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6.2.1Chemical Reactions

Oxidation and Reduction — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 9 of 10 Unit quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Part 1 of 4Three ways to define the terms

DefinitionOxidationReduction
OxygenGain of oxygenLoss of oxygen
HydrogenLoss of hydrogenGain of hydrogen
ElectronsLoss of electronsGain of electrons

Part 2 of 4Remembering and using the electron definition

OIL RIG
  • OIL RIG Oxidation Is Loss of electrons; Reduction Is Gain of electrons.
  • The electron definition is the most general and works for reactions that involve no oxygen at all.
Always together
  • Always together Whenever one substance is oxidised another is reduced, so these reactions are called redox reactions.
  • Electrons lost by one species are gained by another.
Examples
  • Examples Magnesium burning: 2Mg + O2 → 2MgO.
    • Magnesium gains oxygen and loses electrons (Mg → Mg2+ + 2e-), so it is oxidised.
  • Oxygen gains those electrons (O2 + 4e- → 2O2-), so it is reduced.

Part 3 of 4Oxidising and reducing agents

Redox agents
Oxidising agent
  • Oxidising agent A substance that oxidises another by taking electrons or giving oxygen. It is itself reduced.
Reducing agent
  • Reducing agent A substance that reduces another by giving electrons or removing oxygen.
    • It is itself oxidised.
Example: iron(III) oxide and carbon monoxide
  • Example: iron(III) oxide and carbon monoxide Fe2O3 + 3CO → 2Fe + 3CO2.
    • The iron oxide loses oxygen, so it is reduced;
      • carbon monoxide gains oxygen, so it is oxidised and is the reducing agent.
Example: displacement
  • Example: displacement Zn + Cu2+ → Zn2+ + Cu.
    • Zinc loses electrons (oxidised) and copper ions gain them (reduced).

Part 4 of 4Colour tests for redox

1
To show that a substance is a reducing agent, add it to acidified potassium manganate(VII). The purple solution turns colourless as the manganate(VII) is reduced.
2
To show that a substance is an oxidising agent, add it to potassium iodide solution. The colourless solution turns brown as iodide ions are oxidised to iodine.
3
In each case, record the colour before and after, and state which species lost or gained electrons.

Key terms

TermMeaning
OxidationGain of oxygen, loss of hydrogen or loss of electrons.
ReductionLoss of oxygen, gain of hydrogen or gain of electrons.
Redox reactionA reaction in which oxidation and reduction occur together.
Oxidising agentA substance that causes oxidation and is itself reduced.
Reducing agentA substance that causes reduction and is itself oxidised.

Avoid these mistakes

1
Mixing up the directionloss of electrons is oxidation, not reduction.
2
Saying an oxidising agent is oxidised. It is the reducing agent that is oxidised.
3
Using only the oxygen definition for reactions where no oxygen is involved, such as metal displacement.
4
Forgetting that oxidation and reduction always happen at the same time in one reaction.

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