Representation of reactions at electrodes as half equations (HT only): or

The idea

The specification uses 'or' to show two acceptable formats for writing oxidation half equations at the anode. For example, the oxidation of bromide ions can be written by adding electrons to the products (2Br⁻ → Br₂ + 2e⁻) OR by subtracting electrons from the reactants (2Br⁻ - 2e⁻ → Br₂). Both formats correctly represent the loss of electrons.

What earns the marks

• Writes an oxidation half equation by adding electrons to the product side (e.g., 2Br⁻ → Br₂ + 2e⁻).
• Writes an oxidation half equation by subtracting electrons from the reactant side (e.g., 2Br⁻ - 2e⁻ → Br₂).
• Balances the number of atoms of each element on both sides of the half equation.
• Verifies that the total electrical charge is identical on both the reactant and product sides of the half equation.

Common mistakes and how to put them right

• Confusing the signs when moving electrons across the arrow; writing 2Br⁻ + 2e⁻ → Br₂ instead of 2Br⁻ - 2e⁻ → Br₂ for oxidation.

• Forgetting to balance the diatomic molecules, such as writing Br⁻ → Br + e⁻ instead of 2Br⁻ → Br₂ + 2e⁻ when a halogen is produced.

• Confusing oxidation and reduction.

  Put it right: Oxidation (loss of electrons) occurs at the anode and reduction (gain of electrons) at the cathode.

Exam tips

• While both formats for oxidation are acceptable, adding electrons to the product side (e.g., 2Br⁻ → Br₂ + 2e⁻) is often less confusing and reduces sign errors.
• Always check that your half equation produces the correct species, such as a stable diatomic molecule for halogens (e.g., Br₂) or a neutral atom for metals (e.g., Cu).