Redox Reactions

Summary: Oxidation is loss of electrons, gain of oxygen, or loss of hydrogen. Reduction is gain of electrons, loss of oxygen, or gain of hydrogen. OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). Half equations show electron transfer. Oxidising agents accept electrons; reducing agents donate electrons. Tags: igcse chemistry reactions redox Created: 2026-07-14 Last Updated: 2026-07-16

Definitions of Oxidation and Reduction

Redox is short for reduction-oxidation. Every redox reaction involves both processes happening simultaneously — one substance is oxidised while another is reduced.

Oxidation can be defined in three ways:

1. Loss of electrons — e.g., Zn → Zn2+ + 2e-
2. Gain of oxygen — e.g., 2Mg + O2 → 2MgO (magnesium is oxidised because it gains oxygen)
3. Loss of hydrogen — e.g., CH4 + 2O2 → CO2 + 2H2O (carbon is oxidised because it loses hydrogen and gains oxygen)

Reduction can be defined in three ways:

1. Gain of electrons — e.g., Cu2+ + 2e- → Cu
2. Loss of oxygen — e.g., 2CuO + C → 2Cu + CO2 (copper(II) oxide is reduced because it loses oxygen)
3. Gain of hydrogen — e.g., N2 + 3H2 → 2NH3 (nitrogen is reduced because it gains hydrogen)

OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). This is the most fundamental definition and the one most commonly tested.

Oxidising Agents and Reducing Agents

• An oxidising agent (oxidant) is a substance that oxidises another substance and is itself reduced in the process. It accepts electrons.
• A reducing agent (reductant) is a substance that reduces another substance and is itself oxidised in the process. It donates electrons.

Always specify the agent, not just “it oxidises”. For example: “chlorine is the oxidising agent” or “Fe2+ ions act as the reducing agent”.

Identifying Oxidation and Reduction in Reactions

To determine which species is oxidised and which is reduced:

Method 1 — Electron transfer (half equations):

• Write the half equation for each species
• If electrons are on the right, that species is oxidised (loses electrons)
• If electrons are on the left, that species is reduced (gains electrons)

Method 2 — Oxygen/hydrogen transfer:

• Gain of oxygen = oxidation
• Loss of oxygen = reduction
• Loss of hydrogen = oxidation
• Gain of hydrogen = reduction

Method 3 — Oxidation number change (extension):

• Increase in oxidation number = oxidation
• Decrease in oxidation number = reduction
• Uncombined elements have oxidation number 0
• Oxygen in compounds is usually -2; hydrogen is usually +1

Half Equations

A half equation shows either oxidation or reduction separately, including the electrons transferred.

Rules for writing half equations:

1. Write the species on each side (reactant → product)
2. Balance all atoms except O and H
3. Balance O atoms by adding H2O
4. Balance H atoms by adding H+
5. Balance the charge by adding electrons (e-)

Example 1: Zinc and copper(II) sulfate

• Zn → Zn2+ + 2e- (oxidation half equation)
• Cu2+ + 2e- → Cu (reduction half equation)
• Overall: Zn + Cu2+ → Zn2+ + Cu

Example 2: Chlorine and iron(II) ions

• 2Fe2+ → 2Fe3+ + 2e- (oxidation half equation)
• Cl2 + 2e- → 2Cl- (reduction half equation)
• Overall: 2Fe2+ + Cl2 → 2Fe3+ + 2Cl-

Half equations must be balanced for both atoms AND charge. The number of electrons lost in oxidation must equal the number gained in reduction.

Redox and the Reactivity Series (Displacement Reactions)

A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its compound. These are redox reactions.

The reactivity series (most to least reactive): K > Na > Ca > Mg > Al > (C) > Zn > Fe > (H) > Cu > Ag > Au

• A metal higher in the series will displace a metal lower in the series from a solution of its salt
• Carbon can displace metals below it from their oxides

Example: Zinc + copper(II) sulfate → zinc sulfate + copper

• Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
• Ionic equation: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
• Zn is oxidised: Zn → Zn2+ + 2e- (Zn is the reducing agent)
• Cu2+ is reduced: Cu2+ + 2e- → Cu (Cu2+ is the oxidising agent)

Observation: The blue colour of the CuSO4 solution fades (Cu2+ ions are consumed) and a pink-brown deposit of copper metal forms on the zinc.

Redox in Electrolysis

In electrolysis, redox reactions are forced to occur using an electric current:

• Oxidation occurs at the anode (positive electrode): anions lose electrons
  • Example: 2Cl- → Cl2 + 2e- (chloride ions are oxidised to chlorine gas)
• Reduction occurs at the cathode (negative electrode): cations gain electrons
  • Example: Cu2+ + 2e- → Cu (copper ions are reduced to copper metal)

Mnemonic: An Ox — Anode = Oxidation. Red Cat — Reduction = Cathode.

Tests for Oxidising and Reducing Agents

Testing for an Oxidising Agent

An oxidising agent will oxidise certain reagents, producing a colour change:

Explanation: The oxidising agent oxidises iodide ions (I-) to iodine (I2): 2I- → I2 + 2e-. The iodine produced turns the solution brown. If starch is present, a blue-black colour appears.

Testing for a Reducing Agent

A reducing agent will reduce certain reagents, producing a colour change:

Explanation: The reducing agent donates electrons, reducing Mn(VII) to Mn(II) or Cr(VI) to Cr(III), causing the observed colour changes.

Potassium manganate(VII) and potassium dichromate(VI) test for reducing agents. Potassium iodide tests for oxidising agents.

Key Points

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
• Oxidation = loss of electrons / gain of oxygen / loss of hydrogen
• Reduction = gain of electrons / loss of oxygen / gain of hydrogen
• Oxidising agent: accepts electrons, is itself reduced
• Reducing agent: donates electrons, is itself oxidised
• Electrolysis: Anode = Oxidation (An Ox), Cathode = Reduction (Red Cat)
• Potassium manganate(VII): purple → colourless with a reducing agent
• Potassium dichromate(VI): orange → green with a reducing agent
• Potassium iodide: colourless → brown with an oxidising agent
• More reactive metals displace less reactive metals — these are redox reactions
• Half equations must be balanced for atoms AND charge

Key Concepts from Past Papers

• Oxidation: loss of electrons / gain of oxygen / increase in oxidation number
• Reduction: gain of electrons / loss of oxygen / decrease in oxidation number
• Oxidising agent: a substance that oxidises another substance and is itself reduced / accepts electrons
• Reducing agent: a substance that reduces another substance and is itself oxidised / donates electrons
• Redox reaction: a reaction in which both oxidation and reduction occur
• Zn to Zn2+ because of electron loss
• H+ accepts electrons (from zinc atoms)
• Copper oxide / CuO — oxygen removed (from copper oxide) / oxidation number of copper decreases / copper ions gain electrons

Keywords from Past Papers

oxidation, electrons, oxygen, oxide, because, increased, decreases, copper, loss, hydrogen, loses, lead, electron, ion, zinc

Related Notes

• Rates of Reaction
• Reversible Reactions
• Electrolysis
• IGCSE-Chem-Index

Sources

• OpenStax Chemistry 2e — [Chapter 17: Electrochemistry], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Redox Reactions], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 6: Chemical Reactions (Redox), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 22: Oxidation-Reduction Reactions], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q33(a)(i) (0m), Q33(a)(ii) (2m)
• 0620/32 Feb/March 2015: Q44(b)(iv) (2m)
• 0620/32 Feb/March 2019: Q66(b)(i) (1m)
• 0620/33 October/November 2015: Q33(b)(iv) (0m)
• 0620/33 October/November 2017: Q55(b)(i) (1m)
• 0620/33 October/November 2019: Q88(b)(i) (1m)
• 0971/31 May/June 2018: Q33(b)(ii) (1m)
• 0971/32 October/November 2023: Q55(b)(ii) (1m)

Common Misconceptions