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Practical 4

Aim : To investigate oxidation and reduction in the transfer of electron at adistance.

Materials : 2.0 mol dm-3 sulphuric acid, 0.5 mol dm-3 freshly prepared iron(II)

sulphate solution, 0.2 mol dm-3 acidified potassium manganate(VII)solution, 0.5 mol dm-3 potassium iodide solution, 0.2 mol dm-3 acidifiedpotassium dichromate(VI) solution, 0.2 mol dm-3 potassium thiocyanatesolution, 1 % starch solution.

Apparatus : U-tube, galvanometer, connecting wires with crocodile clips, carbonelectrodes, retort stand and clamp, test tube, dropper, stoppers with onehole.

Procedure:

1. Clamp a U-tube to a retort stand.

2. Pour dilute sulphuric acid into the U-tube until its level are 6 cm away from themouths of the U-tube.

3. Using a dropper, carefully add 0.5 mol dm-3 iron(II) sulphate solution to one of thearms of the U-tube until the layer of iron(II) sulphate solution reaches the heightof 3 cm.

4. In a similar manner as in step 3, add 0.2 mol dm-3 acidified potassium dichromate(VI) solution to other arm of the U-tube.

5. Place a carbon electrode in each arm of the U-tube.6. Connect the electrodes to a galvanometer as shown in Figure 4. Based on the

deflection of the galvanometer, determine the electrodes that act as the positiveterminal and negative terminal.

7. Left the set-up aside for 30 minutes, observe if there is any change.

8. Using a clean dropper, remove 1 cm3 of iron(II) sulphate solution from the U-tubeand place it in a test tube. Add a few drops of 0.2 mol dm -3 potassium thiocyanatesolution into the test tube. Observe the changes.

9. Repeat steps 1 to 7 using 0.5 mol dm-3 potassium iodide solution and 0.2 mol dm-3 acidified potassium manganate(VII) solution to replace the iron(II) sulphatesolution and acidified potassium dichromate(VI) solution respectively.

10. Repeat step 8 to test the potassium iodide solution with 1 % starch solution.

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Questions:

1. (i) Write the redox equations for both the redox reactions carried out in thisexperiment. (4m)(ii) In each of the reaction, state the oxidizing agent and redusing agent. (4m)

2. State the direction of flow electrons in each of the redox reaction carried out inthis experiment. (2m)

3. State the function of the sulphuric acid used. (1m)

4. State another two pairs of solution that can be used for the experiment above.(2m)

5. Explain why potassium bromide solution cannot be used to replace sulphuricacid?

(2m)

Figure 4: Oxidation and reduction in the transfer of electron at a distance.

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