DP Chemistry (first assessment 2025)
Question 19M.1A.SL.TZ2.27
| Date | May 2019 | Marks available | [Maximum mark: 1] | Reference code | 19M.1A.SL.TZ2.27 |
| Level | SL | Paper | 1A | Time zone | TZ2 |
| Command term | Question number | 27 | Adapted from | N/A |
27.
[Maximum mark: 1]
19M.1A.SL.TZ2.27
The following equation represents the dissociation of water at 25 °C.
2H2O (l) H3O+ (aq) + OH− (aq) ΔH = +56 kJ
Which changes occur as the temperature increases?
A. [H3O+] increases and pH will decrease.
B. [H3O+] decreases and pH will increase.
C. [H3O+] increases and pH will increase.
D. [H3O+] decreases and pH will decrease.
[1]
Markscheme
A
Examiners report
This is one of the more challenging questions on the paper. 62 % of the candidates obtained the correct answer. The most commonly chosen distractor was C, where the increase in the concentration of H3O+ was recognized by applying Le Chatelier’s principle but the effect on pH was incorrect.
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–].