Representation of reactions at electrodes as half equations (HT only) — AQA GCSE Chemistry
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Representation of reactions at electrodes as half equations (HT only) explained
At the cathode, which is the negative electrode, positively charged ions move towards it because opposite charges attract.
Read the full explanation
Each positive ion gains one or more electrons from the electrode and becomes a neutral atom or molecule. Gaining electrons is reduction, so every cathode reaction in electrolysis is a reduction. For example, in copper chloride solution, Cu²⁺(aq) gains two electrons to form copper metal: Cu²⁺(aq) + 2e⁻ → Cu(s). In dilute sulfuric acid, 2H⁺(aq) + 2e⁻ → H₂(g). You show this by writing a balanced half equation with the correct charges and state symbols, and higher-tier questions assess whether you can construct and interpret such equations.
Your focus
- Describe the movement of positive ions to the negative cathode.
- Explain why cathode reactions are reductions in terms of electron gain.
- Construct balanced half equations for reactions at the cathode.
Representation of reactions at electrodes as half equations (HT only) exam tips
Marking Points
- State that the cathode is the negative electrode and that positive ions move towards it.
- Explain that positive ions gain electrons at the cathode and that electron gain is reduction.
- Write a balanced cathode half equation showing the ion, electrons and product with correct formulae and state symbols.
- Balance the number of electrons gained with the charge on the ion, for example two electrons for a 2⁺ ion.
- Link the cathode half equation to the overall electrolysis reaction and to the product observed.
Examiner Tips
- 💡Check the charge balance: the total charge on the left must equal the total charge on the right.
- 💡Use the phrase gain electrons equals reduction to justify why a cathode reaction is a reduction.
- 💡Include state symbols in half equations because they show the form of each substance.
Common Mistakes
- Writing the cathode as the positive electrode; correct this by remembering that the cathode is negative and attracts positive ions.
- Showing electrons lost at the cathode; correct this by writing electrons as gained, for example Cu²⁺(aq) + 2e⁻ → Cu(s).
- Omitting state symbols or charges in the half equation; correct this by including (aq), (s), (g) or (l) and the ion charge where needed.