electrolysis

6 Electrochemistry

Electrolytes – substances in molten state
or aqueous solution that can conduct
electricity due to the presence of free
moving ions.
(ionic compounds, acids, alkali)

Electrolysis (电解）

Electrolysis of molten compounds

Electrolysis of molten lead(ii) bromide
• Half equation at anode : 2Br- → Br2 + 2e-

• Half equation at cathode : Pb2+ + 2e- → Pb

• Half equation show the discharge of
ions

Electrolysis of Aqueous Solution

• An aqueous solution is solution of water of a
substance. For example, if you heat sodium
chloride until it melts, it is called molten sodium
chloride, but if you dissolve sodium chloride in
water, it is called aqueous sodium chloride.

• Electrolysis of aqueous solution is different from
electrolysis of molten electrolyte because an
aqueous solution contain more types of ions. (H+
and OH- ions)

Selective Discharge

• When the ions move to the anode and
cathode, only 1 negative ion and 1 positive ion
will be selected to be discharged, and this is
called selective discharge.

• There are three main factors that affect the
selective discharge in electrolysis:

– position of ion in the electrochemical series
– the concentration of ion
– the type of electrode

Factors Affecting the Selective
Discharge – Electrochemical Series

• During electrolysis, the ion in the lower
position will be selected to be discharge.

Exm : electrolysis of CuSO4 solution

• the ions that present in the solution are H+ ,
SO42-, Cu2+ and OH–

• At anode (Positive Electrode)
• Tathterancetegdattivoethioenasn(oSOde42.- and OH–) will be
• OelHec–tirooncshiesmloicwael rsethriaens,ShOe4n2-cieotnhseinOH– ions will be

selected to be discharged at anode.
• The OH– ions discharged to form water and

oxygen gas.
• 4OH– → 2H2O + O2 + 4e
• Observation:

Colourless gas bubbles are released around
anode. When a glowing wooden splinter is
inserted into the test tube that contain the gas,
the wooden splinter light up.

• At cathode (negative Electrode)

• The positive ions (Cu2+ and H+) will be
attracted to the cathode.

• Cu2+ ions is lower than H+ ions in
electrochemical series, hence the Cu2+ ions
will be selected to be discharged at cathode.

• Cu2+ + 2e → Cu

• Observation:
Brown solid deposited at cathode

Factors Affecting the Selective
Discharge – Concentration

• If the concentration of a particular ion (Cl-, Br-
, I-)is high, it may be selected to be discharged
even though it is higher in the electrochemical
series compares with another ion present in
the solution.

Exm : hydrochloric acid (HCl)

Electrolysis of Diluted Hydrochloric Electrolysis

Acid of Concentrated Hydrochloric Acid

Ions presence at Ions presence at
Anode: Cl–, OH– Anode: Cl–, OH–

Cathode: H+ Cathode: H+

Observation at Observation at

Anode: Anode:

Colourless gas bubbles is produced. A greenish yellow gas produced. When a

When a glowing wooden splinter is blue litmus paper is inserted into the test

inserted into the test tube that contain the tube that contain the gas, the blue litmus

gas, the splinter is rekindled. paper turn red and then become

Cathode colourless.

Colourless gas is produced. When a Cathode

lighted wooden splinter is brought close Colourless gas is produced. When a

to the test tube that contain the gas, a lighted wooden splinter is brought close to

“pop” sound produced.. the test tube that contain the gas, a “pop”

sound produced.

Half Equation of the Reaction at Half Equation of the Reaction at
Anode: 4OH– → 2H2O + O2 + 4e Anode: 2Cl– → Cl 2+ 2e
Cathode: 2H+ + 2e → H2 Cathode: 2H+ + 2e → H2

Factors Affecting the Selective
Discharge – Types of Electrode

• If an active electrode (copper, silver..) is used,
no ion discharge at anode. Instead, the active
anode ionised.

• Example: Electrolysis of Copper(II) Sulphate
with Carbon/Copper Electrodes

Using Carbon Electrode Using Copper Electrode

Ions presence at Ions presence at

Anode: Anode:
OH–, SO42- OH–, SO42-
Cathode: Cathode:
H+, Cu2+ H+, Cu2+

Observation at Observation at

Anode: Anode:

Colourless gas is produced. When a glowing The electrode becomes thinner.

wooden splinter is inserted into the test Cathode

tube that contains the gas, the glowing The electrode becomes thicker.

splinter light up. Colour of the Solution:

Cathode Remain unchanged

The electrode become thicker.

Colour of the Solution:

intensity of blue colour solution decrease

Half Equation of the Reaction at Half Equation of the Reaction at
Anode:
Anode: Cu → Cu2+ + 2e
4OH– → 2H2O + O2 + 4e Cathode:
Cathode: Cu2+ + 2e → Cu
Cu2+ + 2e → Cu

• Note:
For the electrolysis of copper(II) sulphate
solution by using copper electrodes, both the
hydroxide and sulphate ions are not selected
to be discharged, Instead, the copper anode
ionises to form copper(II) ions. The electrode
has taken part in the reaction!