DP Chemistry (first assessment 2025)
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 |
b(i).
[Maximum mark: 2]
19M.2.SL.TZ2.b(i)
Predict, referring to Equilibrium (2), how the added sodium hydrogencarbonate affects the pH.(Assume pressure and temperature remain constant.)
[2]
Markscheme
«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.
Examiners report
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.
Syllabus sections
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.5—The ion product constant of water, Kw, shows an inverse relationship between [H+] and [OH–]. Kw = [H+] [OH–]. Recognize solutions as acidic, neutral and basic from the relative values of [H+] and [OH–].
Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Ch.telier’s principle to predict and explain responses to changes of systems at equilibrium.