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Date May 2021 Marks available 2 Reference code 21M.2.hl.TZ1.6
Level HL Paper 2 Time zone TZ1
Command term Draw Question number 6 Adapted from N/A

Question

When dinitrogen pentoxide, N2O5, is heated the colourless gas undergoes thermal decomposition to produce brown nitrogen dioxide:

N2O5 (g) → 2NO2 (g) + 12O2 (g)

Data for the decomposition at constant temperature is given.

Suggest how the extent of decomposition could be measured.

[1]
a.

Plot the missing point on the graph and draw the best-fit line.

[2]
b(i).

Outline why increasing the concentration of N2O5 increases the rate of reaction.

[1]
b(ii).

Write the rate expression for this reaction.

[1]
b(iii).

Calculate the value of the rate constant, k, giving its units.

[3]
b(iv).

Markscheme

use colorimeter
OR
change in colour
OR
change in volume
OR
change in pressure ✔

Accept suitable instruments, e.g. pressure probe/oxygen sensor.

a.

point correct ✔

straight line passing close to all points AND through origin ✔


Accept free hand drawn line as long as attempt to be linear and meets criteria for M2.

b(i).

greater frequency of collisions «as concentration increases»
OR
more collisions per unit time «as concentration increases» ✔


Accept “rate/chance/probability/likelihood” instead of “frequency”.

Do not accept just “more collisions”.

b(ii).

rate = k[N2O5] ✔

b(iii).

krateN2O5

«k0.75×10-3«moldm-3min-1»25×10-3«moldm-3»= » 0.030 «min–1» ✔

min–1


M1 can be awarded from correct M2 if not explicitly stated.

Accept k = gradient.

Accept values in the range 0.028–0.032.

Award [3] for correct final answer.

b(iv).

Examiners report

[N/A]
a.
[N/A]
b(i).
[N/A]
b(ii).
[N/A]
b(iii).
[N/A]
b(iv).

Syllabus sections

Core » Topic 6: Chemical kinetics » 6.1 Collision theory and rates of reaction
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