What is the half-equation for the reduction of MnO₄⁻ in acid?

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

What is the half-equation for the reduction of Cr₂O₇²⁻ in acid?

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

What is the half-equation for the oxidation of H₂O₂ to O₂?

H₂O₂ → O₂ + 2H⁺ + 2e⁻

What is the order for balancing a half-equation in acid?

Balance the element, then O with H₂O, then H with H⁺, then charge with e⁻.

How do you combine two half-equations?

Multiply so that the electrons are equal, add them and cancel the electrons and any species on both sides.

What is the overall equation for MnO₄⁻ with Fe²⁺ in acid?

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

What is the overall equation for MnO₄⁻ with H₂O₂ in acid?

2MnO₄⁻ + 6H⁺ + 5H₂O₂ → 2Mn²⁺ + 8H₂O + 5O₂

How can you check that a half-equation is balanced?

The total charge must be the same on both sides, as well as the numbers of each atom.

Half-equations and balancing redox equationsAQA A-Level Chemistry: Flashcards

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What is a half-equation?

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What is a half-equation?: An equation that shows either oxidation or reduction on its own, with the electrons included.
Where do electrons appear in an oxidation half-equation?: On the right-hand side.
Where do electrons appear in a reduction half-equation?: On the left-hand side.
What is the half-equation for the reduction of MnO₄⁻ in acid?: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
What is the half-equation for the reduction of Cr₂O₇²⁻ in acid?: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
What is the half-equation for the oxidation of Fe²⁺?: Fe²⁺ → Fe³⁺ + e⁻
What is the half-equation for the oxidation of H₂O₂ to O₂?: H₂O₂ → O₂ + 2H⁺ + 2e⁻
What is the order for balancing a half-equation in acid?: Balance the element, then O with H₂O, then H with H⁺, then charge with e⁻.
How do you combine two half-equations?: Multiply so that the electrons are equal, add them and cancel the electrons and any species on both sides.
What is the overall equation for MnO₄⁻ with Fe²⁺ in acid?: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
What is the overall equation for MnO₄⁻ with H₂O₂ in acid?: 2MnO₄⁻ + 6H⁺ + 5H₂O₂ → 2Mn²⁺ + 8H₂O + 5O₂
What is the mole ratio of Fe²⁺ to MnO₄⁻ in a titration?: 5 : 1
How can you check that a half-equation is balanced?: The total charge must be the same on both sides, as well as the numbers of each atom.

Exam questions on Half-equations and balancing redox equations

Bromine is extracted from seawater by bubbling chlorine gas through it. The overall reaction is Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
Write the half-equation for the reduction of chlorine and explain why electrons do not appear in the overall equation.2 marks
In acidic solution, orange dichromate(VI) ions, Cr₂O₇²⁻, are reduced to green chromium(III) ions, Cr³⁺, and water is formed. A student reacts acidified dichromate(VI) ions with a solution of iron(II) ions, which are oxidised to iron(III) ions, Fe³⁺.
The iron(II) half-equation is Fe²⁺ → Fe³⁺ + e⁻. Combine it with the dichromate(VI) half-equation to give the overall ionic equation.2 marks
Iron(II) ions in a solution are titrated against 0.0200 mol dm⁻³ acidified potassium manganate(VII). In acidic solution the MnO₄⁻ ion is reduced to Mn²⁺ and the Fe²⁺ ion is oxidised to Fe³⁺. A 25.0 cm³ sample of the iron(II) solution required 22.40 cm³ of the manganate(VII) solution for complete reaction.
Write the half-equation for the reduction of the MnO₄⁻ ion in acid and the half-equation for the oxidation of Fe²⁺, and combine them to give the overall ionic equation.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).