Redox Equations

← Back to Electrochemistry

 

Write balanced half-equations for the two reacting species. Multiply each half-equation to make the number of electrons equal. Combine the two half-equations. Simplify if necessary. Change to alkaline medium by neutralising H+ with OH– if required.

 

How to balance half-equations:

Steps	Half-equation
1. Write skeletal half-equations and balance the atoms being oxidised /reduced. Do not include spectator ions.	MnO4– → Mn2+
2. Balance O atoms by adding H2O	MnO4– → Mn2+ + 4H2O
3. Balance H atoms by adding H+	MnO4– + 8H+ → Mn2+ + 4H2O
4. Balance charges by adding e–	MnO4– + 8H+ + 5e– → Mn2+ + 4H2O

 

Example:

 

MnO₄^– + Br^– → MnO₂ + BrO₃^– (under alkaline conditions)

Steps	Half-equation/ Overall equation
1. Write balanced half-equations for the two reacting species.	MnO4– + 4H+ + 3e– → MnO2 + 2H2O Br– + 3H2O → BrO3– + 6H+ + 6e–
2. Multiply each half-equation to make the number of electrons equal.	2MnO4– + 8H+ + 6e– → 2MnO2 + 4H2O Br– + 3H2O → BrO3– + 6H+ + 6e–
3. Combine the two half-equations. Simplify if necessary.	2MnO4– + 8H+ + Br– + 3H2O → 2MnO2 + 4H2O + BrO3– + 6H+ 2MnO4– + 2H+ + Br– → 2MnO2 + H2O + BrO3–
4. Neutralise H+ by adding OH–.	Add 2OH– to both sides to neutralise the 2H+ on the LHS 2MnO4– + 2H+ + 2OH– + Br– → 2MnO2 + H2O + BrO3– + 2OH– 2MnO4– + H2O + Br– → 2MnO2 + BrO3– + 2OH–

 

Practice:

---

Balance the following equation under acidic and alkaline conditions:

MnO₄^– + C₂O₄^2– → Mn²⁺ + CO₂

 

Answer:

Acidic: 16H⁺ + 2MnO₄^– + 5C₂O₄^2– → 2Mn²⁺ + 8H₂O + 10CO₂

Alkaline: 8H₂O + 2MnO₄^– + 5C₂O₄^2– → 2Mn²⁺ + 16OH^– + 10CO₂