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Redox Chemistry

7. Redox Chemistry

  • SEC G3 Combined Science Chemistry component 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
  • describe the use of aqueous potassium iodide and acidified potassium manganate(VII) in testing for oxidising and reducing agents from the resulting colour changes.

Redox becomes much easier when you choose one model, follow the change in both directions and only then name the agents. This lesson moves from oxygen transfer to electrons and oxidation states before applying the same reasoning to test-tube evidence.

1. Definition

A redox reaction is a reaction in which oxidation and reduction happen together.

  • Oxidation: gain of oxygen, loss of hydrogen, loss of electrons, or an increase in oxidation state.
  • Reduction: loss of oxygen, gain of hydrogen, gain of electrons, or a decrease in oxidation state.

2. Key Ideas

Question cueMethod to use
Oxygen or hydrogen is transferredTrack gain or loss of oxygen/hydrogen
Ions or half-equations are shownTrack electrons
Formulae or oxidation numbers are givenTrack oxidation-state changes
An agent is requestedThe oxidising agent is reduced; the reducing agent is oxidised
Electron mnemonic

OIL RIG: Oxidation Is Loss; Reduction Is Gain — of electrons.

3. Detailed Explanations

For the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s):

Zn(s) → Zn²⁺(aq) + 2e⁻

Zinc loses electrons, so it is oxidised and is the reducing agent.

Cu²⁺(aq) + 2e⁻ → Cu(s)

Copper(II) ions gain electrons, so they are reduced and are the oxidising agent.

For oxidation states, an element has oxidation state 0, a simple ion has its ionic charge, oxygen is usually -2, and hydrogen is usually + 1. The sum is 0 in a neutral compound and equals the overall charge in an ion.

Use the whole equation as a consistency check. If one species loses electrons, another must gain the same total number because electrons are transferred, not created. The species that is oxidised is the reducing agent; the species that is reduced is the oxidising agent. This apparently reversed naming becomes easier when you ask what each species does to the other one. In an oxidation-state answer, show the value before and after so the direction of change is visible rather than merely naming a process.

Two prescribed tests connect visible evidence to redox:

  • Add aqueous potassium iodide to test for an oxidising agent: the colourless solution turns brown as iodine forms.
  • Add acidified potassium manganate(VII) to test for a reducing agent: purple to colourless.

4. Common Mistakes

  • Saying the oxidising agent is oxidised. It accepts electrons and is reduced.
  • Reversing the direction of electron loss and gain.
  • Treating every oxidation state as a measured ionic charge.
  • Giving only “it changes colour” instead of the initial and final colours.
  • Forgetting to say acidified potassium manganate(VII).

5. Exam Tips

  1. Choose one redox method that matches the information given.
  2. If identifying an agent, state what it does to electrons and what happens to it.
  3. For oxidation-state questions, write the value before and after.
  4. For tests, use the sequence: reagent → observation → inference.

6. Worked Examples

Modelled example 1

Identify an oxidising agent

Core

Problem

In Mg + Cu²⁺ → Mg²⁺ + Cu, identify the oxidising agent and justify the choice using electrons.
Study the worked solution

  1. Track copper(II) ions

    Method

    Give Cu²⁺ two electrons.

    Reason

    The +2 ion must gain two negative charges to form neutral copper.

    Working

    Cu²⁺ + 2e⁻ → Cu.

  2. Name the process

    Method

    Identify electron gain as reduction.

    Reason

    Reduction is gain of electrons.

    Working

    Cu²⁺ is reduced.

  3. Name the agent

    Method

    Identify Cu²⁺ as the oxidising agent.

    Reason

    It accepts electrons from magnesium, causing magnesium to be oxidised.

    Working

    Oxidising agent: Cu²⁺.

Guided practice 2

Track oxygen before naming the agents

About 5 min

Problem

For CuO + H₂ → Cu + H₂O, identify what is oxidised, what is reduced, and both agents.

Follow where oxygen moves

Substance oxidised
Substance reduced
Oxidising agent

Hints

Hint 1: oxygen destination
Oxygen starts in copper(II) oxide and ends in water.
Hint 2: agent rule
The oxidising agent causes oxidation and is itself reduced.
View solution step by step

  1. Track oxygen

    Method

    Move oxygen from copper(II) oxide to hydrogen.

    Reason

    Gain of oxygen is oxidation; loss of oxygen is reduction.

    Working

    Hydrogen gains oxygen; CuO loses oxygen.

  2. Name the agents

    Method

    Call CuO the oxidising agent and H₂ the reducing agent.

    Reason

    CuO oxidises hydrogen and is reduced; hydrogen reduces CuO and is oxidised.

    Working

    CuO: oxidising agent; H₂: reducing agent.

Common misconception 3

Calculate an oxidation state

Find and correct the mistake

Learner working

A student writes x + 4(-2) = 0 for sulfur in SO₄²⁻. Diagnose the error and calculate the correct oxidation state.

Use the written ion charge

Required sum

View solution step by step

  1. Set the ionic total

    Method

    Set the oxidation-state sum equal to -2.

    Reason

    For a polyatomic ion, the sum equals the overall ion charge rather than zero.

    Working

    x + 4(-2) = -2.

  2. Solve and include the sign

    Method

    Balance the four oxygen contributions.

    Reason

    Oxygen contributes -8, so sulfur must be + 6 to leave -2 overall.

    Working

    x-8 = -2; x = +6.

Examiner practice 4

Analyse a halogen displacement

4 marks

Examination question

For Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, identify the species oxidised and reduced and name the oxidising and reducing agents. Justify each answer using electrons. [4 marks]

Write two electron changes and pair each agent

View solution step by step

  1. Track bromide

    1 mark

    Method

    State that bromide ions lose electrons and are oxidised.

    Reason

    Electron loss is oxidation.

    Working

    2Br⁻ → Br₂ + 2e⁻

  2. Name the reducing agent

    1 mark

    Method

    Name Br⁻ as the reducing agent.

    Reason

    It supplies electrons to chlorine and is itself oxidised.

    Working

    Reducing agent: Br⁻.

  3. Track chlorine

    1 mark

    Method

    State that chlorine gains electrons and is reduced.

    Reason

    Electron gain is reduction.

    Working

    Cl₂ + 2e⁻ → 2Cl⁻

  4. Name the oxidising agent

    1 mark

    Method

    Name Cl₂ as the oxidising agent.

    Reason

    It accepts electrons from bromide and is itself reduced.

    Working

    Oxidising agent: Cl₂.

Challenge 5

Report a redox test

Minimal support

Practical-evidence transfer

An unknown decolourises acidified potassium manganate(VII). Report the complete reagent and observation, infer the agent type and explain the electron transfer.

Separate visible evidence from inference

Observation
Unknown agent
Unknown's electron role

Hints

Hint 1: agent rule
The sample causes manganate(VII) ions to be reduced.
View solution step by step

  1. State reagent and observation

    Method

    Name acidified potassium manganate(VII) and purple to colourless.

    Reason

    The full reagent and both colours are separate marking points.

    Working

    Acidified potassium manganate(VII) is decolourised.

  2. Infer the sample role

    Method

    Identify a reducing agent.

    Reason

    The unknown causes manganate(VII) ions to gain electrons and be reduced.

    Working

    Conclusion: reducing agent present.

  3. Explain the paired process

    Method

    Make the sample donate electrons and become oxidised.

    Reason

    A reducing agent supplies the electrons used in the paired reduction.

    Working

    Unknown loses electrons; manganate(VII) gains them.

7. Mind Stretchers

Mind stretcher 1: Why is an oxidising agent reduced?Extension

A chlorine molecule gains electrons from bromide ions. Explain why chlorine is called the oxidising agent even though chlorine itself is reduced.

Show Answer

Chlorine causes bromide ions to lose electrons, so it oxidises bromide. It accepts those electrons and therefore gains electrons itself. Electron gain is reduction, which is why the oxidising agent is reduced.

Mind stretcher 2: Can redox occur without oxygen?Extension

Use Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) to prove that a reaction can be redox even when no oxygen is present.

Show Answer

Zinc loses electrons and is oxidised, while copper(II) ions gain the same number of electrons and are reduced. Oxidation and reduction occur together, so the reaction is redox. Oxygen transfer is one redox model, not a requirement for every redox reaction.

Mind stretcher 3: Choose evidence for an unknown agentExtension

An unknown turns colourless potassium iodide solution brown but does not decolourise acidified potassium manganate(VII). What type of agent does the positive result support, and why must you report the reagent and colour change rather than only writing “redox occurred”?

Show Answer

The brown iodine formed from colourless potassium iodide supports the presence of an oxidising agent: the unknown causes iodide ions to lose electrons. Naming the reagent and exact observation shows the evidence used. “Redox occurred” is an inference and does not record what was actually seen.

8. Quiz

Check 1: Check your understandingCore

Check that you can apply oxygen, hydrogen, electron and oxidation-state definitions without mixing them, identify both agents, and write a complete reagent → observation → inference answer for each prescribed test.

Check your understanding

Practise redox reasoning or try a mixed assessment.