IUPAC convention for writing half-equations for electrode reactions.

The idea

By IUPAC convention every electrode half-equation is written as a reduction, with the electrons on the left-hand side: Ox + ne⁻ ⇌ Red. The zinc half-cell is therefore written Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E⦵ = −0.76 V, never as the oxidation. The tabulated E⦵ belongs to that direction; reverse the equation and the sign of E⦵ reverses with it. A half-equation must balance for atoms and for total charge. In acidic conditions oxygen is balanced with H₂O and hydrogen with H⁺, then electrons are added until the charge on each side is equal: MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O, where both sides carry a charge of +2. Writing every half-equation the same way round is what makes the electrochemical series comparable.

What earns the marks

• Writing the half-equation as a reduction, with the electrons on the left-hand side.
• Balancing oxygen with H₂O and hydrogen with H⁺ in acidic conditions.
• Adding electrons until the total charge is equal on both sides.
• Using the equilibrium sign and correct state symbols.

Common mistakes and how to put them right

• Writing Zn(s) → Zn²⁺(aq) + 2e⁻ and still quoting E⦵ as −0.76 V for that direction.

  Put it right: The sign must be reversed.

• Reversing a half-equation, changing the sign of E⦵, and then subtracting as well, so the sign is changed twice.

• Balancing the atoms but leaving the charges unequal, for example MnO₄⁻ + 8H⁺ + 4e⁻ → Mn²⁺ + 4H₂O.

  Put it right: Check total charge on both sides.

• Adding OH⁻ to balance oxygen when the question states acidic conditions.

  Put it right: Use H₂O and H⁺ instead.

Exam tips

• Put the electrons on the left every time, then count the total charge on each side as a check before moving on.
• Balance in the order: atoms other than H and O, then O with H₂O, then H with H⁺, then charge with electrons.