EFFECT OF ELECTRICITY ON SUBSTANCES
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.
Summary of the effects of 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.
dealing with this unit the teacher should have knowledge on the following:
(i) Definitions of strong and weak electrolytes, conductors, non- conductors
(ii) Examples of strong and weak electrolytes, non-electrolytes, conductors and
The conditions required for electrolysis to take place such as:
1- Diagram showing the requirements for electrolysis
(iv) The factors which affect discharge of an ion at an electrode.
of the ion in the activity series: Illustrate this factor using
copper(II) sulphate with carbon electrodes or dilute sulphuric acid using
of electrolytes: Illustrate this factor using dilute and concentrated
hydrochloric acid, dilute and concentrated sodium chloride solutions.
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.
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.
2- Electrolysis of copper(II) sulphate using graphite
Sources of ions
Cu2+(aq) + SO42-(aq)
H+(aq) + OH-(aq)
At the anode hydroxide, OH- and sulphate, SO42–
ions are present.
Since the hydroxide ions, OH- is lower in the activity series than SO42–
is preferentially selected for discharge. The equation for the reaction is:
2H2O (l) + O2(g) + 4e
OH– ions give up their excess electrons to the anode to form
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
+ 2e Cu(s)
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
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