ELECTROLYSIS
Electrolysis means ‘to split using electricity.’
The process involves chemical changes which occur when an electric current is passed through
an electrolyte. This chemical changes involves redox reactions. An electrolysis reaction occurs
in a piece of equipment called an electrolytic cell.
An electrolytic cell contains;


A DC (direct current) power supply.
An electrolyte – a liquid containing ions which are free to move. An electrolyte can be
either a molten ionic compound or a solution containing dissolved ions.
Two electrodes- carry current to and from the electrolyte.
The negative electrode is the cathode – This is where reduction reaction occurs.
The positive electrode is the anode – This is where oxidation reaction occurs.
The electrodes are usually made of inert materials such as platinum, Pt, or graphite, C, which
are not reduced or oxidized in an electrolysis reactions.
When a current is supply to the cell:
-
Cations (positive ions) move or migrate towards the cathode (which is the negative
electrode) to get reduced.
Anions (negative ions) move towards the anode (the positive electrode) to get oxidise.
A DIAGRAM OF A SIMPLE ELECTROLYTIC CELL.
ELECTROLYSIS OF MOLTEN OR MELTED IONIC COMPOUNDS.
When ionic compounds are melted, the ions (cations and anions) are free to move around
because they are in liquid form.
During the electrolysis:
The cations (metal ions) migrate towards the cathode where reduction occurs and reduced to
metal. The anions usually migrates to the anode, which the positive electrode (oxidation
occurs) and oxidized to gases.
Example: Electrolysis of melted or molten sodium chloride.
Anode reaction:
-
Chloride ions (Cl-) migrates towards the positive electrode (anode) where oxidation
occurs and oxidizes to chlorine (Cl2) gas.
Bubbles of chlorine gases will be observed at the end of the positive electrode that is
dipped into the solution (see diagram above).
Cathode reaction:
-
Sodium ions (Na+) migrates towards the cathode (the negative electrodes) where
reduction occurs and reduces to sodium metal.
A deposit of sodium metal will be observed at end of the negative electrode that is
dipped into the liquid.
Balanced Overall Equation:
ELECTROLYSIS OF DILUTE AQUEOUS IONIC COMPOUNDS.
An aqueous solution contains water and ions which are free to move. When an aqueous
solution is electrolyzed, some of the ions are oxidized and others reduced. The reactions
depend on the ease with which different ions can be reduced or oxidized compared to water
(H2O).
For metal ions, this relates to the position of their corresponding metal on the activity series;
ions of metal ‘lower down’ the activity series are more easily reduce than ions further ‘up’.
Example: Electrolysis of dilute aqueous sodium chloride.
During electrolysis:
Anode Reactions:
-
-
Chlorides ions and hydroxide ions (from water molecule) migrates towards the positive
electrode to get oxidise, however, water molecule oxidises to oxygen gas in preference
to chloride ions at the anode. This is because the solution is dilute thus, few ions of
chloride present than water molecules.
Bubbles of oxygen gas are seen or observed near the positive electrode.
Cathode Reactions:
-
Sodium ions and hydrogen ions (from water molecule) migrates towards the negative
electrode. Water (H2O) reduces to hydrogen gas at the cathode in preference to sodium
ions. The reason why water oxidises instead of sodium ion is because, sodium ions are
very hard to reduce or sodium metal is very reactive hence, reacts immediately with
hydroxide ions remaining in solution near the cathode electrode to produce sodium
hydroxide).
-
Bubbles of hydrogen gases are observed at the negative electrode.
Balanced overall equations:
ELECTROLYSIS OF AQUEOUS CONCENTRATED IONIC COMPOUND.
Example: Electrolysis of Brine (Concentrated sodium chloride)
Cathode Reaction:
-
Na+ and H+ (from water) are attracted to the negative electrode where reduction occurs.
Water (H2O) molecule reduces to hydrogen (H2) gas in preferences to sodium ion. The
reason why water oxidises instead of sodium ion is because, sodium ions are very hard
to reduce or sodium metal is very reactive hence, reacts immediately with hydroxide
ions remaining in solution near the cathode electrode to produce sodium hydroxide).
-
Bubbles of hydrogen gases are observed at the negative electrode.
Anode Reaction:
-
-
Chlorides ions and hydroxide ions (from water molecule) are attracted towards the
positive electrode where oxidation occurs.
Chloride ions oxidises to chlorine gas in preference to water molecules at the anode.
This is because the solution is concentrated thus more ions of chloride are present than
water molecules.
Bubbles of chlorine gas are seen or observed near the positive electrode.
Balanced Overall Equation:
EXERCISES
1.
a) Draw a simple electrolytic cell of a melted sodium chloride and label the following:
Anode, cathode, electrolyte, anions, cations, direction of flow of electrons and current.
(skill level 2)
b) Describe observation at the electrodes during electrolysis of melted sodium
chloride. (skill level 2)
c) Explain the observations at the electrodes during electrolysis of melted sodium
chloride. (skill level 3)
d) Write balanced half-equations for the anode and cathode reactions and balance the
overall equation. (skill level 3)
2.
a) Draw a simple electrolytic cell of a dilute aqueous sodium chloride and label: anode,
cathode, electrolyte, anions, cations, direction of flow of electrons and current. (skill
level 2)
b) Describe observation at the electrodes during electrolysis of dilute aqueous sodium
chloride. (skill level 2)
c) Explain the observations at the electrodes during electrolysis of dilute aqueous
sodium chloride. (skill level 3)
d) Write balanced half-equations for the anode and cathode reactions and balance the
overall equation. (skill level 3)
3.
a) Draw a simple electrolytic cell of a aqueous concentrated sodium chloride and label
the following: Anode, cathode, electrolyte, anions, cations, direction of flow of
electrons and current. (skill level 2)
b) Describe observation at the electrodes during electrolysis of aqueous concentrated
sodium chloride. (skill level 2)
c) Explain the observations at the electrodes during electrolysis of concentrated
aqueous sodium chloride. (skill level 3)
d) Write balanced half-equations for the anode and cathode reactions and balance the
overall equation. (skill level 3)