Introduction to Electrochemistry

The branch of chemistry in which we study the relationship between electrical energy and chemical change (i.e., generation of electricity by spontaneous redox reaction.) is called electrochemistry. The substances which allow an electric current to flow through them are called electrical conductors; while those which do not allow any electric current to flow through them are called non-conductors or insulators.

Electrical conductors are of two types:

(i)     Metallic or electronic conductors: In general, metals belong to this category. The metals remain unchanged during the flow of current except warming. Here transfer of electric current is due to transfer of electrons without any transfer of matter. Example: Cu, Ag, Al, etc.

• Graphite also conducts electricity due to presence of free $e^-$e in its hexagonal sheet like structure.

(ii)     Electrolytic conductors: The substances which in fused state or in aqueous solution allow the electric current to flow accompanied by chemical decomposition are called electrolytes. On the other hand substances which in fused state or in aqueous solution do not allow the electric current to flow are called non-electrolytes. Usually, electrovalent compounds are good electrolytes and covalent compounds are non-electrolytes (Glycerol, glucose etc.).

The metal rod or foil dipped in electrolyte, through which the electric current is conveyed, is called electrode. The electrode on which negative ions (anions) arrive are called positive electrode or anode. Similarly the electrode on which positive ions (cations) arrive are called negative electrode or cathode.

Metallic conduction

1. Conduction is due to movement of electrons

2. No chemical change takes place

3. No transfer of matter Occurs

4. Resistance $\propto$Temperature

5. Faraday’s law is not followed

Electrolytic conduction

1. Conduction is due to movement of ions

2. Ions get oxidized or reduced at electrodes

3. Transfer of Matter occurs in the form of ions

4. Resistance $\propto \dfrac{1}{Temperature}$

5. Faraday’s law is followed

Some practical units used in electrochemistry:

(i)  Coulomb: It is the unit of electricity and may be defined as, “The amount of electricity which is required to deposit by electrolysis 0.001118 g of Ag from a solution of Ag ions”.

1 Faraday (F) = 965000 Coulomb

= $1.6 \times 10^{-19} \times 6.023 \times 10^{23}$ electrocharge

(ii)  Ampere: It is the unit for measuring the rate of flow of electrons, i.e.,,

Ampere = Columb / Second

(iii)  Ohm: It is the unit of resistance and may be defined as, “The electrical resistance which is offered to a steady current by ha uniform column of mercury cross section $(cm^2)$ of length 106.3 cm and weighing 14.4521 g at 273 K”.

(iv)   Volt: It is the unit of electrical potential and may be defined as, “The difference of potential steadily applied to a conductor, which has a resistance of 1 ohm and produces a current of 1 ampere”.

Thus,

$Jole = Colume \times Volt$

(v)    Watt: It is the unit of electric power and may be defined as, “The rate of supply of electric power when one joule of electrical energy is made available per second”.

Thus,

$watt = \dfrac{jole}{second} = \dfrac{coulomb \times Volt}{second} \\[3mm] = ampere \times volt$

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