Electrolysis of molten ionic compounds — AQA GCSE Combined Science
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Electrolysis of molten ionic compounds explained
When a simple ionic compound like lead bromide is melted, its ions become free to move, allowing it to conduct electricity.
Read the full explanation
During electrolysis with inert electrodes, positive metal ions (Pb²⁺) are attracted to the negative cathode, producing lead metal. Negative non-metal ions (Br⁻) are attracted to the positive anode, producing bromine vapour. The inert electrodes do not react. The overall process decomposes the molten compound into its constituent elements. (HT only) At the cathode, metal ions gain electrons (reduction) to form metal atoms, e.g., Pb²⁺ + 2e⁻ → Pb. At the anode, non-metal ions lose electrons (oxidation) to form molecules, e.g., 2Br⁻ → Br₂ + 2e⁻.
Students should be able to predict the products of the electrolysis of binary ionic compounds in the molten state.
A binary ionic compound contains a metal cation and a non-metal anion. In the molten state, its ions are free to move, so electrolysis decomposes it into its elements. To predict the products, identify the ions from the formula, then apply charge attraction: the positive metal ion goes to the negative cathode to form the metal; the negative non-metal ion goes to the positive anode to form the non-metal. For example, molten sodium chloride gives sodium at the cathode and chlorine at the anode. (HT only: At the cathode, positive ions gain electrons to form atoms. At the anode, negative ions lose electrons to form atoms). The method works for any binary molten ionic compound, including lead bromide and potassium iodide.
Your focus
- Describe the products formed at the cathode and anode when a molten ionic compound is electrolysed.
- Explain why a simple ionic compound must be molten to undergo electrolysis.
- (HT only) Write balanced ionic half-equations for the oxidation and reduction reactions at the electrodes.
Show all 6 objectives
- Deduce the ions present in a binary ionic compound from its formula.
- Predict the element formed at each electrode when a named molten binary ionic compound is electrolysed.
- (HT only) Explain the formation of elements at electrodes in terms of electron transfer.
Electrolysis of molten ionic compounds exam tips
Marking Points
- Molten ionic compounds conduct electricity because their ions are free to move and carry charge.
- Positive metal ions are attracted to the negative cathode, where the metal is produced (e.g., lead).
- Negative non-metal ions are attracted to the positive anode, where the non-metal is produced (e.g., bromine).
- Inert electrodes are used because they do not react with the electrolyte or the products.
- (HT only) At the cathode, positive ions gain electrons (reduction), and at the anode, negative ions lose electrons (oxidation).
- Identify the metal cation and non-metal anion from the compound's formula, for example MgCl₂ gives Mg²⁺ and Cl⁻.
- Predict that the metal cation is discharged at the negative cathode because opposite charges attract.
- Predict that the non-metal anion is discharged at the positive anode.
- Name the elements formed and state that the molten compound is decomposed into its elements.
- (HT only) Explain that metal ions gain electrons at the cathode (reduction) and non-metal ions lose electrons at the anode (oxidation).
Examiner Tips
- 💡State the electrode and the product together, for example 'lead is produced at the cathode', to clearly link charge attraction to the product.
- 💡Remember that metals always form at the cathode and non-metals at the anode during the electrolysis of molten simple ionic compounds.
- 💡(HT only) Use the acronym OILRIG to remember that Oxidation Is Loss of electrons (at the anode) and Reduction Is Gain of electrons (at the cathode).
- 💡Write the ions first, then draw or state the electrode each ion moves to before naming the product.
- 💡For a formula such as Al₂O₃, use the charges Al³⁺ and O²⁻ to predict aluminium at the cathode and oxygen at the anode.
- 💡(HT only) Remember OIL RIG to help identify oxidation at the anode and reduction at the cathode.
Common Mistakes
- Saying positive ions go to the positive anode. Correction: opposite charges attract, so positive ions move to the negative cathode and negative ions move to the positive anode.
- Believing solid lead bromide conducts electricity. Correction: the ions in a solid lattice are fixed; the compound must be molten for ions to move and carry charge.
- (HT only) Writing the anode product as single atoms (e.g., Br). Correction: halogens form diatomic molecules, so the product is Br₂ and the half-equation is 2Br⁻ → Br₂ + 2e⁻.
- Swapping the electrode products: correct this by remembering that the metal always forms at the cathode and the non-metal at the anode in a binary molten compound.
- Forgetting that some non-metal products are diatomic: correct this by writing Cl₂, Br₂ or I₂ rather than Cl, Br or I.
- Using the compound's formula as the product, for example saying 'sodium chloride' forms at an electrode: correct this by naming the elements sodium and chlorine separately.