Half-Equation – IGCSE Chemistry Definition

A half-equation shows what happens at a single electrode during electrolysis — specifically, the gain or loss of electrons by the ions. Writing and balancing half-equations is a key Supplement skill on the 0620 syllabus. Examiners award marks separately for the correct formula, the correct number of electrons, and correct balancing.

The 0620 definition

A half-equation is an equation that shows the transfer of electrons at one electrode during electrolysis. Electrons are shown as e⁻.

Half-equations at the cathode (reduction)

Cations gain electrons. Electrons appear on the left side of the equation.

Reaction	Half-equation
Copper deposited	Cu²⁺ + 2e⁻ → Cu
Lead deposited	Pb²⁺ + 2e⁻ → Pb
Silver deposited	Ag⁺ + e⁻ → Ag
Hydrogen gas produced	2H⁺ + 2e⁻ → H₂
Aluminium deposited	Al³⁺ + 3e⁻ → Al

Half-equations at the anode (oxidation)

Anions lose electrons. Electrons appear on the right side of the equation.

Reaction	Half-equation
Chlorine gas produced	2Cl⁻ → Cl₂ + 2e⁻
Bromine produced	2Br⁻ → Br₂ + 2e⁻
Oxygen gas produced	4OH⁻ → 2H₂O + O₂ + 4e⁻

How to write a half-equation

1. Write the ion on one side and the product on the other
2. Balance the atoms (you may need to double the ions for diatomic products like Cl₂)
3. Add electrons to balance the charge
4. Check: total charge on the left = total charge on the right

Example: chloride ions at the anode

• Start: Cl⁻ → Cl₂ (chlorine is diatomic, so need 2 Cl⁻)
• Balance atoms: 2Cl⁻ → Cl₂
• Balance charge: left side = 2 x (-1) = -2. Right side = 0. Add 2e⁻ to the right.
• Final: 2Cl⁻ → Cl₂ + 2e⁻

Worked exam question

During the electrolysis of dilute sulfuric acid: (a) Write the half-equation at the cathode. (1) (b) Write the half-equation at the anode. (2) (c) State which half-equation represents oxidation. (1)

Mark scheme

(a) 2H⁺ + 2e⁻ → H₂ [1]

(b) 4OH⁻ → 2H₂O + O₂ + 4e⁻ [1 for correct formula/products, 1 for balance]

(c) The anode half-equation [1] — because electrons are lost / appear on the right / oxidation is loss of electrons

Common exam mistakes

• Putting electrons on the wrong side. At the cathode, electrons are gained (left side). At the anode, electrons are lost (right side).
• Forgetting to make gaseous products diatomic. Chlorine is Cl₂ not Cl; hydrogen is H₂ not H; oxygen is O₂ not O.
• Not balancing the charge. If you have 2Cl⁻ on the left (charge = -2) and Cl₂ on the right (charge = 0), you need 2e⁻ on the right to balance.