8.3

Question 1

8.2 Electrochemical cells 3 (microscale)• To set up an electrochemical cell and measure voltages using microscale apparatus.

Answer 1

A potential difference is set up between two half cells which is at a maximum when no current is flowing. This is called the cell potential. The e.m.f. of the cell can be measured using a high-resistance voltmeter. As no current is drawn by the voltmeter, each electrode is in equilibrium.

Question 2

Equipment

Answer 2

• safety spectacles
• Petri dish
• strip of filter paper
• dropping pipettes
• multimeter with probes

Question 3

Procedure

Answer 3

Read the information below and decide how you will show your results.

Then record your measurements of voltage in a suitable format.

1. Place a plastic Petri dish on a flat surface. Place a strip of filter paper in the dish.
2. Add 2–4 drops of copper(II) sulfate (CuSO4) solution to one end of the strip of filter paper. Place a small piece of copper foil on top of the copper(II) sulfate solution.
3. Add 2–4 drops of zinc sulfate (ZnSO4) solution to the opposite end of the strip of filer paper. Place a small piece of zinc foil, or a zinc granule, on top of the zinc sulfate solution.
4. Carefully, add drops of potassium nitrate solution to the centre of the filter paper. Allow the solution to spread out so that it just touches the other two solution areas.
5. Set the multi-meter to a convenient scale, e.g. 2000 mV.
6. Place one probe on the top (i.e. dry) surface of each of the metals and take a reading.
7. This method can be repeated with other half-cells. If you lay another strip of filter paper at right angles across the original strip, you can easily try out different combinations.

Question 4

1. Why does the potassium nitrate solution have to spread out to touch the other solutions at each end of the filter paper?

Answer 4

1. The potassium nitrate solution acts as the salt bridge. In order to do so, it must be in contact with each half-cell solution.

Question 5

2. Write overall reactions for each cell prepared.

Answer 5

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Question 6

3. Outline a plan to use this method to determine how the cell potential depends on the ion concentration in one of the half-cells. How would you display the results from such an investigation?

Answer 6

3. Learners may develop a plan to create a range of dilutions of, for example, CuSO4(aq) and measure the cell potential against a Zn2+|Zn half-cell of constant concentration. In practice, subsequent dilutions by a factor of 10 may be required to show significant changes in potential, but it can be illustrative to allow different groups to attempt different approaches and compare results.
   Learners can display the results from this investigation in a graph. If using factor 10 dilutions, the best way to do this is to plot the voltage against the log of the metal ion concentration; the resulting plot should be linear.