 

    • Iron forms iron(II, III) oxide, Fe3O4 

  • Since this knowledge is beyond the scope of the specification, you should achieve the marks in an exam for forming the typical oxides, e.g.

    • Sodium oxide, Na2O

    • Iron(III) oxide, Fe2O3 

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

Worked Example

Combustion of metals and non-metals

  1. Potassium produces a lilac flame when it burns in air to form potassium oxide, K2O. Write a chemical equation for this reaction.

  2. Write a chemical equation for the combustion of white phosphorous, P4, to form phosphorous(V) oxide, P4O10.

Answer 1: 

  • The chemical symbol for potassium is K

  • The chemical formula for oxygen is O2 

  • The chemical formula of potassium oxide is given as K2O

  • So, the unbalanced chemical equation is:

K + O2 K2O

  • Doubling the K2O balances the oxygen atoms

  • Consequently, four potassium atoms are required on the reactant side to balance the equation

  • The balanced chemical equation is:

4K + O2 → 2K2O

Answer 2: 

  • The chemical formula for white phosphorous is given as P4

  • The chemical formula for oxygen is O2 

  • The chemical formula of phosphorus(V) oxide is given as P4O10 

  • So, the unbalanced chemical equation is:

P4 + O2  P4O10 

  • The phosphorous atoms are balanced on both sides of the equation

  • There are 10 atoms of oxygen on the products side of the equation, which means that 5O2 are required on the reactant side to balance the equation

  • The balanced chemical equation is:

P4 + 5O2 → P4O10 

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)

Examiner Tips and Tricks

Be careful when balancing equations for the combustion of alcohol, as students often forget to count the oxygen in the alcohol

  • 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

Worked Example

Complete combustion of hydrocarbons and alcohols

Write chemical equations for the complete combustion of:

  1. Propane, C3H8 

  2. Propan-1-ol, C3H7OH

Answer 1: 

  • Since this is complete combustion, the products will be carbon dioxide and water

  • So, the unbalanced chemical equation is:

C3H8 + O2 → CO2 + H2O

  • The 3 carbons in propane will form 3CO2 

  • The 8 hydrogens in propane will form 4H2O

  • This updates the unbalanced chemical equation to:

C3H8 + O2 → 3CO2 + 4H2O

  • There are now 10 oxygens in total on the product's side, which means that 5O2 are required on the reactant side to balance the equation

  • The balanced chemical equation is:

C3H8 + 5O2 → 3CO2 + 4H2O

Answer 2: 

  • Since this is complete combustion, the products will be carbon dioxide and water

  • So, the unbalanced chemical equation is:

C3H7OH + O2 → CO2 + H2O

  • The 3 carbons in propan-1-ol will form 3CO2 

  • The 8 hydrogens in propan-1-ol will form 4H2O

  • This updates the unbalanced chemical equation to:

C3H7OH + O2 → 3CO2 + 4H2O

  • There are now 10 oxygens in total on the product's side AND one oxygen on the reactants side

    • This means that 4½O2 are required on the reactant side to balance the equation

  • The balanced chemical equation is:

C3H7OH + 4½O2 → 3CO2 + 4H2O

OR

giving whole number coefficients
2C3H7OH + 9O2 → 6CO2 + 8H2O

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener