Chemistry Electrolysis

EFFECT OF ELECTRICITY ON SUBSTANCES

(a) You should be able to perform the following electrolytic processes

Electrolysis of Dilute sulphuric acid (electrolysis of acidified water)

Apparatus / chemicals – Dilute sulphuric acid, dry cells, platinum or carbon electrodes, boiling tubes, corks, wooden splint and plastic mineral water bottle.

Procedure

1. Prepare a dilute solution of sulphuric acid and fill a coulometer with it.
2. Arrange the electrodes as shown in the diagram.
3. Switch on the current and allow electrolysis to occur until reasonable volumes of gas are collected in the tubes.
4. Compare the volumes of the gas formed in the tubes.
5. Test the gases using a glowing and burning splint.

Figure 5- Electrolysis of acidified water

Electrolysis of sodium chloride solution

Apparatus /Chemicals – same as in (a) above except use sodium chloride solution instead of dilute sulphuric acid.

Procedure: same as in (a) above

Electrolysis of molten or fused lead(II) bromide

Apparatus and chemicals: Crucible, lead(II) bromide crystals, heat source, carbon electrodes.

Caution: Bromine vapour is poisonous. Avoid inhaling to much of it. Keep the windows open to ensure proper ventilation.

Procedure

1. Heat lead (II) bromide in a fume cupboard in hard glass test tube or crucible until it melts.
2. Electrolyze the molten solution using carbon electrodes

(iii)     Note what is observed at the electrodes.

(a) The teacher should be able to perform the following electrolytic processes

Electrolysis of Dilute sulphuric acid (electrolysis of acidified water)

Apparatus / chemicals – Dilute sulphuric acid, dry cells, platinum or carbon electrodes, boiling tubes, corks, wooden splint and plastic mineral water bottle.

Procedure

1. Prepare a dilute solution of sulphuric acid and fill a coulometer with it.
2. Arrange the electrodes as shown in the diagram.
3. Switch on the current and allow electrolysis to occur until reasonable volumes of gas are collected in the tubes.
4. Compare the volumes of the gas formed in the tubes.
5. Test the gases using a glowing and burning splint.

Electrolysis of sodium chloride solution

Apparatus /Chemicals – same as in (a) above except use sodium chloride solution instead of dilute sulphuric acid.

Procedure: same as in (a) above

Electrolysis of molten or fused lead(II) bromide

Apparatus and chemicals: Crucible, lead(II) bromide crystals, heat source, carbon electrodes.

Caution: Bromine vapour is poisonous. Avoid inhaling to much of it. Keep the windows open to ensure proper ventilation.

Procedure

1. Heat lead(II) bromide in a fume cupboard in hard glass test tube or crucible until it melts.
2. Electrolyze the molten solution using carbon electrodes

(iii)     Note what is observed at the electrodes

Figure 6- Electrolysis of molten lead(II)bromine

Electrolysis of copper (II) sulphate solution using copper electrodes

Apparatus and chemicals: Copper foils as electrodes, copper(II) sulphate solution, Dry cells or car battery.

Procedure

(i) Prepare a solution of copper(II) sulphate solution

(ii) Weigh the electrodes before electrolysis is carried out.

(iii) Complete the circuit and allow electrolysis for some time (about 10 minutes).

(iv) Weigh the electrodes again and then compare the masses. Comment on the masses you obtained.

Electrolysis of copper (II) sulphate solution using copper electrodes

Apparatus and chemicals: Copper foils as electrodes, copper(II) sulphate solution, Dry cells or car battery.

Procedure

(i) Prepare a solution of copper(II) sulphate solution

(ii) Weigh the electrodes before electrolysis is carried out.

(iii) Complete the circuit and allow electrolysis for some time (about 10 minutes).

(iv) Weigh the electrodes again and then compare the masses. Comment on the masses you obtained.

EFFECT OF ELECTRICITY ON SUBSTANCES

PAPER1: MULTIPLE CHOICE QUESTIONS

 

1. Which of the following solutions is not an electrolyte?

Potassium chloride solution Copper(II) sulphate crystals Molten zinc bromide Dilute hydrochloric acid

 

2. Which of the following ions is not present in a dilute solution of sulphuric acid?

H⁺ SO₄^2- OH^- Na⁺

3. Which of the following is not true of the electrolysis of copper(II) sulphate solution using carbon electrodes?

The mass of the anode decreases  Copper is deposited at the cathode Oxygen gas is formed at the anode The mass of the anode increases

4. Which of the following pairs of electrolytes give a similar product at the anode using platinum as the electrodes?

Dilute sulphuric acid and copper(II) sulphate solution Sodium hydroxide solution and sodium chloride solution Dilute sulphuric acid and concentrated hydrochloric acid Copper(II) sulphate solution and potassium chloride solution.

 

5. Which of these is not a factor that affects discharge of an ion during electrolysis?

Nature of electrode Position of the element in the reactivity series. Concentration of the electrolyte The nature of the crystals of the compound.

STRUCTURED QUESTIONS

6. (a)  Explain why solid sodium chloride is a very poor electrical conductor while if melted, it conducts             electricity readily.

   (b) (i) State the products of electrolysis of molten sodium chloride and give equations for reactions that             occur at the electrodes.

        (ii) Name the different products obtained when concentrated sodium chloride is electrolyzed.

            Explain your answers. Include equations in your answer.

7.A current of 8 amperes were passed for 80 minutes through a solution of copper(II) sulphate using platinum electrodes.

(a)  State what was observed at

      (i)  the cathode                             (ii) the anode

(b)   Write equation for the reaction taking place at the

       (i)  the cathode                             (ii)  the anode

(c)       Calculate

            (i)        the mass of solid deposited at cathode.

            (ii)       the volume of the product formed at the anode at STP.

Glossary

Definitions

(a) Conductors are solid substances except mercury that allow electric current to pass through them. For example metals like copper and aluminium, and non-metal like graphite (carbon).

(b) Non-conductors are solid or liquid substances that do not allow an electric current to pass through them. For example diamond (carbon), rubber/plastics, benzene, dry wood etc.

(c) Electrolysis – Is the decomposition of an electrolyte by passage of electric current through it.

(d) Electrolyte – Is a compound which when in solution or in molten state conducts an electric current and is decomposed by it.

Strong electrolyte is one that is completely ionized when in solution or molten form. Examples include sodium chloride solution, dilute hydrochloric acid and sodium hydroxide solution.

Weak electrolyte is one that is partially ionized when in solution or molten form. Examples include ammonia solution, ethanoic acid solution and water.

(e) Non- electrolyte – Is a compound which when in solution or molten state does not conduct electricity.

Non-electrolytes include sugar solution, alcohol and urea.

(f) Electrodes – Are two pieces of metal or graphite rods by which electrons enter and or leave an electrolyte.

(g) Amode is the positively charged electrode by which electrons leave the electrolyte.

(h) Cathode is the negatively charged electrode by which the electrons enter the electrolyte.

(i) Anion is a negatively charged ion that moves to the anode during electrolysis.

(j) Cation is a positively charged ion that moves to the cathode during electrolysis.

(k) Electroplating is the coating of one metal with another by the process of electrolysis.

(l) Faraday’s laws of electrolysis – The two laws are

     (i) The mass of a substance liberated during electrolysis is directly proportional to the quantity of electricity passed.

     (ii) When the same quantity of electricity is passed through different electrolytes, the amount of the substances liberated in moles is inversely proportional to the charges on their ions.