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
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
Potassium produces a lilac flame when it burns in air to form potassium oxide, K2O. Write a chemical equation for this reaction.
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:
Propane, C3H8
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
You've read 0 of your 5 free revision notes this week
Unlock more, it's free!
Did this page help you?