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
Representation of reactions at electrodes as half equations (HT only) covers 2 specification points, including During electrolysis, at the cathode (negative… and At the anode (positive electrode), negatively….
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Open a point below for its explanation and revision guidance.
During electrolysis, at the cathode (negative electrode), positively charged ions gain electrons and so the reactions are reductions.
At the cathode, which is the negative electrode, positively charged ions move towards it because opposite charges attract. 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.
At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations.
At the anode, which is the positive electrode, negatively charged ions move towards it because opposite charges attract. Each negative ion loses one or more electrons to the electrode and becomes a neutral atom or molecule. Losing electrons is oxidation, so every anode reaction in electrolysis is an oxidation. For example, in copper chloride solution, chloride ions are oxidised: 2Cl⁻(aq) → Cl₂(g) + 2e⁻. In dilute sulfuric acid, hydroxide ions are oxidised: 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻. You show this by writing a balanced half equation with 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.
Show all 6 objectives
- Describe the movement of negative ions to the positive anode.
- Explain why anode reactions are oxidations in terms of electron loss.
- Construct balanced half equations for reactions at the anode.
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.
- State that the anode is the positive electrode and that negative ions move towards it.
- Explain that negative ions lose electrons at the anode and that electron loss is oxidation.
- Write a balanced anode half equation showing the ion, product and electrons with correct formulae and state symbols.
- Balance the number of electrons lost with the charge on the ions, for example two chloride ions lose two electrons to form one chlorine molecule.
- Link the anode 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.
- 💡Check the charge balance: the total charge on the left must equal the total charge on the right.
- 💡Use the phrase lose electrons equals oxidation to justify why an anode reaction is an oxidation.
- 💡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.
- Writing the anode as the negative electrode; correct this by remembering that the anode is positive and attracts negative ions.
- Showing electrons gained at the anode; correct this by writing electrons as lost, for example 2Cl⁻(aq) → Cl₂(g) + 2e⁻.
- Forgetting to balance atoms and charge in the half equation; correct this by checking that atoms and total charge are equal on both sides.