AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrolysis of molten ionic compounds: 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.
When a simple ionic compound like lead bromide is melted, its ions become free to move, allowing it to conduct electricity.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a simple ionic compound like lead bromide is melted, its ions become free to move, allowing it to conduct electricity. 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⁻.
What earns the marks
- 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).
Common mistakes and how to put them right
Saying positive ions go to the positive anode.
Put it right: 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.
Put it right: 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).
Put it right: Halogens form diatomic molecules, so the product is Br₂ and the half-equation is 2Br⁻ → Br₂ + 2e⁻.
Exam 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).
Try the lesson
The start of the Electrolysis of molten ionic compounds: 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. lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of molten lead bromide, at which electrode is lead produced?
Listen
When a simple ionic compound like lead bromide is melted, its ions become free to move, allowing it to conduct electricity. 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⁻. A strong answer earns marks for these 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. A common slip: Saying positive ions go to the positive anode. Instead, opposite charges attract, so positive ions move to the negative cathode and negative ions move to the positive anode. Exam tip: State the electrode and the product together, for example 'lead is produced at the cathode', to clearly link charge attraction to the product.
See it
Key words
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Cathode
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Anode
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Reduction
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See every definition
- Cathode
- The negative electrode, where positive metal ions are attracted and the metal is produced.
- Anode
- The positive electrode, where negative non-metal ions are attracted and the non-metal is produced.
- Reduction
- At the cathode, positive ions gain electrons to form metal atoms.
Label the diagram
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1Box 1
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3Box 3
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