8.2

Question 1

8.2 Electrochemical cells 2 Aims

• To set up an electrochemical cell and measure voltages.

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

KNO3(aq)

Answer 2

Oxidising

Question 3

Equipment

Answer 3

• safety spectacles
• voltmeter – range 0 – 1 V
• 2 x wire and crocodile clip
• copper strip (clean)
• carbon rod
• beaker (100cm3)
• filter paper
• scissors
• 2 x measuring cylinder (50cm3)
• Emery paper

Question 4

Procedure

Answer 4

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

Then record your measurements of voltage in a suitable format.1 Clean the metal strips by rubbing them with emery paper, rinsing in distilled water, and drying them.
2 Pour 50cm3 aqueous copper(II) sulfate into a beaker and place the copper metal strip into it. This forms the copper half-cell. You can bend the foil over the edge of the beaker and hold it in place with the crocodile clip.
3 Pour 50cm3 of the aqueous zinc(II) sulfate into a second beaker and place the zinc metal strip into the other beaker. This is the zinc half-cell.
4 Soak a strip of filter paper in saturated potassium nitrate(V) solution, and use it as a salt bridge to connect the solutions in the two beakers.
5 Complete the circuit by connecting the copper and zinc strips to the voltmeter as shown below.
6 Record the cell potential of the zinc–copper cell, and note carefully which half-cell is positive and which negative.
7 You also have access to iron, aqueous iron(II) sulfate, magnesium and aqueous magnesium sulfate. Prepare further cells using all combinations of the metal–metal ion half cells possible. Record the cell potential of each cell and note which cell is positive and which is negative.
8 The standard electrode potential for Cu2+(aq) + 2e–  Cu(s) is +0.34 V.
Use this information and your measured cell potentials to determine the standard electrode potential the other redox systems used in the half-cells. 
Add this information to your results.
9 Do not throw away the metal–metal ion solutions, but return them to the containers provided. Are you sure about this?

Question 5

1. What assumption has been made when calculating the standard electrode potentials of the redox systems?

Answer 5

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Question 6

2. Write overall cell reactions for each cell prepared.

Answer 6

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Question 7

Do not throw away the metal–metal ion solutions, but return them to the containers provided. Are you sure about this?

Answer 7

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