The Faraday constant is the charge in coulombs, C, carried by 1 mole of electrons. (a) A student plans an electrolysis experiment to determine the Faraday constant. The student was supplied with the following. $1.0\text{ mol dm}^{-3}$ copper(II) sulfate clean, dry copper foil electrodes, labelled ‘anode’ and ‘cathode’ balance stop-clock ammeter other equipment suitable for carrying out electrolysis Draw a labelled diagram of the apparatus and chemicals the student should use…

Chemistry 9701/53 — May/June 2018

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Q1PlanningAnalysis, Conclusions and EvaluationFree sample

The Faraday constant is the charge in coulombs, C, carried by 1 mole of electrons.

(a)

A student plans an electrolysis experiment to determine the Faraday constant.

The student was supplied with the following.

$1.0\text{ mol dm}^{-3}$ copper(II) sulfate
clean, dry copper foil electrodes, labelled ‘anode’ and ‘cathode’
balance
stop-clock
ammeter
other equipment suitable for carrying out electrolysis

Draw a labelled diagram of the apparatus and chemicals the student should use in their electrolysis experiment. Include in your diagram the circuit connecting the anode and cathode.

(b)

Two of the hazards of using copper(II) sulfate solution are given below.

For each hazard, state a precaution, other than eye protection and a lab coat, that the student should take when carrying out the experiment.

hazard: copper(II) sulfate solution causes skin irritation

precaution ..................................................................................................................................

....................................................................................................................................................

hazard: copper(II) sulfate solution is toxic to aquatic life

precaution ..................................................................................................................................

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(c)

The student carried out the electrolysis for exactly 30 minutes with a current of 0.5A.

After the electrolysis was finished, the student removed the electrodes.
The electrodes were then carefully washed in water and then dipped in propanone.
The electrodes were dried by allowing the propanone to evaporate.

State the measurements the student would need to record to calculate the mass change of an electrode. Include the appropriate unit.

(d)

Calculate the charge passed through the copper(II) sulfate solution during the electrolysis experiment using the formula shown.

\text{charge (C)} = \text{current (A)} \times \text{time (s)}

charge passed = .............................. C

(e)

The mass change of the anode was $-0.282\text{ g}$ .

Calculate the amount, in mol, of copper lost from the anode. Give your answer to 3 significant figures.
[ $A_r$ : Cu, 63.5]

moles of copper lost from the anode = .............................. mol

(f)

Use your answers to (d) and (e) to calculate the charge required to remove 1 mole of copper from the anode.

charge required to remove 1 mole of copper = .............................. C

(g)

The theoretical charge required to remove 1 mole of copper from the anode into solution as copper(II) ions is $193\,000\text{ C}$ .
The Faraday constant is $96\,500\text{ C mol}^{-1}$ .

Explain why the theoretical charge is twice the Faraday constant.

....................................................................................................................................................

(h)

A possible source of error is not drying the anode at the start of the experiment.

Explain the effect, if any, on the calculated value of the Faraday constant if the anode is wet at the beginning of the experiment but dry at the end.

effect ..........................................................................................................................................

explanation ................................................................................................................................

....................................................................................................................................................

(i)

The student wanted to ensure that the anode was completely dry at the end of the experiment and decided to evaporate off the propanone using a blue Bunsen flame. The student noticed some blackening of the surface of the copper.

Suggest what caused this blackening.

....................................................................................................................................................

(j)

The student calculated the mass change of the anode and the cathode after the experiment was complete.

\text{mass change of anode} = -0.282\text{ g} \quad \text{mass change of cathode} = +0.217\text{ g}

Suggest one reason why the mass gained at the cathode is not the same as the mass lost at the anode. Assume the student has recorded the mass changes correctly.

....................................................................................................................................................

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