Electrolysis

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Question 18

An industrial chemist investigates the electrorefining of nickel using nickel electrodes immersed in an aqueous nickel(II) sulfate solution.

The chemist uses the following experimental method:

Weigh two clean, dry strips of nickel.
Use one strip as the anode (positive electrode) and the other as the cathode (negative electrode).
After electrolysis, wash both nickel strips with ethanol (a volatile solvent that boils at $78∘C78^\circ\text{C}$ ).
Dry the strips thoroughly and record their final masses.

The ionic half-equations for the reactions occurring at the electrodes are:

Positive electrode: $Ni(s)−2e−→Ni2+(aq)\text{Ni}(s) - 2\text{e}^- \rightarrow \text{Ni}^{2+}(aq)$
Negative electrode: $Ni2+(aq)+2e−→Ni(s)\text{Ni}^{2+}(aq) + 2\text{e}^- \rightarrow \text{Ni}(s)$

Suggest why the nickel strips would dry more quickly when rinsed with ethanol rather than with water.

[1]

The chemist's initial results are recorded in the table below:

Electrode	Mass of electrode before electrolysis in $g\text{g}$	Mass of electrode after electrolysis in $g\text{g}$
Positive electrode	$15.80$	$14.95$
Negative electrode	$12.10$	$12.82$

Calculate the increase in mass, in grams, of the negative electrode.

[1]

Suggest two practical reasons why the increase in mass of the negative electrode was less than the decrease in mass of the positive electrode.

[2]

The chemist then investigates the effect of varying the quantity of electrical charge, measured in faradays, passed through the cell. One faraday represents the charge of one mole of electrons. The results of these trials are displayed below:

| Experiment | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | | :--- | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | | Electrical charge in faradays | 0.02 | 0.04 | 0.06 | 0.08 | 0.10 | 0.12 | 0.14 | 0.16 | 0.18 | | Increase in mass in $g\text{g}$ | 0.587 | 1.174 | 1.761 | 2.348 | 2.935 | 3.522 | 5.120 | 4.696 | 5.283 |

Identify which experiment gave an anomalous result.

[1]

Suggest why a line of best fit plotted for this dataset must pass through the origin $(0, 0)$ .

[1]

Explain how the data shows that the increase in mass is directly proportional to the electrical charge passed.

[2]

Calculate the expected increase in mass, in grams, of the negative nickel electrode if an electrical charge of $0.22$ faradays was passed.

[2]

An industrial chemist investigates the electrorefining of nickel using nickel electrodes immersed in an aqueous nickel(II) sulfate solution.

The chemist uses the following experimental method:

Weigh two clean, dry strips of nickel.
Use one strip as the anode (positive electrode) and the other as the cathode (negative electrode).
After electrolysis, wash both nickel strips with ethanol (a volatile solvent that boils at $78∘C78^\circ\text{C}$ ).
Dry the strips thoroughly and record their final masses.

The ionic half-equations for the reactions occurring at the electrodes are:

Positive electrode: $Ni(s)−2e−→Ni2+(aq)\text{Ni}(s) - 2\text{e}^- \rightarrow \text{Ni}^{2+}(aq)$
Negative electrode: $Ni2+(aq)+2e−→Ni(s)\text{Ni}^{2+}(aq) + 2\text{e}^- \rightarrow \text{Ni}(s)$

Electrode

Mass of electrode before electrolysis in

g\text{g}

Mass of electrode after electrolysis in

g\text{g}

Positive electrode

15.80

14.95

Negative electrode

12.10

12.82

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