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Date	May 2022	Marks available	2	Reference code	22M.2.hl.TZ1.2
Level	HL	Paper	2	Time zone	TZ1
Command term	Suggest	Question number	2	Adapted from	N/A

Question

Magnesium is a reactive metal often found in alloys.

Magnesium is sometimes used as a sacrificial anode to protect steel from corrosion.

A graph of the volume of gas produced by reacting magnesium with a large excess of 1 mol dm^–3 hydrochloric acid is shown.

Suggest an experiment that shows that magnesium is more reactive than zinc, giving the observation that would confirm this.

[2]

Calculate the standard potential, in V, of a cell formed by magnesium and steel half-cells. Use section 24 of the data booklet and assume steel has the standard electrode potential of iron.

[1]

b(i).

Calculate the free energy change, ΔG^⦵, in kJ, of the cell reaction. Use sections 1 and 2 of the data booklet.

[2]

b(ii).

This cell causes the electrolytic reduction of water on the steel. State the half-equation for this reduction.

[1]

b(iii).

Use the graph to deduce the dependence of the reaction rate on the amount of Mg.

[1]

c(i).

The reaction is first order with respect to HCl. Calculate the time taken, in seconds (s), for half of the Mg to dissolve when [HCl] = 0.5 mol dm^–3.

[1]

c(ii).

Carbonates also react with HCl and the rate can be determined by graphing the mass loss. Suggest why this method is less suitable for the reaction of Mg with HCl.

[1]

c(iii).

Markscheme

Alternative 1

put Mg in Zn²⁺(aq) ✔

Zn/«black» layer forms «on surface of Mg» ✔

Award [1 max] for “no reaction when Zn placed in Mg²⁺(aq)”.

Alternative 2

place both metals in acid ✔

bubbles evolve more rapidly from Mg
OR
Mg dissolves faster ✔

Alternative 3

construct a cell with Mg and Zn electrodes ✔

Accept “electrons flow from Mg to Zn”.

Accept Mg is negative electrode/anode
OR
Zn is positive electrode/cathode

bulb lights up
OR
shows (+) voltage
OR
size/mass of Mg(s) decreases <<over time>>
OR
size/mass of Zn increases <<over time>>

Accept other correct methods.

Cell potential: «(–0.45 V – (–2.37 V)» = «+»1.92 «V» ✔

b(i).

«ΔGº = -nFEº»
n = 2
OR
ΔGº = «-»2×96500×1.92 / «-»370,560 «J» ✔

-371 «kJ» ✔

For n = 1, award [1] for –185 «kJ».

Award [1 max] for (+)371 «kJ»

b(ii).

2 H₂O + 2 e^- → H₂ + 2 OH^- ✔

Accept equation with equilibrium arrows.

b(iii).

independent / not dependent ✔

Accept “zero order in Mg”.

c(i).

«2×170 s» = 340 «s» ✔

Accept 320 – 360 «s».

Accept 400 – 450 «s» based on no more gas being produced after 400 to 450s.

c(ii).

«relative/percentage» decrease in mass is «too» small/«much» less ✔

Accept “«relative/percentage» uncertainty in mass loss «too» great”. OR “density/molar mass of H₂ is «much» less than CO₂”.

c(iii).

Examiners report

Mediocre performance; some experiments would not have worked such as adding magnesium to zinc salt without reference to aqueous environment, adding Zn to magnesium ions, or Mg combustion reaction being more exothermic. In the last one, an inference wad made instead of identifying an observation or measuring temperature using a thermometer or a temperature probe.

Good performance; instead of E° = 1.92 V, answer such as −1.92 V + or −2.82 V showed a lack of understanding of how to calculate E° cell.

b(i).

Satisfactory performance; two major challenges in applying the equation ΔG° = −nFE° from the data booklet included:

Using n = 1, not 2, the number of electrons transferred in the redox reaction.

ΔG° unit from the equation is in J; some did not convert J to kJ as asked for.

b(ii).

Mediocre performance; some candidates had difficulty writing the reduction half-equation for water, the typical error included O₂(g) gas in the reactant or product, rather than H₂(g) in the product or including an equation with Fe(s) and H₂O(l) as reactants.

b(iii).

Candidates found this to be a tough question (see comments for parts (ii) and (iii)).

c(i).

Mediocre performance in calculating time from the graph for the data provided. Some wrote the rate expression, which only contains [HCl] and not mass or amount in mol Mg (as a solid, [Mg] is constant). This presented a challenge in arriving at a reasonable answer.

c(ii).

Poorly done; many candidates did not grasp the question and answer it appropriately. Candidates generally did not realize that decrease in mass (due to H₂(g) as a product for the reaction of Mg with HCl) is «too» small/«much» less compared to that of CO₂(g) from the reaction of carbonates with HCl.

c(iii).

Syllabus sections

Core » Topic 9: Redox processes » 9.1 Oxidation and reduction

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