FARADAY'S LAWS OF ELECTROLYSIS
When electric current is passed through an electrolyte solution, the ions of electrolyte undergoes chemical changes at the respective electrodes. The chemical reaction carried out by passing electricity is called as electrolysis.
There are two important laws of electrolysis proposed by Faraday and the relation between the amount of products liberated at the electrodes and the quantity of electricity passed through the electrolyte is established based on these laws.
Faraday's First law :
The mass of the substance (m) liberated at the electrodes during the electrolysis is directly proportional to the quantity of electricity (Q) that passes through the electrolyte.
Faraday's Second law :
When the same quantity of electricity passes through solutions of different electrolytes, the amounts of the substances liberated at the electrodes are directly proportional to their chemical equivalents.
Electrochemical equivalent defined in First law
If m is the mass of substance (in grams) deposited on electrode by passing Q coulombs of electricity, then
m a Q ... First law
We know that Q = It
Or m = Zit
where Z is the constant known as the Electrochemical equivalent of the substance (electrolyte). If I = 1 ampere and t = 1 second, then
m = Z
Thus, the electrochemical equivalent is the amount of a substance deposited by 1 ampere current passing for 1 second (i.e., one coulomb)
The Electrical unit Faraday
It has been found experimentally that the quantity of electricity required to liberate one gram equivalent of a substance is 96,495 coulombs. This quantity of electricity is known as Faraday and is denoted by the symbol F.
It is obvious that the quantity of electricity needed to deposit 1 mole of the substance is given by the expression.
Quantity of electricity = n x F
where n is the valency of its ion. Thus the quantity of electricity required to discharge :
one mole of Ag+ = 1 x F = 1F
one mle of Cu2+ = 2 x F = 2F
one mole of Al3+ = 3 x F = 3F
We can represent the reactions on the cathode as :
Ag+ + e = Ag
Cu2+ + 2e = Cu
Al3+ + 3e = Al
It is clear that the moles of electrons required to discharge one mole of ions Ag+, Cu2+ and Al3+ is one, two and three respectively. Therefore it means that the quantity of electricity in one Faraday is one mole of electrons. Now we can say that,
1 Faraday = 96,495 coulombs = 1 Mole electrons
Importance of the First law of Electrolysis
With the help of the first law of electrolysis we are able to calculate :
i. the value of electrochemical equivalents of different substances ; and
ii. the mass of different substances produced by passing a known quantity of electricity through their solutions.
96495 coulomb of electricity liberates one gram equivalent weight of the element.
1 coulomb current liberates = equivalent mass / 96495 g.coulomb-1
Z = electrochemcial equivalent mass = ( equivalent mass / 96495 ) x 10-3 kg.coulomb-1
1 Faraday equal the quantity of electricity produced by 1 mole of electrons.
Charge of an electron = 96495 / 6.023 x1023 = 1.602 x 10-19 Coulomb.