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Date	May 2019	Marks available	2	Reference code	19M.2.sl.TZ1.4
Level	SL	Paper	2	Time zone	TZ1
Command term	Calculate	Question number	4	Adapted from	N/A

Question

This question is about peroxides.

Hydrogen peroxide decomposes to water and oxygen when a catalyst such as potassium iodide, KI, is added.

2H₂O₂ (aq) $\xrightarrow{{{\text{KI (aq)}}}}$ O₂ (g) + 2H₂O (l)

Suggest why many chemicals, including hydrogen peroxide, are kept in brown bottles instead of clear colourless bottles.

[1]

In a laboratory experiment solutions of potassium iodide and hydrogen peroxide were mixed and the volume of oxygen generated was recorded. The volume was adjusted to 0 at t = 0.

The data for the first trial is given below.

Plot a graph on the axes below and from it determine the average rate of formation of oxygen gas in cm³ O₂ (g) s⁻¹.

Average rate of reaction:

[3]

b(i).

Additional experiments were carried out at an elevated temperature. On the axes below, sketch Maxwell–Boltzmann energy distribution curves at two temperatures T₁ and T₂, where T₂ > T₁.

[2]

b(ii).

Apart from a greater frequency of collisions, explain, by annotating your graphs in (b)(ii), why an increased temperature causes the rate of reaction to increase.

[2]

b(iii).

MnO₂ is another possible catalyst for the reaction. State the IUPAC name for MnO₂.

[1]

b(iv).

Comment on why peracetic acid, CH₃COOOH, is always sold in solution with ethanoic acid and hydrogen peroxide.

H₂O₂ (aq) + CH₃COOH (aq) $\rightleftharpoons$ CH₃COOOH (aq) + H₂O (l)

[1]

Sodium percarbonate, 2Na₂CO₃•3H₂O₂, is an adduct of sodium carbonate and hydrogen peroxide and is used as a cleaning agent.

M_r (2Na₂CO₃•3H₂O₂) = 314.04

Calculate the percentage by mass of hydrogen peroxide in sodium percarbonate, giving your answer to two decimal places.

[2]

Markscheme

decomposes in light [✔]

Note: Accept “sensitive to light”.

points correctly plotted [✔]

best fit line AND extended through (to) the origin [✔]

Average rate of reaction:
«slope (gradient) of line =» 0.022 «cm³ O₂ (g) s⁻¹» [✔]

Note: Accept range 0.020–0.024cm³ O₂ (g) s⁻¹.

b(i).

peak of T₂ to right of AND lower than T₁ [✔]

lines begin at origin AND T₂ must finish above T₁ [✔]

b(ii).

E_a marked on graph [✔]

explanation in terms of more “particles” with E ≥ E_a
OR
greater area under curve to the right of E_a in T₂ [✔]

b(iii).

manganese(IV) oxide
OR
manganese dioxide [✔]

Note: Accept “manganese(IV) dioxide”.

b(iv).

move «position of» equilibrium to right/products [✔]

Note: Accept “reactants are always present as the reaction is in equilibrium”.

M (H₂O₂) «= 2 × 1.01 + 2 × 16.00» = 34.02 «g» [✔]

«% H₂O₂ = 3 × $\frac{{34.02}}{{314.04}}$ × 100 =» 32.50 «%» [✔]

Note: Award [2] for correct final answer.

Examiners report

The explanation that the brown bottle prevented light causing a decomposition of the chemical was well answered but some incorrectly suggested it helped to stop mixing up of chemicals e.g. acid/water/peroxide.

The graphing was disappointing with a surprising number of students missing at least one mark for failing to draw a straight line or for failing to draw the line passing through the origin. Also some were unable to calculate the gradient.

b(i).

The drawing of the two curves at T1 and T2 was generally poorly done.

b(ii).

Explaining why temperature increase caused an increase in reaction rate was generally incorrectly answered with most students failing to mention “activation energy” in their answer or failing to annotate the graph.

b(iii).

Many could correctly name manganese(IV)oxide, but there were answers of magnesium(IV) oxide or manganese(II) oxide.

b(iv).

Suggesting why peractic acid was sold in solution was very poorly answered and only a few students mentioned equilibrium and, if they did, they thought it would move to the left to restore equilibrium.

Calculating the % by mass was generally well answered although some candidates started by using rounded values of atomic masses which made their final answer unprecise.

Syllabus sections

Core » Topic 1: Stoichiometric relationships » 1.2 The mole concept

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