OUR LADY OF THE ROSARY COLLEGE CHEMISTRY PRACTICAL MANUAL

Advanced Level, F.6

Experiment should be carried out under the supervision of a Chemistry teacher

Safety precaution must be noted.

Division of work: two students per group

A Quantitative Study of Electrolysis

Introduction

The relationship between the mass of a metal deposited during electroplating and the quantity of electricity passed can be studied by measuring the changes in mass of the copper electrodes during the electrolysis of copper(II) sulphate solution for a certain time using a constant current.

Pre-laboratory work

  1. How can you obtain the value of electric charge from the value of electric current?
  2. State and explain Faraday's first and second law of electrolysis.
  3. In the course of the electrolysis, you are advised to keep CONSTANT electric current. Explain.
  4. In the course of the electrolysis, you are advised that do NOT change the following:

(a)the position of the electrodes and (b) the rheostat

Explain.

  1. Why is it necessary to clean the copper foil? What is the purpose of using propanone?

Materials required

0.1 M silver nitrate solution, silver wire, electrode holder, 1.0 M copper(II) sulphate (VI) solution, washing acetone (propanone), copper foil, balance, readable 0.001 g, sand paper, dilute hydrochloric acid, 4 connecting wires, 2 of them equipped with clean crocodile clips, rheostat, 1 A- 5 A ammeter, stopwatch. Hair-dryer, DC power supply(0-24 V)

Technician's note

  1. Please check the conductivity of the electric wire, especially the crocodile clips.

Use of the electrode holder is essential in this experiment. Without the holder, the results may be very poor.

Procedure A (and B) To verify Faraday's second law of electrolysis

  1. Clean carefully the copper foil cathode and anode by rubbing with steel wool (or sand paper).
  1. Place them under bench hydrochloric acid for about 3 minutes. Wash them under tap water thoroughly.
  1. Wash the copper foil by using washing acetone (propanone). Hold the clean copper foils by edge. Grease from your bare hands may make the effect of electrolysis poor. Wave the copper foils in the air for 1 - 2 minutes to vaporize most of the propanone.
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  1. Place the copper foils in a clean watch glass and weigh the copper cathode and anode separately.
  2. Fill a 200 cm3 beaker with 1.0 M copper(II) sulphate solution to one-half and insert the electrodes. Use an electrode holder to hold them in place.
  3. Connect the electrodes for electrolysis as in the above diagram. Start taking time when the circuit is closed. Adjust the rheostat to give a current of 0.20 ampere.
  4. Allow the current to pass through the solution for 20 minutes, keeping the current at 0.20 ampere by adjusting the rheostat when necessary.
  5. At the end of 20 minutes disconnect the circuit and rinse the copper electrodes with water. The anode will need a strong jet of water from the tap, followed by a firm wipe with clean cloth to remove the film on surface.
  6. Rinse each electrode twice with washing acetone (propanone), wave in the air for 1 - 2 minutes.
  7. Re-weigh each electrode . Give your data in Table B.

Procedure B

Repeat procedure A, starting with step 2 for silver and silver nitrate solution. Give your data in Table C

Procedure C To verify Faraday's first law of electrolysis

Repeat procedure B but change the electric current. Weigh one of the electrode before and after electrolysis for 20 minutes only.

Data and Results

Table A

Current (A) / Duration (min)
Electrolysis of copper
Electrolysis of silver

Table B

CATHODE (1) / ANODE (2)
Before
electrolysis / After
electrolysis / Before electrolysis / After
electrolysis
Mass of copper + watch glass (g)
Mass of watch glass (g)
Mass of copper foil (cathode)/g
Mass of copper deposited/reacted (g)

Table C

CATHODE (3) / ANODE (4)
Before
electrolysis / After
electrolysis / Before electrolysis / After
electrolysis
Mass of silver + watch glass (g)
Mass of watch glass (g)
Mass of silver foil (cathode)/g
Mass of silver deposited/reacted (g)

Table D

Current (A) / 0.10 / 0.20 / 0.30 / 0.40
Duration (min)
Mass of silver reacted (g)

Calculation and questions

  1. Are the masses in (1) and (2) equal? Are the masses in (3) and (4) equal? Explain your answers.
  2. Calculate number of moles of metal formed / dissolved per 1 Faraday (i.e. 96500 coulumbs of electricity.
  3. Verify Farady's first and second law of electrolysis by using your results.
  4. Discuss sources of error and suggest method to minimize the errors.