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Learning Objectives

Preferential Discharge of ions at electrodes

Preferential Or Selective Discharge of ions a electrodes

➢ When two or more ions of the

Similar charge are present in a

solution of an electrolyte, under

identical conditions

➢ Competing for discharge at the

same electrode

➢ one of them gets preferentially

discharged

Selective discharge of ions depends on three factors :

A. The relative position of ions in the

electrochemical series

B. The relative concentration of ions

C. The nature of electrodes

a) the relative position of ions in the electrochemical series

If All other factors are constant any ion

lower in electrochemical series gets

discharged at the relevant electrode in

preference to those above it.

Electrolysis Dil CuSO4 Soln

Dissociation of Electrolytes

Ions migrating to Cathode Cu2+ H+

Ions Discharge at Cathode Cu2+

Ions migrating to anode SO42-

Ions discharged at anode OH-


If All other factors are constant any ion lower in

electrochemical series gets discharged at the

relevant electrode in preference to those above

it.

Electrolysis Dil H2SO4 Soln


Ions migrating to Cathode Only H+

Ions Discharge at Cathode H+

Ions migrating to anode SO42- OH-



If All other factors are constant any ion

lower in electrochemical series gets

discharged at the relevant electrode in

preference to those above it.

Electrolysis Dil NaCl Soln


Ions migrating to Cathode Na+ ,H+

Ions Discharge at Cathode H+

Ions migrating to anode Cl-, OH-


b) concentration of ions in the electrolyte

➢ If an electrolyte has a much higher

concentration of a particular ion

➢ that ion will be discharge at the

relevant electrode

➢ In preference to those ions which

are lower in the electrochemical

series but are present along with.


Example :

Electrolysis of Concentrated NaCl

solution

At cathode :

➢ Both Na+ and H3O+migrate to cathode

➢ but concentration of Na+ is much higher

than that of H3O+

➢ Na+ discharge in preference to H3O+

irrespective of its higher position than

H3O+ in electrochemical series


Example :

Electrolysis of Concentrated NaCl

solution

At anode :

➢ Both Cl- and OH- migrate to the anode.

➢ But the concentration of Cl- is much

higher than that of OH-

➢ Hence Cl- is discharged ,although OH-

has lower position in the electrochemical

series

c) nature of electrodes used

Electrodes used in the process of

electrolysis are either inert or active

Inert Electrodes

➢ Electrodes which will not take part in

electrolytic reaction

➢ electrodes like graphite and platinum


Active Electrodes

➢ Electrodes which will take part in

electrolytic reaction

➢ Different products from inert

electrodes are formed

➢ Electrodes like Cu, Ni, Ag,


Example :

In the electrolysis of CuSO4 solution

With Active Cu Electrodes

➢ Cu atom is deposited at cathode

➢ Cu2+ are formed at the anode


Example :


With Inert Pt Electrodes


Example :


With Inert Pt Electrodes

➢ oxygen is formed at the anode➢ copper atom is deposited at the

cathode

Examples of Electrolysis

1. Electrolysis of molten lead

bromide

1. Electrolysis of acidified water

using Pt electrodes

1. Electrolysis of Copper Sulphate

solution using Pt anode & Copper

Or Pt Cathode

1. Electrolysis of Aqueous copper

sulphate using Copper electrodes

1. Electrolyte : molten lead bromide

(PbBr2)

2. Temperature: Above 380 °C , the

melting point of PbBr2 is maintained by

continuous heating of a burner.

Electrolysis of molten lead bromide

3.Electrolytic cell : Crucible made of Silica


4. Electrodes :The cathode and anode are both made of graphite plates which are inert they are connected by copper wire to the two ends of a battery

5. Current : 12 volt ,3 ampere (from a battery)6. Ions present : Pb2+ & Br-


Electrode reactions

At cathode :

At anode :


The overall reaction

Observations :

1. Dark reddish brown fumes of

bromine evolve at the anode.

2. Greyish lead is formed on the

cathode


Note :

1. Since silica is not reactive hence an

electrolytic cell is made up of silica

■ It can withstand high

temperature and is almost

non conductor of electricity.

■ The silica crucible is heated

slowly from outside


Note :

A. Solid lead bromide is a non- conductor of

electric current since it’s ions are not free

but held together by an electrostatic force

of attraction.

B. Ions become free in fused or molten lead

bromide. Hence the crucible is heated from

its outside to keep lead bromide in the

molten state.


Note :

2. Graphite anode is preferred to other inert

electrodes such as platinum since graphite

is unaffected by the reactive bromine

vapours.


To do at home

1. What are inert & active electrodes.

2. On what factors does the discharge of ions depends.