1.1. INTRODUCTION
Electrochemistry is a branch of chemistry, which deals with the chemical applications of
electricity. Electrochemistry deals with the chemical reactions produced by passing electric
current through an electrolyte or the production of electric current through chemical
reactions.
1.1.1 Conductors
A substance or material that allows electric current to pass through it is called a conductor.
The ability of a material to conduct electric current is called conductance.
All metals, graphite, fused salts, aqueous solutions of acids,
1.1.2
bases, etc.,
Non-Conductors (or) Insulators
Materials which do not conduct electric current are called non-conductors or insulators.
Plastics, wood, most of the non metals, etc.,
1.1.3
Types of Conductors
The conductors are broadly classified into two types as follows.
1. Metallic conductors (or) Electronic conductors
Metallic conductors are solid substances, which conduct electric current due to the movement
of electrons from one end to another end. The conduction decreases with increase of
temperature.
All metals, graphite.
2. Electrolytic Conductors
Electrolytic conductors conduct electric current due to the movement of ions in solution or in
fused state. The conduction increases with increase of temperature.
Acids, bases, electrovalent substances.
1.2 ELECTRODE POTENTIAL
A metal (M) consists of metal ions (Mn+) with valence electrons. When the metal (M) is placed
in a solution of its own salt, any one of the following reactions will occur.
(i)
Positive metal ions may pass into the solution.
M −−−−−>Mn+ + ne− (oxidation)
(ii)
Positive metal ions from the solution may deposit example-1
over the metal.
Mn+ + ne− −−−−−> M (reduction)
Examples – 1 :
Zn electrode dipped in ZnSO4 solution
When Zn electrode is dipped in ZnSO4 solution, Zn goes into the solution as Zn2+ ions. Now,
the Zn electrode attains a negative charge, due to the accumulation of valence electrons on the
metal. The negative charges developed on the electrode attract the positive ions from solution.
Due to this attraction the positive ions remain close to the metal. (Fig. 1.1.a)
Example -2
Cu electrode dipped in CuSO4 solution
When Cu electrode is dipped in CuSO4 solution, Cu2+ ions from the solution deposit over
the metal. Now, the Cu electrode attains a positive charge, due to the accumulation of Cu2+
ions on the metal. The positive charges developed on the electrode attract the negative
ions from solution. Due to this attraction, the negative ions remain close to the metal.
(Fig. 1.1.b)
Thus, a sort of layer (positive (or) negative ions) is formed all around the metal. This layer is
called Helmholtz electrical double layer. This layer prevents further passing of the positive ions
from or to the metal. A difference of potential is consequently set up between the metal and the
solution. At equilibrium, the potential difference becomes a constant value, which is known as
the electrode potential of a metal.
Thus, the tendency of an electrode to lose electrons is called the oxidation potential, and the
tendency of an electrode to gain electrons is called the reduction potential.
Factors affecting electrode potential
The rate of the above reactions depend on
i.
The nature of the metal.
ii.
The temperature.
iii.
The concentration of metal ions in solution.
1.2.1Single electrode potential (E)
It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in
contact with a solution of its own salt.
1.2.2Standard electrode potential (E°)
It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in
contact with a solution
of its own salt of 1 molar concentration at 25°C.
1.2.3Nernst equation for electrode potential
Consider the following redox reaction
Mn+ + ne− −−−−−>M
For such a redox reversible reaction, the free energy change (∆G) and its equilibrium
constant (K) are inter related as
1.2.1Single electrode potential (E)
It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in
contact with a solution of its own salt.
1.2.2 Standard electrode potential (E°)
It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in
contact with a solution
of its own salt of 1 molar concentration at 25°C.