Question 19M.2.hl.TZ2.c(ii)
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.hl.TZ2.c(ii) |
| Level | hl | Paper | 2 | Time zone | TZ2 |
| Command term | Explain | Question number | c(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.
This decomposition obeys the rate expression:
= k[N2O]
It has been suggested that the reaction occurs as a two-step process:
Step 1: N2O (g) → N2 (g) + O (g)
Step 2: N2O (g) + O (g) → N2 (g) + O2 (g)
Explain how this could support the observed rate expression.
[2]
1 slower than 2
OR
1 rate determinant step/RDS [✔]
1 is unimolecular/involves just one molecule so it must be first order
OR
if 1 faster/2 RDS, second order in N2O
OR
if 1 faster/2 RDS, first order in O [✔]
Most students were able to identity step 1 as the RDS/slow but few mentioned unimolecularity or referred vaguely to NO2 as the only reagent (which was obvious) and got only 1 mark.
