Ionic Half-Equations

In electrolysis, changes at each electrode are written as ionic half-equations. Think of them as a “score sheet” showing how electrons move. Each half-equation shows either gaining electrons (reduction) or losing electrons (oxidation).

Key ideas

• Cathode (negative): cations gain electrons — reduction.
• Anode (positive): anions lose electrons — oxidation.
• Electrons are written as e⁻. Put them on the correct side.
• Atoms and total charge must balance on both sides.

How to write a half-equation

1. Decide which ion is discharged and what it becomes (metal, non‑metal, hydrogen, oxygen).
2. Balance atoms (except H and O at first).
3. Balance charge by adding electrons: add e⁻ to the more positive side.
4. For aqueous solutions, you may need H₂O, H⁺, or OH⁻ (common ones are shown below).
5. Check atoms and total charge are the same on both sides.

Common half-equations to learn

• Metal cation to metal (cathode): $\text{M}^{n+} + ne^- \rightarrow \text{M}$
• Hydrogen from water (cathode, aqueous): $2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2 + 2\text{OH}^-$
• Halide to halogen (anode): $2\text{X}^- \rightarrow \text{X}_2 + 2e^-$ (X = Cl, Br, I)
• Oxygen from hydroxide (anode, aqueous): $4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4e^-$

Misconceptions to avoid

• Putting electrons on the wrong side. Oxidation produces electrons; reduction uses electrons.
• Forgetting to balance charge as well as atoms.
• Including spectator ions (they do not appear in half-equations).