ExpertMedium1 markShort Answer
AQA GCSE · Question 05.3 · Chemical changes
Name the product formed at the negative electrode when aqueous calcium chloride solution is electrolysed.
Name the product formed at the negative electrode when aqueous calcium chloride solution is electrolysed.
How to approach this question
1. Identify all the positive ions present in aqueous calcium chloride solution. There are Ca²⁺ ions from the salt and H⁺ ions from the water.
2. The negative electrode (cathode) attracts positive ions.
3. The rule for electrolysis of aqueous solutions at the negative electrode is: the less reactive element is produced. Compare the reactivity of calcium and hydrogen.
4. The less reactive element will be discharged. Name this element.
Full Answer
Hydrogen
In an aqueous solution of calcium chloride (CaCl₂), there are four ions present:
- From CaCl₂: Ca²⁺ (positive) and Cl⁻ (negative)
- From water (H₂O): H⁺ (positive) and OH⁻ (negative)
At the negative electrode (cathode), the positive ions (cations) are attracted: Ca²⁺ and H⁺.
The rule for discharge at the cathode is that the ion of the less reactive element will be reduced. We compare the reactivity of calcium and hydrogen. Calcium is a very reactive metal, much more reactive than hydrogen. Therefore, hydrogen ions are discharged in preference to calcium ions.
The reduction of hydrogen ions produces hydrogen gas: 2H⁺(aq) + 2e⁻ → H₂(g).
Common mistakes
✗ "Calcium" - Calcium is more reactive than hydrogen, so it remains in solution.
✗ "Chlorine" - Chlorine is a gas produced at the positive electrode.
✗ "Hydroxide" - This is a negative ion and would go to the positive electrode.
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