Electrolysis of molten ionic compounds — AQA GCSE Chemistry
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Electrolysis of molten ionic compounds explained
Electrolysis of molten ionic compounds covers 2 specification points, including When a simple ionic compound (eg lead bromide)… and Students should be able to predict the….
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Open a point below for its explanation and revision guidance.
When a simple ionic compound (eg lead bromide) is electrolysed in the molten state using inert electrodes, the metal (lead) is produced at the cathode and the non-metal (bromine) is produced at the anode.
Electrolysis of a molten simple ionic compound, such as lead bromide, uses inert electrodes to produce the metal at the cathode and the non-metal at the anode. When lead bromide is melted, it dissociates into Pb²⁺ and Br⁻ ions. These ions are free to move and carry charge. At the negative cathode, Pb²⁺ ions gain electrons to form lead metal. At the positive anode, Br⁻ ions lose electrons to form bromine molecules. The products are lead, a silvery liquid metal, and bromine, a brown vapour. Inert electrodes, such as graphite, are used so they do not react with the products or electrolyte. Higher Tier students must also be able to represent these electron transfers using half equations.
Students should be able to predict the products of the electrolysis of binary ionic compounds in the molten state.
In a binary ionic compound, only two ions are present. 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
- Predict the products of electrolysis of a molten binary ionic compound.
- Describe the observations at each electrode during the electrolysis of molten lead bromide.
- Explain the role of inert electrodes and the need for the molten state.
Show all 6 objectives
- 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 electrolyte as molten lead bromide, which contains mobile Pb²⁺ and Br⁻ ions.
- State that lead is produced at the cathode (negative electrode) as positive lead ions are attracted to it.
- State that bromine is produced at the anode (positive electrode) as negative bromide ions are attracted to it.
- Explain that inert electrodes are used so they do not participate in the reactions.
- Higher Tier only: Write the half equations for the reactions at the electrodes (e.g., Pb²⁺ + 2e⁻ → Pb).
- 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
- 💡Name the products and the electrodes clearly: lead at cathode, bromine at anode.
- 💡Explain why the compound must be molten: to free the ions so they can move to the electrodes.
- 💡Higher Tier students should practice writing balanced half equations for the discharge of ions.
- 💡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
- Confusing which product forms at which electrode. Correction: metals form at the cathode; non-metals form at the anode.
- Thinking that solid lead bromide can be electrolysed. Correction: it must be molten to allow ions to move.
- Using reactive electrodes instead of inert ones. Correction: inert electrodes such as graphite are used to avoid unwanted reactions.
- 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.