DP Chemistry (first assessment 2025)

Question 21M.2.HL.TZ1.6

Date	May 2021	Marks available	[Maximum mark: 8]	Reference code	21M.2.HL.TZ1.6
Level	HL	Paper	2	Time zone	TZ1
Command term	Calculate, Draw, Outline, Suggest, Write	Question number	6	Adapted from	N/A

[Maximum mark: 8]

21M.2.HL.TZ1.6

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

N₂O₅(g) → 2NO₂(g) + $\frac{1}{2}$ O₂(g)

Data for the decomposition at constant temperature is given.

(a)

Suggest how the extent of decomposition could be measured.

[1]

Markscheme

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

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

(b(i))

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

[2]

Markscheme

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(ii))

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

[1]

Markscheme

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(iii))

Write the rate expression for this reaction.

[1]

Markscheme

rate = k[N₂O₅] ✔

(b(iv))

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

[3]

Markscheme

k = $\frac{∆ rate}{∆ [N_{2} O_{5}]}$ ✔

«k = $\frac{0.75 \times 10^{- 3} « mol {dm}^{- 3} \min^{- 1} »}{25 \times 10^{- 3} « mol {dm}^{- 3} »}$ = » 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.

Syllabus sections

Tools » Tool 1: Experimental techniques

Reactivity 2. How much, how fast and how far? » Reactivity 2.2—How fast? The rate of chemical change » Reactivity 2.2.1—The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

Tools » Tool 3: Mathematics

Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Ch.telier’s principle to predict and explain responses to changes of systems at equilibrium.

Reactivity 2. How much, how fast and how far? » Reactivity 2.2—How fast? The rate of chemical change » Reactivity 2.2.10—The order of a reaction with respect to a reactant is the exponent to which the concentration of the reactant is raised in the rate equation. The order with respect to a reactant can describe the number of particles taking part in the rate determining step. The overall reaction order is the sum of the orders with respect to each reactant. Sketch, identify and analyse graphical representations of zero, first and second order reactions.

Reactivity 2. How much, how fast and how far? » Reactivity 2.2—How fast? The rate of chemical change » Reactivity 2.2.11—The rate constant, k, is temperature dependent and its units are determined from the overall order of the reaction. Solve problems involving the rate equation, including the units of k.

Question 21M.2.HL.TZ1.6

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