Question 22M.2.sl.TZ2.5
| Date | May 2022 | Marks available | [Maximum mark: 4] | Reference code | 22M.2.sl.TZ2.5 |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Deduce | Question number | 5 | Adapted from | N/A |
Molten zinc chloride undergoes electrolysis in an electrolytic cell at 450 °C.
Deduce the half-equations for the reaction at each electrode.
[2]
Cathode (negative electrode):
Zn2+ + 2e− → Zn (l) ✔
Anode (positive electrode):
2Cl− → Cl2 (g) + 2e−
OR
Cl− → ½ Cl2 (g) + e− ✔
The half-equations were often incorrect. The average mark was 0.8 out of 2, and the correlation to high scoring candidates was strong as expected. Many candidates started the half-equations with the elements and gave the ions as products. We also saw some scripts with Cl instead of Cl2 as the product. Some of the candidates thought the zinc ion was Zn+ instead of Zn2+. Some candidates reversed the anode and cathode equations earning only 1 of the 2 marks.
Deduce the overall cell reaction including state symbols. Use section 7 of the data booklet.
[2]
ZnCl2 (l) → Zn (l) + Cl2 (g)
balanced equation ✔
correct state symbols ✔
Accept ionic equation.
The performance was weak on this part-question as well. The overall equations did not balance atoms or charges on many of the incorrect answers. For the state symbols, many candidates used the aqueous state symbol, some gave the chloride ion a gaseous state symbol, and some candidates still had a solid zinc product even though they were directed to use the melting point of zinc in the data booklet. 12% of the candidates did not answer the question and the average mark was 0.5 out of 2 marks.
