Question 19M.2.sl.TZ2.b(i)
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.sl.TZ2.b(i) |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Predict | Question number | b(i) | Adapted from | N/A |
Carbonated water is produced when carbon dioxide is dissolved in water under pressure.
The following equilibria are established.
Soda water has sodium hydrogencarbonate, NaHCO3, dissolved in the carbonated water.
Predict, referring to Equilibrium (2), how the added sodium hydrogencarbonate affects the pH.(Assume pressure and temperature remain constant.)
[2]
«additional HCO3–» shifts position of equilibrium to left [✔]
pH increases [✔]
Note: Do not award M2 without any justification in terms of equilibrium shift in M1.
This was one of the most challenging questions on the paper that required application of Le Chatelier’s Principle in an unfamiliar situation. Most candidates did not refer to equilibrium (2), as directed by the question, and hence could not gain any marks. Some candidates stated that NaHCO3 was an acid and decreased pH. Some answers had contradictions that showed poor understanding of the pH concept.
