Question 19M.2.sl.TZ2.2a
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.sl.TZ2.2a |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Explain | Question number | a | 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.
Explain why, as the reaction proceeds, the pressure increases by the amount shown.
[2]
increase in the amount/number of moles/molecules «of gas» [✔]
from 2 to 3/by 50 % [✔]
About a quarter of the candidates gave the full answer. Some only gained the first marking point (M1) by recognizing the increase in the number of moles of gas. Some candidates wrote vague answers that did not receive credit such as “pressure increases as more gaseous products form” without explicitly recognizing that the reactants have fewer moles of gas than the products. Some candidates mistook it for a system at equilibrium when the pressure stops changing (although a straight arrow is shown in the equation). A teacher commented that the wording of the question was rather vague “not clear if question is asking about stoichiometry (i.e. how 200 & 300 connect to coefficients) or rates (i.e. explain graph shape)”. We did not see a discussion of the slope of the graph with time and most candidates understood the question as it was intended.
