The three main factors that affect the electrolysis of an aqueous solution are: 1) The position of ions in the electrochemical series, which determines which ions will be reduced or oxidized at the electrodes. 2) The concentration of ions or electrolytes in the solution. Higher concentrations result in greater rates of reaction. 3) The type of electrodes used, as certain electrodes may dissolve and alter the composition of the solution during electrolysis.

1. Rossita Radzak SASER
ELECTROLYSIS OF AQUEOUS SOLUTIONS
Factors that affect the electrolysis of an aqueous solution:
(a) Position of ions in the electrochemical series (ECS).
(b) Concentration of ions / anion in the solution / electrolytes.
(c) Type of electrodes used in the electrolysis.
a. Position of ions in the electrochemical series (ES).
Cation Anion
K+ F-
Na+ SO42-
Ca2+ NO3-
Mg2+ Cl-
Al3+ Br-
Zn2+ I-
Fe2+ OH-
Sn2+
Pb2+ Ease of
H+ discharge
Cu2+ increases
Ag+
Electrochemistry
Series
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2. Rossita Radzak SASER
Hydrogen gas Oxygen gas
Potassium nitrate Carbon electrodes
solution,
0.5 mol dm-3
Figure 2 Electrolysis of potassium nitrate solution
KNO3 solution consists of K+ , H+, NO3- and OH-
Cathode: Anode:
K+ and H+ NO3- and OH-
H+ is selected for discharged as it is OH- is selected for discharged as it is
lower than K+ in ECS. lower than NO3 - in ECS.
b. Concentration of ions in the solution / electrolytes.
2H+ + 2e  H2 4OH-  2H2O + O2 + 4e
Hydrogen gas is collected at the Oxygen gas is collected at the anode.
cathode. Hydrogen gas 2 volume. Oxygen gas 1 volume.
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3. Rossita Radzak SASER
(b) Concentration of ions in the solution / electrolytes.
Sodium chloride
solution, 1.0 mol dm-3
Carbon
electrodes
Figure 3 Electrolysis of sodium chloride solution
From sodium chloride Na+, Cl-
From water H+ , OH-
CATHODE ANODE:
Na+ , H+ Cl- , OH-
Both types of cations are attracted to the Both anions are attracted to the anode,
cathode, H+ ions are selected for discharged Chloride ions are discharged because the
as they are lower in the ECS. concentration of Cl- ions are higher than of
OH- ions.
2H+ + 2e  H2 (g) 2Cl-  Cl2 (g) + 2e
Gas bubbles are formed. When a lighted Bubbles pungent yellowish-green gas is
wooden splinter is place near the mouth of produced. The gas changed the damp blue
the test tube, a `pop` sound is produced. litmus paper to red and bleached it. Chlorine
Hydrogen gas is evolved. gas is formed.
Solution becomes alkaline by presence of sodium hydroxide.
NaOH(aq)
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4. Rossita Radzak SASER
(c) Type of electrodes used in the electrolysis.
Copper electrodes
Copper (II)
sulphate solution
0.5 mol dm-3
Figure 4 Electrolysis of copper (II) sulphate solution using copper electrodes
From copper (II) sulphate Cu2+ , SO42-
From water H+ , OH-
CATHODE: ANODE:
Cu2+ ions and H+ ions move to the OH- ions and SO42- ions move to the anode.
cathode. Cu2+ ion which is at a lower Copper electrode dissolves /copper atom
position than H+ ion in the ECS is ionizes and releases 2 electrons to form
selected to be discharged. Cu2+ ions.
Cu2+ (aq) + 2e  Cu (s) Cu  Cu2+ + 2e
Brown deposit/ solid is formed. The mass The mass of the anode decreases and the
of the cathode increases and the cathode anode becomes thinner.
becomes thicker.
The colour intensity of blue solution dose not change because the concentration of Cu2+ ions
in the solution remains unchanged. This is because the rate of formation of copper (II) ions,
at the anode is the same as the rate of discharge of copper (II) ions, at the cathode.
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