Electrolysis of molten ionic compounds — AQA GCSE Chemistry
Test yourself on Electrolysis of molten ionic compounds with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Electrolysis of molten ionic compounds explained
In a binary ionic compound, only two ions are present.
Read the full explanation
When the solid is melted, these ions become free to move and carry charge. At the cathode (negative electrode), the positive metal ion gains electrons to form the metal: for PbBr₂, Pb²⁺ + 2e⁻ → Pb. At the anode (positive electrode), the negative non-metal ion loses electrons to form the non-metal: 2Br⁻ → Br₂ + 2e⁻. Thus electrolysis of molten lead bromide gives lead and bromine. The method is: identify the ions, assign each to the electrode that attracts it, then write the product. If the compound contains a metal and a non-metal, the metal appears at the cathode and the non-metal at the anode. This skill is assessed by asking you to name or write the products for a given molten binary compound, often with equations.
Your focus
- Identify the ions present in a binary ionic compound.
- Predict the product at each electrode when the compound is molten.
- Write balanced half-equations for the reactions at the cathode and anode.
Electrolysis of molten ionic compounds exam tips
Marking Points
- Identify the positive metal ion and the negative non-metal ion in the binary compound.
- State that the positive metal ion is attracted to the cathode and gains electrons to form the metal.
- State that the negative non-metal ion is attracted to the anode and loses electrons to form the non-metal.
- Write balanced half-equations for each electrode, including state symbols where appropriate.
- Name the products correctly, for example lead at the cathode and bromine at the anode for molten lead bromide.
- Explain that the ions must be free to move, which occurs in the molten state but not in the solid state.
Examiner Tips
- 💡Underline the metal and non-metal in the formula before predicting products.
- 💡Use the phrase 'positive ions move to the cathode' to make your reasoning clear.
- 💡Check that each half-equation is balanced for both atoms and charge.
Common Mistakes
- Thinking that the metal forms at the anode: correct this by recalling that positive ions move to the negative electrode (cathode).
- Writing the non-metal as a single atom rather than a diatomic molecule: correct this by using the formula of the molecule, such as Br₂ or Cl₂.
- Forgetting to balance the electrons in half-equations: correct this by ensuring the number of electrons lost equals the number gained in the overall process.
- Assuming that water is present: correct this by noting that the compound is molten and anhydrous, so water is not a reactant.