Question 19M.2.hl.TZ2.g(ii)
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.2.hl.TZ2.g(ii) |
| Level | hl | Paper | 2 | Time zone | TZ2 |
| Command term | Deduce | Question number | g(ii) | Adapted from | N/A |
The thermal decomposition of dinitrogen monoxide occurs according to the equation:
2N2O (g) → 2N2 (g) + O2 (g)
The reaction can be followed by measuring the change in total pressure, at constant temperature, with time.
The x-axis and y-axis are shown with arbitrary units.
Dinitrogen monoxide has a positive standard enthalpy of formation, ΔHfθ.
Deduce, giving reasons, whether altering the temperature would change the spontaneity of the decomposition reaction.
[3]
exothermic decomposition
OR
ΔH(decomposition) < 0 [✔]
TΔSθ > ΔHθ
OR
ΔGθ «= ΔHθ – TΔSθ» < 0 «at all temperatures» [✔]
reaction spontaneous at all temperatures [✔]
Though the question asked for decomposition (in bold), most candidates ignored this and worked on the basis of a the ΔH of formation. However, many did write a sound explanation for that situation. On the other hand, in quite a number of cases, they did not state the sign of the ΔH (probably taking it for granted) nor explicitly relate ΔG and spontaneity, which left the examiner with no possibility of evaluating their reasoning.
