CHEMISTRY

ELECTROLYSIS

ACTIVITIES

Introduction
Students' Activities
Exercise
Glossary

THE EFFECT OF ELECTRICITY ON SUBSTANCES

INTRODUCTION

In chemistry, you might have learnt that some reactions proceed easily (spontaneously) either naturally or on application of heat. Other reactions can only occur (need to be forced to proceed) by supplying energy with an externally applied electric current, for example, a battery. This process is called electrolysis.

Electrolysis is important in that it is used in the extraction of reactive metals, such as potassium, sodium, calcium, magnesium and aluminium; electroplating metals to improve their appearance and reduce corrosion/prevent rusting and in refining metals such as copper. For example, copper is mined in Kilembe and is transported to Jinja for purifying electrolytically. Electroplating can be done at home once you have the necessary substances to use and intend to start a small business, for example, coating iron or aluminium bangles with copper.

See Table 1: Summary of the effects of electrolysis

ELECTROLYSIS

This unit deals with

(i) Characterisation of substances as electrolytes, non – electrolytes, conductors and non-conductors.

(ii) Selective discharge of an ion at an electrode.

(iii) Reactions at the electrodes during electrolysis.

(iv) Laws of electrolysis.

(v) Electrochemical cells.

In dealing with this unit the teacher should have knowledge on the following:

(i) Definitions of strong and weak electrolytes, conductors, non-  conductors and electrolysis.

(ii) Examples of strong and weak electrolytes, non-electrolytes, conductors and non-conductors.

 (iii) The conditions required for electrolysis to take place such as:             

  • Source of current (direct current)

  • Electrodes (Points at which electricity enter and leave the solution)

  • Electrolyte (in solution or molten state).

Figure 1- Diagram showing the requirements for electrolysis


(iv)   The factors which affect discharge of an ion at an electrode. These include:

       Position of the ion in the activity series: Illustrate this factor using copper(II) sulphate with carbon  electrodes or dilute sulphuric acid using platinum electrodes.

       Concentration of electrolytes: Illustrate this factor using dilute and concentrated hydrochloric acid, dilute and concentrated sodium chloride solutions.

       Caution: if you are performing the experiment take note that chlorine is poisonous. Therefore, you should avoid inhaling too much of it. In order to do so keep the windows open to ensure proper ventilation.

       Nature of electrodes: This could be illustrated by using copper(II) sulphate solution with copper electrodes.

  • Reactions at the electrodes in terms of electron transfer. Explain the reactions taking place in view of the ions discharged using dilute sulphuric acid, copper(II) sulphate solution,  sodium chloride solution, concentrated sodium chloride solution (brine) to establish the role of water. For example, electrolysis of copper(II) sulphate solution.

Figure 2- Electrolysis of copper(II) sulphate using graphite electrodes


Sources of ions 
  

CuSO4 (aq)        Cu2+(aq) + SO42-(aq)


H2O(l)       H+(aq)  + OH-(aq)

Reactions

At the anode hydroxide, OH- and sulphate, SO42– ions are present.

Since the hydroxide ions, OH- is lower in the activity series than SO42– .

It is preferentially selected for discharge. The equation for the reaction is:

4OH (aq)      2H2O (l)  +  O2(g)  +   4e

OH ions give up their excess electrons to the anode to form oxygen gas
At the cathode hydrogen, H+ and copper(II), Cu2+ ions are present but since Cu2+ is lower in the activity series than H+ it is discharged. Copper(II) ions take up electrons to form copper metal.

Cu2+(aq) +  2e   Cu(s)

Figure 3 - Movement of ions in an incomplete and complete circuit

The arrows indicate the direction of movement of ions when charged plates are placed in solution.

(vi)      State the Faraday's laws of electrolysis and solve problems involving mass or the volume of product.

(vii)    State applications of electrolysis, for example,
            -           In extraction of metals – Sodium and aluminium. The first five metals in the activity series are extracted by electrolysis.

            -           In electroplating.

            -           Purification of metals for example copper.

            -           Manufacture of sodium hydroxide and chlorine.

(viii)   Construct and explain the working of a simple electrochemical cell.

For example, the Daniel cell consisting of Zinc rod dipping in zinc(II) sulphate solution and copper rod dipping in copper(II) sulphate solution.


Figure  4- A simple Daniel Cell

Teachers' Notes
Scheme of work
Lesson Plan
Learners' activities