Electrolysis: Electrical Conduction through Liquid, and Gases. SS3 Physics Lesson Note

CONTENT

• Definition of simple terms
• Faraday’s laws of electrolysis
• Simple calculations
• Applications of electrolysis

DEFINITION OF SIMPLE TERMS

Electrolysis – This is the process whereby a liquid conducts electricity by the movement of positive and negative ions within the liquid while undergoing chemical changes.

Electrolytes – These are liquid, which allows electricity through them is called electrolytes. Such electricity is salt solutions, alkalis and dilute acids (acidulated water).

Non-Electrolytes – are liquids, which do not allow electricity to pass through them. Such liquids include distilled water, alcohol, liquid paraffin and sugar solution.

NOTE: Metals and hydrogen are deposited at the cathode, while non-metals and oxygen are deposited on the anode. The anode may dissolve in solution. 

Electrolysis does not manufacture electric charges and it is the “splitting’ of compounds by electricity. E.g. water decomposes into oxygen and hydrogen by electric current.

FARADAY’S LAWS OF ELECTROLYSIS

Faraday’s first law states that the mass of a substance liberated during the process of electrolysis is proportional to the quantity of electricity passed through the electrolyte.

Faraday’s second law of electrolysis states that the relative masses of substances liberated by the same quantity of electricity are proportional to their chemical equivalents.      

SIMPLE CALCULATIONS

 If M is the mass of substance deposited when a current q flows for time t, then the quantity of electricity which flows is It, and

m = Z It.

Where, Z = electrochemical equivalent of the substance

Z =    m

It

I = current in A

t = time in seconds

m = mass of substance in grams

APPLICATIONS OF ELECTROLYSIS

In industry, electrolysis is used in the electroplating of metals, purification of metals and electrolytic production of metals from compounds.

1. Electroplating
2. The purification of metals
3. The electrolytic preparation of metals from compounds 

CLASSWORK

1. What is electrolysis? Mention at least two uses of electrolysis
2. What is an electrolyte?
3. State Faraday’s laws of electrolysis
4. A current of 3A maintained for 50 minutes deposits 3.048g of zinc at the cathode. Determine the electrochemical equivalent of zinc.

ASSIGNMENT

SECTION A

1. In an electrolysis experiment, a cathode of mass 5g is found to weigh 5.01g after a current of 5A flows for 50 seconds. What is the electrochemical equivalent of the deposited substance (a) 0.00004gC-1 (b) 0.00002 gC-1 (c) 0.02500 gC-1 (d) 0.00001 gC-1
2. The electrochemical equivalent of a metal is 0.126×10-6kgC-1. The mass of the metal that a current of 5A will deposit from a suitable bath in 1 hour is (a) 2.268×10-3kg (b) 0.378 x10-3kg (c) 0.0378 x10-3kg (d) 0.227 x10-3kg
3. The electrochemical equivalent of platinum is 5×10-7kgC-1. To plate out 1.0kg of platinum, a current of 100A must be passed through an appropriate vessel for (a) 5.6hours (b) 56 hours (c) 1.4×104hours (d) 2.0×104hours
4. Which of the following statements about the defects of simple cells is not correct? (a) Polarisation defect is minimised by the use of manganese oxide as depolarizer (b) Polarisation may also be reduced by brushing the plates occasionally  (c) Local action occurs because zinc is not pure (d) Local action also occurs. After all, hydrogen bubbles accumulate at the plate.
5. Which of the following instruments is most accurate for comparing e.m.f of two cells? (a) Wheatstone bridge (b) galvanometer   (c) potentiometer (d) metre bridge

SECTION B

1. What is meant by the statement “the electrochemical equivalent of copper is 0.000 33 g /coulomb”?
2. In electrolysis of copper tetraoxosulphate (vi) using copper electrodes, 1.53g of copper wire is deposited in 30 minutes. Determine the average current used (z=3.29 x 10-4)
3. (a) Name two industrial applications of electrolysis. (b) Calculate the time to deposit 1.56g of nickel using a current of 2.34A in a nickel-plating process. (Assume that 1/20g of nickel is deposited per ampere-hour)