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Practice & Assess

Redox Reactions

Balance redox equations with the ion-electron method and observe metal displacement reactions

Ion-Electron Method

Skeleton→Assign O.N.→Half-equations→Balance→Final

Medium

Skeleton redox equation

Pick a preset example to begin.

Mn O_{4}^{-} + F e^{2 +} \to M n^{2 +} + F e^{3 +}

1
Write the unbalanced skeleton equation (main species only).
2
Assign oxidation numbers; identify oxidised and reduced species.
3
Write separate oxidation and reduction half-equations.
4
In acidic medium add H⁺ and H₂O; in alkaline add OH⁻ and H₂O; balance charge with e⁻.
5
Equalise electrons, then add the half-equations for the final answer.

HKDSE Tip

O.N. changes: $Oxidation = increase in oxidation number (O.N.). Reduction = decrease in oxidation number (O.N.).$
When to use: $The ion-electron method suits multi-electron redox pairs (e.g. MnO₄⁻/Fe²⁺, Cr₂O₇²⁻/I⁻).$
Medium: $Always identify acidic vs alkaline medium before adding H⁺ or OH⁻.$