Electrolysis
IONIC THEORY
The theory states that electrolytes are made up of ions, which are built up in certain patterns
called crystal lattice. When these substances dissolve in water or melt, the structure is
destroyed and the ions are set free to move.
Noted: Concentrated mineral acids such as tetraoxosulphate (IV) acid, hydrochloric acid and
trioxonitrate (V) acid do not contain ions but they consist of molecules. However, when they
are diluted, the molecular structure is destroyed and ions are formed.
When an electric current is passed through an electrolyte solution, the ions of the electrolyte undergo
chemical changes at the respective electrodes. The chemical reaction carried out by passing electricity is
called electrolysis.
Terminologies used in electrolysis
Electrolysis: is the decomposition of a compound in solution or molten state by passing
electricity through it.
Conductor: is a solid substance that allows electricity to pass through it. E.g. all metals, graphite.
Non-conductor or insulator: is a solid substance that does not allow electricity to flow through
it.
Electrolyte: is a substance which, when dissolved or molten, conducts electricity and is
decomposed by it. E.g. all acids, alkalis, ionic compounds.
Non-electrolyte: is a compound which cannot conduct electricity be it in molten or solution
state. E.g. urea, ethanol, benzene, cane sugar.
Electrode: a graphite or metal pole (rod) or plate through which the electric current enters or
leaves the electrolyte.
Cathode: a negative electrode which leads electrons into the electrolyte.
Anode: a positive electrode which leads electrons out of the electrolyte.
Ion: a positively or negatively charged atom or radical (group of atoms).
Cation: a positive ion which moves to the cathode during electrolysis.
Anion: a negative ion which moves to the anode during electrolysis.
The electrolytic cell (voltameter)
The apparatus in which electrolysis is carried out is called electrolytic cell. A battery supplies the
direct current. Graphite electrodes carry the current into and out of the liquid electrolyte.
Graphite is chosen because it is inert. It will not react with the electrolyte or with products of
electrolysis. Electrons flow from the negative terminal (cathode) of the battery around the circuit
and back to the positive terminal (anode). In the electrolyte it is the ions that move to carry the
current.
The electrolytic cell
PREFERENTIAL (SELECTIVE) DISCHARGE OF IONS
When a salt such as sodium chloride is dissolved in water, its ions are set free to move. The
water in which the salt dissolved also ionizes.
Substance Ions produced
NaCl Na+ and Cl -
H2O
H+ and OH-
Then at each electrode, we get more than one ion for discharge, but only one is supposed to be
discharged.
Cathode: Na+ and H+
Anode: Cl - and OH-
In situations like this, the order of discharge of the ions at the electrode will depend on:
1. the position of the metal ion or radical in the electrochemical series;
2. the concentration ions to be discharged;
3. the nature of the electrodes used.
1. The position of ion or radical in the electrochemical series
If all other factors are constant, any ion will be discharged from solution in preference to those
above it. For example, in sodium hydroxide solution, containing H+ (from water) and Na+ (from
the salt), H+ discharges in preference to Na+.
In copper tetraoxosulphate(IV) solution, containing OH- (from water) and SO42- as negative
ions, OH- is discharged in preference to SO42-.
2. The concentration or nature of electrolyte: ions with greater concentration are
discharges if their position is close.
Electrolysis of concentrated solution of sodium chloride using platinum electrodes
Cathode Anode
Na+and H+ Cl- and OH-
H+ ions are discharged in preference Cl- ions are discharged in preference to OH- ions since
to Na+ ions since Na+ and H+ are Cl- and OH- ions are very close to each other in the e. c.
very far from each other in e. c.s s (and because there are more Cl- ions in the solution)
2H+ + 2e- → H2(g) 2Cl- → Cl2(g) + 2e-
Result
At cathode H2(g) is given off
At anode Cl2(g) is given off
The solution gradually becomes more alkaline as the electrolysis goes on because Na + and
OH- ions remain in solution as caustic soda (sodium hydroxide) solution. This is the main method
used in the manufacture of sodium hydroxide in industry.
Na+(aq) + OH-(aq) → NaOH (aq)
Fused or molten sodium chloride
NaCl → Na+ + Cl-
During this electrolysis: [there is no preferential discharge]
Cathode Anode
Na+ + e- → Na(l) 2Cl- → Cl2(g) + 2e-
Result
At cathode sodium is liberated (deposited)
At anode chlorine is given off
Electrolysis of dilute tetraoxosulphate (VI) acid (H2SO4) using platinum electrodes
H2SO4 → 2H+ + SO42-
H2O ⇔ H+ + OH-
Cathode Anode
SO42- and OH-
2H+ + 2e- → H2(g)
4OH- → 2H2O(l) + O2(g) + 4e-
Result
At cathode H2(g) is given out
At anode O2(g) is given out
The solution becomes acidic at the end of electrolysis because of the acidic ions (SO 42-) left in the
final solution
Electrolysis of copper (II) tetraoxosulphate (VI) using platinum electrode
CuSO4 → Cu2+ + SO42-
H2O ⇔ H+ + OH-
Cathode Anode
H+ and Cu2+ SO42- and OH-
Cu2+ + 2e- → Cu(s) 4OH-→ 2H2O(l) + O2(g) + 4e-
Result
At cathode copper is deposited
At anode oxygen is given out
The solution turns colourless and acidic to litmus due to SO42- and H- ions remaining in the
solution, both of which are acidic in nature and are equivalent to aqueous tetraoxosulphate (VI)
acid.
2H+ + SO42- → H2SO4
With copper electrodes
CuSO4 → Cu2+ + SO42-
H2O ⇔ H+ + OH-
Cathode Anode
H+ and Cu2+ SO42- and OH-
Cu2+ + 2e- → Cu(s)
Cu → Cu2+ + 2e-
SO42- and OH- ions move to the anode. However, these ions are not discharged but remain in
solution. Instead, copper atoms of the anode lose electrons and go into solution as copper ions
because copper lose electron more readily.
Cu → Cu2+ + 2e-
Result
At cathode copper is deposited
At anode copper dissolves (ionizes)
The concentration of the solution remains constant since for every one mole of Cu2+ discharged,
there is one mole of Cu ionized into one mole of Cu2+ ions. This method is applied in the
purification of impure copper where the impure copper is made the anode, pure copper the
cathode, and CuSO4 solution the electrolyte.
ELECTROMETALLURGY/extraction of metals
Many of the highly reactive metals such as metal of group 1 or group 2 are extracted from their
ores by electrolysis of their molten ores. This is because chemical reduction of their compounds
is either chemically not viable or highly expensive. Metals like sodium and magnesium are
manufactured by the electrolysis of their molten chlorides. Pure aluminium is obtained from a
solution of its oxide in molten cryolite.
ELECTROREFINING OF METALS/purification of metals
Refining of many of the crude metals such as copper, silver; aluminium, etc. is carried out by the
process of electrolysis. In this process, the block of crude metal is made anode while a thin sheet
of pure metal is made cathode and the electrolyte is aqueous solution of salt of the metal. As the
electrolysis proceeds metal from anode dissolves whereas it gets deposited in pure form at
cathode. The impurities settle down just below anode in the form of anode sludge.
MANUFACTURE OF CHEMICAL SUBSTANCES
Electrolysis is used in the manufacture of some important substances such as hydrogen, chlorine,
sodium hydroxide, sodium oxochlorate (I) and oxygen.
Electroplating
Electroplating is an art of coating a layer of costlier metal like gold, silver, etc. over the cheaper
metal like iron. The purpose of electroplating is
• protection of cheaper metal like iron from corrosion
• beautification of articles like, earrings, bangles, tings, parts of wrist watches, etc.
• repair of the broken parts of delicate machinary where welding is not possible.
The article to be plated is thoroughly cleaned with H2SO 4 and washed with distilled water. It is
then made the cathode of the electrolytic cell. The anode is pure sheet of metal to be coated or
plated. The electrolyte is a solution of a salt of the metal to be plated. During electrolysis, the
metal to be electroplated is transferred from the anode to the cathode.
