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3.2.2—Half-equations

Syllabus
First assessment 2025
Objective
3.2.2
Level
HL

Redox Half-Equations

Separate oxidation and reduction, balance atoms, add H2O and H+ in acidic solution as needed, balance charge with electrons, then multiply to cancel electrons before adding.

A valid full redox equation conserves atoms and charge and contains no uncancelled electrons.

For MnO₄⁻ → Mn²⁺ in acid, balance O with 4H₂O, H with 8H⁺ and charge with 5e⁻: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. After combining halves, cancel electrons and any identical H⁺ or H₂O, then recheck both atoms and net charge.

To adapt an acidic half-equation to neutral or basic conditions, first balance it with H₂O, H⁺ and e⁻. Add the same number of OH⁻ to both sides to neutralize every H⁺, replace H⁺+OH⁻ by H₂O, then cancel water appearing on both sides. Recheck atoms and total charge; do not leave free H⁺ in a stated neutral medium unless the chemistry justifies it.

Balancing Redox Equations

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

The reaction continues until the violet colour disappears. The thiosulfate ion, S2O32−\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}, is oxidized to SO2\mathrm{SO}_{2}, and Fe3+\mathrm{Fe}^{3+} is reduced to Fe2+\mathrm{Fe}^{2+}. Deduce the oxidation half-equation, and the overall redox equation for this second step of the reaction.

Oxidation half-equation:
Overall redox equation:

Electron Transfer Reactions Summary

Retrieve the route: assign oxidation states, balance half-equations, predict displacement, label cells, trace electrons and ions, follow organic redox pathways, calculate potentials and choose electrolysis products.

Check electron loss/gain, anode/cathode versus polarity, spontaneous sign, salt-bridge direction, ions present, organic functional-group direction and object-cathode placement.