DP Chemistry (first assessment 2025)
Question 19M.2.HL.TZ2.3a(ii)
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.HL.TZ2.3a(ii) |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Write | Question number | a(ii) | Adapted from | N/A |
a(ii).
[Maximum mark: 2]
19M.2.HL.TZ2.3a(ii)
(a(ii))
Dinitrogen monoxide in the stratosphere is converted to nitrogen monoxide, NO (g).
Write two equations to show how NO (g) catalyses the decomposition of ozone.
[2]
Markscheme
NO (g) + O3 (g) → NO2 (g) + O2 (g) [✔]
NO2 (g) + O3 (g) → NO (g) + 2O2 (g) [✔]
Note: Ignore radical signs.
Accept equilibrium arrows.
Award [1 max] for NO2 (g) + O (g) → NO (g) + O2 (g).
Examiners report
Many candidates recalled the first equation for NO catalyzed decomposition of ozone only. Some considered other radical species.
Syllabus sections
Reactivity 1. What drives chemical reactions? » Reactivity 1.4—Entropy and spontaneity (Additional higher level) » Reactivity 1.4.2—Change in Gibbs energy, ΔG, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, ΔH, change in entropy, ΔS, and absolute temperature, T. Apply the equation ΔG⦵ = ΔH⦵ − TΔS⦵ to calculate unknown values of these terms.
Reactivity 1. What drives chemical reactions? » Reactivity 1.4—Entropy and spontaneity (Additional higher level) » Reactivity 1.4.3—At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative. Interpret the sign of ΔG calculated from thermodynamic data. Determine the temperature at which a reaction becomes spontaneous.