Oxidation is gain of oxygen, reduction is loss
Definition
Oxidation
The gain of oxygen by a substance in a chemical reaction.
Definition
Reduction
The loss of oxygen by a substance in a chemical reaction.
- Burning magnesium in air forms magnesium oxide, so magnesium gains oxygen and is oxidised:
2Mg(s)+O2(g)→2MgO(s)2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}2Mg(s)+O2(g)→2MgO(s)
- Rusting, respiration and the combustion of fuels are other everyday oxidations, because oxygen is added to a substance in each one.
- Heating black copper(II) oxide in a stream of hydrogen removes its oxygen and leaves pink-brown copper, so copper(II) oxide is reduced.
- Removing oxygen from a metal oxide to leave the metal is the most important use of reduction, and it is how iron is obtained from its ore.
- Both words describe what happens to a whole substance, so the substance that gains oxygen is oxidised and the substance that loses oxygen is reduced.
Key Idea
Oxidation adds oxygen to a substance and reduction takes oxygen away, so the two words always describe opposite changes.
Oxidation and reduction happen together in oxygen transfer
Definition
Redox reaction
A reaction in which one substance is oxidised while another is reduced.
- In CuO+H2→Cu+H2O\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}CuO+H2→Cu+H2O, the oxygen that leaves copper(II) oxide is gained by hydrogen.
- Copper(II) oxide is reduced to copper, while hydrogen is oxidised to water.
- Oxygen cannot vanish or appear from nowhere, because atoms are conserved in every reaction.
- Whenever one substance loses oxygen, another substance in the same reaction must gain it.
- The name redox is built from the start of the words reduction and oxidation.
- Heating a metal oxide with carbon is another redox reaction, because carbon takes the oxygen and becomes carbon dioxide: 2CuO+C→2Cu+CO22\text{CuO} + \text{C} \rightarrow 2\text{Cu} + \text{CO}_22CuO+C→2Cu+CO2
Common Mistake
Do not say that copper loses oxygen, because the substance that loses oxygen is copper(II) oxide and copper is the product.
Identifying the species oxidised and the species reduced
Definition
Species
Any atom, molecule or ion that takes part in a chemical reaction.
- Pick out each reactant that takes part in the oxygen transfer and find the product it becomes.
- A species that ends up with more oxygen has been oxidised.
- A species that ends up with less oxygen has been reduced.
- When one reactant is oxygen gas, it is the source of the oxygen, so the species to name as oxidised is the one it combines with.
- In the combustion of hydrogen, 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2H2+O2→2H2O, hydrogen is the species oxidised to water.
Example
- In a blast furnace, iron(III) oxide reacts with carbon monoxide, Fe2O3+3CO→2Fe+3CO2\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2Fe2O3+3CO→2Fe+3CO2; identify the species oxidised and the species reduced.
- Iron(III) oxide, Fe2O3\text{Fe}_2\text{O}_3Fe2O3, becomes iron, Fe\text{Fe}Fe, so it loses oxygen and is reduced.
- Carbon monoxide, CO\text{CO}CO, becomes carbon dioxide, CO2\text{CO}_2CO2, so it gains oxygen and is oxidised.
The oxygen test works for any oxygen transfer
- When methane burns, CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}CH4+2O2→CO2+2H2O, its carbon and hydrogen both gain oxygen, so methane is oxidised.
- Magnesium keeps burning in carbon dioxide, leaving black specks of carbon:
2Mg+CO2→2MgO+C2\text{Mg} + \text{CO}_2 \rightarrow 2\text{MgO} + \text{C}2Mg+CO2→2MgO+C
- Carbon dioxide loses its oxygen and is reduced to carbon, while magnesium gains oxygen and is oxidised to magnesium oxide.
- Magnesium also reacts with steam:
Mg+H2O→MgO+H2\text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2Mg+H2O→MgO+H2
- Here water loses oxygen and is reduced to hydrogen, while magnesium is oxidised.
- Heating zinc oxide with carbon, 2ZnO+C→2Zn+CO22\text{ZnO} + \text{C} \rightarrow 2\text{Zn} + \text{CO}_22ZnO+C→2Zn+CO2, shows zinc oxide being reduced by carbon.
Exam technique
- When a question asks which species is oxidised or reduced, name the compound or element from the equation and give the reason, such as 'carbon dioxide loses oxygen'.
- Give both halves of the answer, because a redox reaction always has one species oxidised and another reduced.
Self review
- What is oxidation in terms of oxygen?
- Why must oxidation and reduction happen in the same reaction?
- In CuO+H2→Cu+H2O\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}CuO+H2→Cu+H2O, which species is reduced?
- When magnesium burns in carbon dioxide, which substance is oxidised?
- Why is it wrong to say that copper loses oxygen when copper(II) oxide is heated with hydrogen?
