AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrolysis of aqueous solutions: This happens because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged.
In an aqueous solution, water ionises slightly: H₂O ⇌ H⁺ + OH⁻.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In an aqueous solution, water ionises slightly: H₂O ⇌ H⁺ + OH⁻. These ions compete with those from the dissolved compound. At the cathode, hydrogen is produced if the metal is more reactive than hydrogen. At the anode, oxygen is produced unless halide ions are present, in which case the halogen is produced. For example, in aqueous sodium chloride, H⁺ is discharged at the cathode to form hydrogen gas (2H⁺ + 2e⁻ → H₂), and Cl⁻ is discharged at the anode to form chlorine gas (2Cl⁻ → Cl₂ + 2e⁻). The remaining Na⁺ and OH⁻ ions stay in solution, forming sodium hydroxide. This explains why the products of aqueous electrolysis often differ from those of the molten compound.
What earns the marks
- State that water molecules ionise to a small extent (H₂O ⇌ H⁺ + OH⁻), adding H⁺ and OH⁻ to the electrolyte.
- Explain that at the cathode, hydrogen is produced if the metal is more reactive than hydrogen.
- Explain that at the anode, oxygen is produced unless halide ions are present, in which case the halogen is produced.
- Describe how in aqueous sodium chloride, H⁺ and Cl⁻ are discharged, leaving Na⁺ and OH⁻ in solution to form sodium hydroxide.
- Write half-equations for the discharge of ions, such as 2H⁺ + 2e⁻ → H₂ and 2Cl⁻ → Cl₂ + 2e⁻.
Common mistakes and how to put them right
Assuming the metal is always produced at the cathode.
Put it right: Hydrogen is produced if the metal is more reactive than hydrogen.
Stating that oxygen is always produced at the anode.
Put it right: If halide ions (like chloride) are present, the halogen (like chlorine) is produced.
Thinking sodium chloride solution leaves sodium chloride behind.
Put it right: H⁺ and Cl⁻ are discharged, leaving Na⁺ and OH⁻ to form sodium hydroxide.
Exam tips
- Always check if the electrolyte is molten or aqueous; if aqueous, remember to consider H⁺ and OH⁻ ions.
- For aqueous sodium chloride, remember the products are hydrogen, chlorine, and sodium hydroxide.
Try the lesson
The start of the Electrolysis of aqueous solutions: This happens because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged. lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of an aqueous solution, water molecules break down to produce which two types of ion?
Listen
In an aqueous solution, water ionises slightly: H₂O ⇌ H⁺ + OH⁻. These ions compete with those from the dissolved compound. At the cathode, hydrogen is produced if the metal is more reactive than hydrogen. At the anode, oxygen is produced unless halide ions are present, in which case the halogen is produced. For example, in aqueous sodium chloride, H⁺ is discharged at the cathode to form hydrogen gas (2H⁺ + 2e⁻ → H₂), and Cl⁻ is discharged at the anode to form chlorine gas (2Cl⁻ → Cl₂ + 2e⁻). The remaining Na⁺ and OH⁻ ions stay in solution, forming sodium hydroxide. This explains why the products of aqueous electrolysis often differ from those of the molten compound. A strong answer earns marks for these points. State that water molecules ionise to a small extent (H₂O ⇌ H⁺ + OH⁻), adding H⁺ and OH⁻ to the electrolyte. Explain that at the cathode, hydrogen is produced if the metal is more reactive than hydrogen. Explain that at the anode, oxygen is produced unless halide ions are present, in which case the halogen is produced. Describe how in aqueous sodium chloride, H⁺ and Cl⁻ are discharged, leaving Na⁺ and OH⁻ in solution to form sodium hydroxide. A common slip: Assuming the metal is always produced at the cathode. Instead, hydrogen is produced if the metal is more reactive than hydrogen. Exam tip: Always check if the electrolyte is molten or aqueous; if aqueous, remember to consider H⁺ and OH⁻ ions.
Key words
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Competing ions in aqueous solutions
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Reactivity and ease of discharge
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Formation of useful by-products
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See every definition
- Competing ions in aqueous solutions
- In an aqueous solution, water molecules break down producing hydrogen ions and hydroxide ions that are discharged, and these ions compete with those from the dissolved compound.
- Reactivity and ease of discharge
- At the cathode, hydrogen is produced if the metal is more reactive than hydrogen; at the anode, oxygen is produced unless halide ions are present, in which case the halogen is produced.
- Formation of useful by-products
- In aqueous sodium chloride, hydrogen ions and chloride ions are discharged, leaving sodium ions and hydroxide ions in solution to form sodium hydroxide.
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