Question 19M.2.sl.TZ2.e
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.sl.TZ2.e |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Annotate, Outline | Question number | e | 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.
The graph below shows the Maxwell–Boltzmann distribution of molecular energies at a particular temperature.
The rate at which dinitrogen monoxide decomposes is significantly increased by a metal oxide catalyst.
Annotate and use the graph to outline why a catalyst has this effect.
[2]
catalysed and uncatalysed Ea marked on graph AND with the catalysed being at lower energy [✔]
«for catalysed reaction» greater proportion of/more molecules have E ≥ Ea / E > Ea
OR
«for catalysed reaction» greater area under curve to the right of the Ea [✔]
Note: Accept “more molecules have the activation energy”.
This question discriminated very well between high-scoring and low-scoring candidates. About half of the candidates annotated the Maxwell-Boltzmann distribution to show the effect of the catalyst. Some left it blank and some sketched a new distribution that would be obtained at a higher temperature instead. The majority of candidates knew that the catalyst provided an alternative route with lower Ea but only stronger candidates related it to the annotation of the graph and used the accurate language needed to score M2. A common mistake was stating that molecules have higher kinetic energy when a catalyst is added.
