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Chemistry · 06 / 10

Redox: follow the electrons

High school · intermediate · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Atoms, ions and moleculesBalance a reaction equation
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Oxidation loses electrons; reduction gains them. A reducing agent donates electrons and is oxidised; an oxidising agent receives them and is reduced. Electrons cancel in the overall equation. Check both atoms and charge. Species and reaction conditions determine the appropriate half-equations.

THE PICTURE TO REMEMBERRedox: follow the electrons
  1. 01Half-equations
  2. 02Equal electrons
  3. 03Add and cancel
  4. 04Atoms and charge

WORKED EXAMPLE

From principle to reasoning

Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu combine into Zn + Cu²⁺ → Zn²⁺ + Cu. Atoms and the total charge +2 are conserved.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

In Fe²⁺ → Fe³⁺ + e⁻, is iron oxidised or reduced?

One hint

Locate the electron.

Reveal and understand the solution

Fe²⁺ loses an electron, so it is oxidised and acts as the reducing agent. +3 − 1 preserves charge +2.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

Is Zn → Zn²⁺ + 2e⁻ oxidation or reduction?

A hint to get started

Check whether electrons are lost or gained.

Step-by-step solution
  1. Zn loses two electrons.
  2. Electron loss is oxidation.
  3. Zn is the reducing agent because it supplies electrons.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Write the reduction half-equation for Cu²⁺/Cu.

A hint to get started

Put electrons on the reactant side.

Step-by-step solution
  1. Cu²⁺ gains two electrons.
  2. Cu²⁺ + 2e⁻ → Cu.
  3. Atoms and charge balance: +2 − 2 = 0.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Combine Zn/Zn²⁺ and Cu²⁺/Cu. Should electrons remain?

A hint to get started

Both half-equations exchange the same number of electrons.

Step-by-step solution
  1. Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.
  2. Add and cancel the electrons.
  3. Zn + Cu²⁺ → Zn²⁺ + Cu, with no free electrons in the net equation.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

The oxidising agent is reduced, rather than oxidised.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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