Balancing redox equations
The half-reaction method splits a redox reaction into an oxidation and a reduction half. Balance atoms other than H and O, then O using H₂O, then H using H⁺, then charge using electrons. Match the electrons and add.
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Analogy
Like balancing two seesaws before connecting them: make each side level, then join them.
Example
Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂OOxidation: Fe²⁺ → Fe³⁺ + e⁻ (× 5)Total: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
In practice
Chemists use this to balance titrations such as permanganate with iron(II), which is used in iron tests.
Practice
Flashcards
- In half-reaction balancing, how are oxygen atoms balanced in acid?By adding H₂O.
Interview questions
- Balance the reaction of MnO₄⁻ with Fe²⁺ in acid and name the oxidising agent.
- Write two half-reactions
- Balance O with H₂O and H with H⁺
- Match the electrons (5 for 1)
- MnO₄⁻ is the oxidising agent
Questions people ask
How does balancing change in basic solution?
First balance as if the solution were acidic. Then add the same number of OH⁻ ions as there are H⁺ ions to both sides. H⁺ and OH⁻ combine to make water. Finally cancel any water appearing on both sides to give the simplest equation.
Why must the electrons cancel?
Electrons are not free in solution. Every electron lost in the oxidation half must be gained in the reduction half. Multiplying each half so they exchange the same number of electrons makes sure charge is conserved, and then the electrons drop out of the final equation.