Oxidation and reduction
Gain or loss of oxygen
In terms of oxygen in a chemical reaction:
- oxidationThe gain of oxygen, or loss of electrons, by a substance during a chemical reaction. is the gain of oxygen
- reductionThe loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. is the loss of oxygen
For example, magnesium reacts with copper(II) oxide:
magnesium + copper(II) oxide → magnesium oxide + copper
In this reaction:
- magnesium is oxidiseChemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons. to form magnesium oxide
- copper(II) oxide is reduceA substance is reduced if it loses oxygen or gains electrons. to form copper
This is an example of a redox reactionOxidation and reduction always take place together. The combined reaction is called a redox reaction. because reduction and oxygen happen at the same time. Also in this reaction:
- magnesium acts as a reducing agentA substance that loses electrons very easily (for example, group 1 elements). because it can reduce copper(II) oxide
- copper(II) oxide acts as an oxidising agentA substance that can oxidise other substances in chemical reactions. because it can oxidise magnesium
Question
Aluminium reacts with zinc oxide:
aluminium + zinc oxide → aluminium oxide + zinc
Explain which substance is oxidised and which substance acts as an oxidising agent.
Aluminium is oxidised because it gains oxygen to form aluminium oxide. Zinc oxide acts as an oxidising agent because it can oxidise aluminium.
Question
Carbon monoxide reacts with iron(III) oxide:
carbon monoxide + iron(III) oxide → carbon dioxide + iron
Explain which substance is reduced and which substance acts as a reducing agent.
Iron(III) oxide is reduced because it loses oxygen to form iron. Carbon monoxide acts as a reducing agent because it can reduce iron(III) oxide.
Loss or gain of electrons - Higher
In terms of electrons in a chemical reaction:
- oxidation is the loss of electrons
- reduction is the gain of electrons
These definitions are useful in reactions that do not involve oxygen. For example, magnesium reacts with chlorine to form magnesium chloride:
Mg + Cl2 → MgCl2
Magnesium chloride contains Mg2+ ions and Cl- ions. The balanced equation can be split into two half equations:
Mg → Mg2+ + 2e- oxidation, because magnesium atoms lose electrons
Cl2 + 2e- → 2Cl- reduction, because chlorine gains electrons
Question
Zinc reacts with copper(II) sulfate solution:
Zn + CuSO4 → ZnSO4 + Cu
Explain, in terms of electrons, which substance is oxidised.
Zinc is oxidised because zinc atoms lose electrons to form zinc ions, Zn2+.
Question
Chlorine reacts with sodium bromide solution:
Cl2 + 2NaBr → 2Cl- + Br2
Explain, in terms of electrons, which substance is reduced.
Chlorine is reduced because it gains electrons to form chloride ions, Cl-.
Learn more on redox reactions in this podcast.
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