Unit – Iii Electrochemistry

Electrochemistry
The branch of chemistry deals with the
interconversion of chemical energy into electrical
energy and vice versa.
It deals with the study of the electrical
properties of electrolytes and also the chemical
changes brought about by electrical energy.
 Arrhenius Theory of
Ionization
❖ Postulates /Merits of Arrhenius Theory :
1) When an electrolyte (e.g. salt) is dissolved in water, it
splits into two charged particles.
Ionization
NaCl Na+ + Cl−
Electrolyte Cation
Anion

These particles are called ions and the process is called

Ionization.
The positively charged particles are called cations and
those having negative charge are called anions.

2) Cations are generally metallic radicals obtained by

losing electrons from metallic atoms.
Anions are generally non-metallic radicals obtained
by the gain of electrons from non-metallic atoms or
3) The ions present in the solution constantly reunite
to form neutral molecules. Thus there is a state of
equilibrium between the undissociated molecules
and the ions.
Equilibrium
AB A + + B−
Electrolyte Cation
Anion

4) The number of positive or negative charges on the

cation or anion corresponds to the valency of the
element.
 ELECTROCHEMISTRY

ELECTROCHEMICAL CELL ELECTROLYTIC CELL

Converts chemical energy Converts

electrical energy
to electrical energy to chemical
energy
1) Electrolytic cell:
The device that converts electrical energy
into chemical energy, by means of non-spontaneous
chemical reaction carried out using electrical energy
is called an electrolytic cell.
For example: Electroplating, electro-refining,
electrolysis.

2) Electrochemical cell:
The device that converts chemical energy
into electrical energy by means of spontaneous
chemical reaction is called an electrochemical cell.
For example: Dry cell or Leclanche cell, Daniel cell,
Lead acid storage battery cell, Nickel-cadmium.
❖ Important Terms and their Definitions Used in Electrochemistry

1) Conductor: A substance that allows the

electric current to pass through it.
Ex - All metals, graphite, fused (molten) salts,
aqueous solution of acids, bases, and salts.

2) Insulator: A substance that does not allow the

electric current to pass through it.
Ex. Wood, paper, plastic, or alcohol.
 Type of
conductorsThese are the metallic
a)Metallic conductors:
substances that conduct electricity through them
without undergoing any chemical changes.
Metals and their alloys carry electric current
due to the flow of electrons in the metal atoms.
Ex: - Metals like copper, aluminum silver etc.
b)Electrolytic conductor or Electrolyte: These
are substances that in a fused state or an aqueous
solution liberate ions and allow an electric current
to pass through them resulting in chemical
decomposition.
In this type of conductor, the charge is carried
by ions. i.e. Conduction will not occur unless the
ions are free to move.
Differences between Metallic & Electrolytic
conductor
Sr. Metallic conductor Electrolytic conductor
No

1. These are the substances that These are the substances that in a
conduct electricity due to the fused state or in an aqueous
flow of electrons. solution liberate ions & allow an
Ex: - all metals. electric current to pass through them
resulting in chemical decomposition
Ex: - acids, bases & salts.
2. It does not involve any transfer It involves transfer of matter.
of matter.

3. It shows the decrease in It shows the increases in

conductance (increase in conductance as the temperature
resistance) as the temperature increases.
increases.
 3) Electrolytes: The substances that conduct electricity in a
molten state or in an aqueous solution and undergo chemical
changes are called electrolytes.
Electrolytes are of the following types:
a) Strong electrolytes: Electrolytes that are highly
ionized in solution or in a fused state, hence having a high
degree of ionization are called strong electrolytes.
For example: - Strong acids like HCl, H2 SO4 , and HNO3, strong
bases like NaOH, and KOH & almost all soluble salts. CuSO 4 ,
NaCl, etc.
b) Weak electrolyte: The substance that ionizes to a
small extent in a fused state or an aqueous solution hence
having a low degree of ionization is called a weak electrolyte.
For example: Weak acids like carbonic acid(H2 CO3),acetic
acid(CH3 COOH), and weak bases like NH4 OH, Ca(OH)2 & salts
like BaSO4 , Al(OH)3 etc.
4) Non-electrolyte: The substances that cannot conduct
 Distinguish between Cathode and Anode
Sr. Cathode Anode
No
.
1) It is connected to the It is connected to the
negative terminal of positive terminal of
the battery when the battery when
electric current is electric current is
passed through an passed through an
electrolyte. electrolyte.
2) Spontaneous reduction Spontaneous oxidation
reaction occurs at the reaction occurs at the
cathode. anode.
3) During electrolysis, the During electrolysis, the
flow of electrons is from flow of electrons is from
 Mechanism of Electrolysis
E.g.- Electrolysis of Copper Sulphate using Copper
Electrodes.
Applications of Electrolysis:

Electroplating

Electro-refining
 After passage of electric current, Ag + ions from electrolyte gets
deposit on iron spoon and undergo reduction.
 Construction and Working of Daniel Cell

a)Daniel cell with porous partition b) Daniel cell with salt bridge

The most common example of an electrochemical

or galvanic cell is the Daniel cell.
It consists of a zinc electrode immersed in zinc
sulphate solution (first half cell) and a copper
electrode immersed in a copper sulphate solution
(second half cell).
Both these half-cells are separated by a porous
 Because of its high oxidation potential zinc acts
as an anode and copper acts as a cathode.
1) Reaction at Anode:
At the anode, oxidation takes place, so the
zinc atom undergoes oxidation to form zinc ions
with a liberation of 2e⊝.
Anodic oxidation: Zn → Zn2+ + 2e⊝
As the surface of the zinc rod dissolved into
the solution, the weight of the zinc rod (anode)
decreased after electrolysis.
2) Reaction at Cathode:
At the cathode, reduction takes place, so
the copper ion in the copper sulphate solution
accepts the liberated electrons, undergoes reduction,
 Cathodic reduction: Cu2+ + 2e⊝→ Cu
As the metallic copper gets deposited onto
the surface of the cathode, the weight of the
cathode increases after electrolysis.
The e.m.f. developed is due to the reactions
occurring at the cathode and anode. The e.m.f. of
the cell is 1.1 volts.
Representation of Cell: +
⊝ Zn(s) ZnSO4 (1M) CuSO4 (1M) Cu (s)
Faraday’s Second Law
Statement: “When the same quantity of
electricity is passed through different electrolytes
arranged in series, then the amount of substance
liberated or deposited at the respective electrode
is directly proportional to its chemical
equivalent”.
Calculate the time in second in which 0.3 gm of Copper is
liberated from copper sulphate, when a current of 0.5
ampere is passed. (Equivalent weight of Cu = 31.6 gm)