Faraday's Law of Electrolysis

Faraday's Law of Electrolysis

Faraday established a relation between the quantity of electricity passed and the amount of substance electrolysed called Faraday law of electrolysis.
There are two laws of electrolysis given by Faraday in 1833.
1. Faraday's First Law of Electrolysis
2. Faraday's Second Law of Electrolysis

Faraday's First Law of Electrolysis

The amount of substance obtained or deposited at electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolytic solution.
If W is the amount of a substance deposited at cathode by using Q quantity of electricity then-
W ∝ Q
We know that,
W = ct
where, 'c' is current passed and 't' is time, (i.e. c is passed for t second).
or, W ∝ ct
or, W = Zct
where, 'Z' is electro chemical equivalent of the substance.
If c = 1amp and t = 1sec
then- W = Z
Hence, the amount of substance deposited at electrode is equal to their electro chemical equivalence when 1amp current is passed through electrolytic solution for 1sec.(1 coulumb).
Electro chemical equivalent(Z) of the substance is equal to the amount of substance obtained by passage of 1 coulumb of electricity (1amp for 1sec.) through its electrolytic solution. Hence, one coulumb deposites Z gm. of a substance during electrolysis.

Faraday's Second Law of Electrolysis

When same quantity of electricity is passed through different electrolytes, the amount of substances liberated are proportional to their chemical equivalence.
Chemical equivalence means gram equivalent weight or 1gm equivalent.
W ∝ Q if Q is constant.

If W₁, W₂ and W₃ gm. of different substances are obtained when Q coulumb of electricity is passed through their respective electrolytic solution and E₁, E₂ and E₃ are the chemical equivalents respectively, then-
W₁/E₁ = W₂/E₂ = W₃/E₃
Since weight/Eq. Wt. is number of gm equivalent. Therefore, the same quantity of electricity will deposit equal number of gm. equivalent of all substances during electrolysis.
So, one Faraday deposits one gm. equivalent E of a substance during electrolysis.

Relation Between Chemical Equivalent and Electro-Chemical Euivalent

From second law of electrolysis, it is seen that one Faraday will deposit one gm. equivalent of any substance or, E gm. of substance.
From first law-
one coulumb will deposit Z gm. of the substance
so, 96500 coulumb will deposit 96500Z gm. equivalence
∴ E = 96500Z
or, E = ZF
or, Z = E/F
Since, F is constant
∴ Z ∝ Z
or, Z₁/E₁ = Z₂/E₂
Thus, electrochemical equivalence of a substance is proportional to chemical equivalent.

Application of Faraday's Law of Electrolysis

Following are the applications of Faraday’s law of electrolysis-
1. Removal of metal ions from an aqueous solution
2. Used in redox reactions
3. Used in the manufacturing of heavy water
4. Used in the manufacturing of galvanic cells and fuel cells
5. Avoid metal corrosion by using the technique of electroplating

Significance of Faraday

One Faraday is the quantity of electricity which deposits one gm. of any substance.
This also signifies that-
1. As many Faradays are passed through an electrolytic solution, so many gm. equivalents are deposited.
2. If same number of Faraday is paased through different electrolytic solutions, equal number of gm. equivalent of all substance are obtained.

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