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Date	May 2019	Marks available	3	Reference code	19M.2.hl.TZ1.4
Level	HL	Paper	2	Time zone	TZ1
Command term	Draw	Question number	4	Adapted from	N/A

Question

This question is about the decomposition of hydrogen peroxide.

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).

Two more trials (2 and 3) were carried out. The results are given below.

Determine the rate equation for the reaction and its overall order, using your answer from (b)(i).

Rate equation:

Overall order:

[2]

b(ii).

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(iii).

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

[2]

b(iv).

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

[1]

b(v).

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

H₂O₂ (aq) + CH₃COOH (aq) ⇌ 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).

Rate equation:
Rate = k[H₂O₂] × [KI] [✔]

Overall order:
2 [✔]

Note: Rate constant must be included.

b(ii).

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

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

b(iii).

E_a marked on graph [✔]

explanation in terms of more “particles” with E ≥ E_a

greater area under curve to the right of E_a in T₂ [✔]

b(iv).

manganese(IV) oxide

manganese dioxide [✔]

Note: Accept “manganese(IV) dioxide”.

b(v).

moves «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

There were a couple of comments claiming that this NOS question on “why to store hydrogen peroxide in brown bottles” is not the syllabus. Most candidates were quite capable of reasoning this out.

Most candidates could plot a best fit line and find the slope to calculate an average rate of reaction.

b(i).

Good performance but with answers that either typically included only [H₂O₂] with first or second order equation or even suggesting zero order rate equation.

b(ii).

Fair performance; errors including not starting the two curves at the origin, drawing peak for T2 above T1, T2 finishing below T1 or curves crossing the x-axis.

b(iii).

The majority of candidates earned at least one mark, many both marks. Errors included not annotating the graph with E_a and referring to increase of kinetic energy as reason for higher rate at T2.

b(iv).

A well answered question. Very few candidates had problem with nomenclature.

b(v).

One teacher suggested that “stored” would have been better than “sold” for this question. There were a lot of irrelevant answers with many believing the back reaction was an acid dissociation.

It is recommended that candidates use the relative atomic masses given in the periodic table.

Syllabus sections

Core » Topic 11: Measurement and data processing » 11.2 Graphical techniques

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