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Date November 2020 Marks available 1 Reference code 20N.3.hl.TZ0.13
Level HL Paper 3 Time zone TZ0
Command term Write Question number 13 Adapted from N/A

Question

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

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

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

[1]
a.

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

[3]
b.

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

[1]
c.

Markscheme

Mg(s)+Ni2+(aq)Mg2+(aq)+Ni(s) ✔

Accept a balanced molecular equation such as “Mg+NiSO4MgSO4+Ni”.

a.

EƟ=«2.370.26=»(+)2.11«V» ✔

«Q=0.01000.800=»0.0125 AND «n=»2 ✔

«E=EƟRTnFlnQ=2.118.31×2982×96500 ln0.0125=»(+)2.17«V» ✔


Award [3] for correct final answer.

b.

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/nFlnQ becomes more negative ✔


Accept any correct mathematical explanation using the Nernst equation.

c.

Examiners report

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

a.

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.

b.

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.

c.

Syllabus sections

Additional higher level (AHL) » Topic 19: Redox processes » 19.1 Electrochemical cells
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Additional higher level (AHL) » Topic 19: Redox processes
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