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6.2.1Additional lessons

Oxidation and Reduction — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 84 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 5Redox Definitions

6.2.1 Oxidation and Reduction

A useful memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.

Oxidation
  • Oxidation is loss of electrons, and the oxidation number of the species increases.
Reduction
  • Reduction is gain of electrons, and the oxidation number of the species decreases.
Redox reaction
  • Oxidation and reduction always occur together, because electrons lost by one species are gained by another.
Oxygen and hydrogen
  • Gain of oxygen or loss of hydrogen is oxidation; loss of oxygen or gain of hydrogen is reduction.

Part 2 of 5Oxidation Number Rules

Oxidation Number Rules
Elements
  • An atom in an uncombined element has oxidation number 0, for example Mg(s) or O₂.
Simple ions
  • The oxidation number of a monatomic ion equals its charge, so Fe³⁺ is +3.
Oxygen and hydrogen
  • Oxygen is usually −2 and hydrogen is usually +1, with exceptions such as peroxides (O is −1) and metal hydrides (H is −1).
Compounds and ions
  • The sum of the oxidation numbers equals zero in a compound or the charge on an ion.
Roman numerals
  • The oxidation number is written in the name of metals that form several ions, for example iron(III) oxide.
SpeciesElementOxidation number
MnO₄⁻Mn+7
Cr₂O₇²⁻Cr+6
SO₄²⁻S+6
NH₃N−3

Part 3 of 5Oxidising and Reducing Agents

1
An oxidising agent takes electrons from another species, so it is itself reduced.
2
A reducing agent donates electrons to another species, so it is itself oxidised.
3
To identify them, assign oxidation numbers to every element on each side of the equation.
4
The species whose oxidation number rises is oxidised and is therefore the reducing agent.
5
The species whose oxidation number falls is reduced and is therefore the oxidising agent.

Part 4 of 5Worked Example

Question In the reaction Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s), identify what is oxidised and reduced, and name the oxidising and reducing agents.

  1. Mg changes from 0 to +2, so its oxidation number rises: Mg is oxidised.
  2. Cu²⁺ changes from +2 to 0, so its oxidation number falls: Cu²⁺ is reduced.
  3. The species oxidised is the reducing agent and the species reduced is the oxidising agent.

Answer

  • Mg is oxidised and is the reducing agent;
    • Cu²⁺ is reduced and is the oxidising agent.

Part 5 of 5Exam Checklist & Tips

6.2.1 – Key Points for the Exam

  • Oxidation
    • Loss of electrons;
      • an increase in oxidation number.
  • Reduction
    • Gain of electrons;
      • a decrease in oxidation number.
  • Oxidising agent A species that gains electrons and is reduced.
  • Reducing agent A species that loses electrons and is oxidised.
  • Oxidation number The charge an atom would have if all its bonds were ionic.

Common Mistakes

  • Mixing up the agent with the process: the oxidising agent is reduced, not oxidised.
  • Forgetting that the oxidation number sum must equal the overall charge of the ion.
  • Giving oxygen −2 in all compounds, including peroxides such as H₂O₂.
  • Stating that oxidation numbers can only be whole positive numbers; they can be zero or negative.

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