ExpertMedium1 markMultiple Choice
AQA GCSE · Question 05.4 · Chemical changes
What is the half equation for the reaction at the positive electrode when aqueous calcium chloride solution is electrolysed?
What is the half equation for the reaction at the positive electrode when aqueous calcium chloride solution is electrolysed?
Answer options:
A.
2Cl⁻ → Cl₂ + 2e⁻
B.
Cl₂ + 2e⁻ → 2Cl⁻
C.
4OH⁻ → O₂ + 2H₂O + 4e⁻
D.
O₂ + 2H₂O + 4e⁻ → 4OH⁻
How to approach this question
1. Identify the negative ions present in aqueous calcium chloride solution: Cl⁻ from the salt and OH⁻ from the water.
2. The positive electrode (anode) attracts these negative ions.
3. The rule for discharge at the anode is: if halide ions (like Cl⁻) are present, they are discharged to form the halogen, unless the solution is very dilute. If no halide is present, hydroxide ions are discharged to form oxygen and water.
4. Since Cl⁻ ions are present, they will be discharged.
5. Write the half equation for chloride ions losing electrons (oxidation) to form chlorine gas (Cl₂). Remember that chlorine is diatomic.
Full Answer
A.2Cl⁻ → Cl₂ + 2e⁻✓ Correct
2Cl⁻ → Cl₂ + 2e⁻
In aqueous calcium chloride solution, the negative ions attracted to the positive electrode (anode) are chloride ions (Cl⁻) and hydroxide ions (OH⁻).
The rule for discharge at the anode is that if a halide ion (Cl⁻, Br⁻, I⁻) is present at a reasonable concentration, it will be discharged in preference to hydroxide ions.
Therefore, chloride ions are oxidised. Each chloride ion loses one electron to become a chlorine atom. Since chlorine is a diatomic molecule (Cl₂), two chloride ions must react to form one molecule of chlorine gas, releasing two electrons in total.
The half equation for this oxidation process is: 2Cl⁻ → Cl₂ + 2e⁻.
Common mistakes
✗ Choosing the equation for the formation of oxygen (4OH⁻ → O₂ + 2H₂O + 4e⁻). This only happens if there are no halide ions.
✗ Choosing a reduction equation (where electrons are on the left side). Oxidation occurs at the anode.
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