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Question 20N.3.hl.TZ0.13

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Date November 2020 Marks available [Maximum mark: 5] Reference code 20N.3.hl.TZ0.13
Level hl Paper 3 Time zone TZ0
Command term Calculate, Justify, Predict, Write Question number 13 Adapted from N/A
13.
[Maximum mark: 5]
20N.3.hl.TZ0.13

A voltaic cell is made up of nickel and magnesium half-cells.

Mg (s) | Mg2+(aq) | | Ni2+(aq) | Ni (s)

(a)

Write the balanced equation for the reaction in this voltaic cell.

[1]

Markscheme

Mg(s)+Ni2+(aq)→Mg2+(aq)+Ni(s) ✔

Accept a balanced molecular equation such as “Mg+NiSO4→MgSO4+Ni”.

Examiners report

Nearly all were able to write the balanced equation for the reaction occurring in the voltaic cell.

(b)

Calculate the cell potential for 0.0100 mol dm-3 Mg2+(aq) and 0.800 mol dm-3 Ni2+(aq) at 298 K. Use sections 1, 2 and 24 of the data booklet.

[3]

Markscheme

EƟ=«2.37−0.26=»(+)2.11«V» ✔

«Q=0.01000.800=»0.0125 AND «n=»2 ✔

«E=EƟ−RTnF ln Q=2.11−8.31×2982×96 500 ln 0.0125=»(+)2.17«V» ✔


Award [3] for correct final answer.

Examiners report

The cell potential numerical problem was well executed and many scored all three marks, by calculating the final answer as +2.17 V. The most common errors related to either incorrectly calculating Q or identifying n = 2 in the intermediate, numerical step.

(c)

Predict, giving a reason, how an increase in temperature affects the potential of this cell.

[1]

Markscheme

cell potential/E increases AND increasing temperature favours forward reaction
OR
cell potential/E increases AND ∆G becomes more negative
OR
cell potential/E increases AND RT/nF ln Q becomes more negative ✔


Accept any correct mathematical explanation using the Nernst equation.

Examiners report

This was often gained as an easy subsequent mark for those that scored full marks in (b). The majority approached this problem by applying a mathematical explanation using the Nernst equation.

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