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Redox Reactions

Redox reactions: oxidation vs reduction, electron transfer, oxidation state changes, and identifying oxidising and reducing agents in reactions.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • define oxidation and reduction (redox) in terms of oxygen/hydrogen gain/loss
  • define redox in terms of electron transfer and changes in oxidation state
  • identify redox reactions in terms of oxygen/hydrogen gain/loss, electron gain/loss and changes in oxidation state

Redox can be described using oxygen, hydrogen, electrons or oxidation states. The best method depends on the information given, but oxidation and reduction always occur together.

1. Definition

A redox reaction is a reaction where oxidation and reduction happen at the same time.

2. Key Ideas

TermWhat it means (mark-scheme)
Oxidationgain oxygen / lose hydrogen / lose electrons / oxidation state increases
Reductionlose oxygen / gain hydrogen / gain electrons / oxidation state decreases
Oxidising agentcauses oxidation and is reduced (it gains electrons)
Reducing agentcauses reduction and is oxidised (it loses electrons)
Match the definition to the evidence

If ions or half-equations are given, use electron transfer. If oxidation states are shown or can be assigned, state the increase or decrease. For metal oxides, oxygen gain or loss may be the shortest valid explanation.

3. Detailed Explanations

Quick Recall (redox language)
  • Oxidation = loss of electrons; reduction = gain of electrons.
  • Oxidising agent is reduced (gains electrons). Reducing agent is oxidised (loses electrons).
  • Oxidation state increases for oxidation; decreases for reduction.
  • Half-equations show electron transfer clearly (often the cleanest exam method).

A. Oxidation and reduction in terms of oxygen

If a substance gains oxygen, it is oxidised. If it loses oxygen, it is reduced.

Example: H₂(g) + PbO(s) → H₂O(g) + Pb(s)

  • H₂ is oxidised (gains oxygen).
  • PbO is reduced (loses oxygen).
  • H₂ is the reducing agent (it causes PbO to be reduced and is oxidised itself).
  • PbO is the oxidising agent (it causes H₂ to be oxidised and is reduced itself).

B. Oxidation and reduction in terms of hydrogen

If a substance loses hydrogen, it is oxidised. If it gains hydrogen, it is reduced.

Example: H₂S(g) + Cl₂(g) → S(s) + 2HCl(g)

  • H₂S is oxidised (loses hydrogen).
  • Cl₂ is reduced (gains hydrogen).

C. Oxidation and reduction in terms of electron transfer

If a substance loses electrons, it is oxidised. If it gains electrons, it is reduced.

Example: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Half-equations (this is the cleanest way to score marks): Zn(s) → Zn²⁺(aq) + 2e⁻ Cu²⁺(aq) + 2e⁻ → Cu(s)

  • Zn is oxidised (loses 2e⁻) → reducing agent.
  • Cu²⁺ is reduced (gains 2e⁻) → oxidising agent.

D. Oxidation state (oxidation number) method

Oxidation state (aka oxidation number) is the number used to track electron transfer in redox. In redox:

  • oxidation state increases → oxidation
  • oxidation state decreases → reduction

Revise the calculation method in Assigning and Calculating Oxidation States if you need more practice.

The fastest exam method
  1. Assign oxidation states.
  2. Identify what increases (oxidised) and decreases (reduced).
  3. The species reduced is the oxidising agent; the species oxidised is the reducing agent.

4. Common Mistakes

  • Saying “oxidising agent causes oxidation so it is oxidised” (wrong): the oxidising agent is reduced.
  • Calling any reaction with oxygen a redox reaction (incomplete): redox is electron transfer.
  • Forgetting that oxidation and reduction must occur together (if one happens, the other must happen too).
  • Mixing up oxidation state with ionic charge: oxidation state is a bookkeeping number, not always a real charge in covalent molecules.
  • Naming a whole compound when the reacting species is an ion. In the zinc–copper(II) example, Cu²⁺ is the oxidising agent; spectator ions are not involved.

5. Exam Tips

One-line agent rule

Oxidation = loss of electrons. Reduction = gain of electrons. (“OIL RIG” is fine, but write electrons in your answer.)

  • When asked for “identify the oxidising agent/reducing agent”, you must justify with “gains electrons” / “loses electrons” or oxidation state changes.
  • If the question gives ions, write half-equations to show electron transfer.
  • For practical redox evidence, revise the tests for oxidising and reducing agents.

6. Worked Examples

Modelled example 1

Oxygen definition (and agents)

Core

Problem

In H₂(g) + PbO(s) → H₂O(g) + Pb(s), identify what is oxidised and reduced, then name both agents.
Study the worked solution

  1. Track oxygen gain

    Method

    Identify hydrogen as oxidised.

    Reason

    H₂ gains oxygen to form H₂O.

    Working

    H₂: oxidation; therefore H₂ is the reducing agent.

  2. Track oxygen loss

    Method

    Identify lead(II) oxide as reduced.

    Reason

    PbO loses oxygen to form Pb.

    Working

    PbO: reduction; therefore PbO is the oxidising agent.

  3. Check simultaneous changes

    Method

    Pair the two transfers.

    Reason

    Oxygen gained by hydrogen comes from lead(II) oxide, so oxidation and reduction occur together.

    Working

    Reducing agent H₂; oxidising agent PbO.

Guided practice 2

Electron transfer (half-equations)

About 6 min

Problem

For Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), write both half-equations and identify the agents.

Place electrons and assign roles

Zinc half-equation
Copper half-equation
Oxidising agent

Hints

Hint 1: charges
A neutral zinc atom must lose two electrons to become Zn²⁺.
Hint 2: agent rule
The electron acceptor is reduced and is the oxidising agent.
View solution step by step

  1. Write zinc oxidation

    Method

    Place two electrons on the product side.

    Reason

    Zinc loses two electrons as its oxidation state rises from 0 to +2.

    Working

    Zn(s) → Zn²⁺(aq) + 2e⁻; Zn is the reducing agent.

  2. Write copper-ion reduction

    Method

    Place two electrons on the reactant side.

    Reason

    Cu²⁺ gains two electrons to become neutral copper.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s); Cu²⁺ is the oxidising agent.

Common misconception 3

Spot the wrong sentence (and fix it)

Find and correct the mistake

Learner claim

For Zn + Cu²⁺ → Zn²⁺ + Cu, a student writes, “Cu²⁺ is oxidised because it loses electrons.” Correct the species change and agent role.

Follow charge to electron direction

Cu2+ electron change
Process
Agent role

View solution step by step

  1. Use the charge change

    Method

    Give Cu²⁺ two electrons.

    Reason

    The +2 ion becomes neutral copper only by gaining two negative charges.

    Working

    Cu²⁺ + 2e⁻ → Cu.

  2. Name process and agent

    Method

    Call the change reduction and Cu²⁺ the oxidising agent.

    Reason

    Electron gain is reduction; an oxidising agent accepts electrons from the species it oxidises.

    Working

    Cu²⁺ is reduced and oxidises zinc.

Examiner practice 4

Oxidation state method (common exam type)

4 marks

Examination question

In 2KI(aq) + Cl₂(aq) → 2KCl(aq) + I₂(aq), identify what is oxidised and reduced and name both agents using oxidation states. [4 marks]

Write the complete oxidation-state analysis

View solution step by step

  1. Iodide oxidation

    1 mark

    Method

    Track iodine from -1 in I⁻ to 0 in I₂.

    Reason

    An increase in oxidation state is oxidation.

    Working

    -1 → 0: I⁻ is oxidised.

  2. Chlorine reduction

    1 mark

    Method

    Track chlorine from 0 in Cl₂ to -1 in Cl⁻.

    Reason

    A decrease in oxidation state is reduction.

    Working

    0 → -1: Cl₂ is reduced.

  3. Reducing agent

    1 mark

    Method

    Name I⁻ in KI as the reducing agent.

    Reason

    The species that is oxidised causes the paired reduction.

    Working

    I⁻ in KI is the reducing agent because it is oxidised.

  4. Oxidising agent

    1 mark

    Working

    Cl₂ is the oxidising agent because it is reduced.

Challenge 5

Is it redox?

Minimal support

Classification transfer

Which reaction is redox? Justify both classifications. (A) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l); (B) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).

Look for paired electron changes

Reaction A
Reaction B

Hints

Hint 1: do not use reaction label alone
Neutralisation or gas formation does not by itself decide redox; test whether oxidation states change.
Hint 2: track magnesium and hydrogen
In B, compare Mg before and after, then H in H⁺ and H₂.
View solution step by step

  1. Test neutralisation

    Method

    Classify A as not redox.

    Reason

    No oxidation state changes and no electron transfer occur; H⁺ and OH⁻ form water.

    Working

    Reaction A: acid–base neutralisation, not redox.

  2. Track magnesium

    Method

    Oxidise magnesium from 0 to +2.

    Reason

    Magnesium loses two electrons when forming Mg²⁺.

    Working

    Mg → Mg²⁺ + 2e⁻.

  3. Track hydrogen and classify

    Method

    Reduce H⁺ to H₂ and classify B as redox.

    Reason

    Hydrogen gains the electrons magnesium loses, so the two changes occur together.

    Working

    2H⁺ + 2e⁻ → H₂; Reaction B is redox.

7. Mind Stretchers

Mind stretcher 1: Agent logic (no guessing)Extension

Question: In a redox reaction, the oxidising agent is always reduced. Explain why this must be true using electrons.

Show Answer

An oxidising agent causes oxidation by taking electrons from another species. If it takes electrons, it gains electrons, so it is reduced. There is no other way for it to cause oxidation.

Mind stretcher 2: Two methods, same conclusionExtension

Question: For Zn + Cu²⁺ → Zn²⁺ + Cu, show oxidation/reduction using (i) electrons and (ii) oxidation states. Do you get the same result?

Show Answer

(i) Electron method: Zn → Zn²⁺ + 2e⁻ (oxidation), Cu²⁺ + 2e⁻ → Cu (reduction).
(ii) Oxidation states: Zn goes 0 to + 2 (increase → oxidation), Cu goes + 2 to 0 (decrease → reduction).
Yes: both methods agree because oxidation state changes track electron transfer.

8. Quiz

Quiz Time!

Test yourself on oxidation vs reduction, agents, half-equations, and oxidation state changes.

Go to Quiz Page