Faraday’s Laws of Electrolysis First Law and Second Law

An English physicist and chemist Michael Faraday deduced to fundamental laws of electrolysis. He segregated his laws in two parts. We know these as the First Law and Second Law of Electrolysis.

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Faraday’s First Law of Electrolysis

The electrolysis produces, products. The mass of these products is directly proportional to the quantity of electricity passing through the electrolyte during electrolysis. That means the ions liberated at an electrode depends on the quantity of electric charge passed through the electrolyte.

To understand this let us take an example of electrolysis of copper sulfate (CuSO4) with copper electrodes.

Here we immerse two copper electrodes in dilute copper sulfate (CuSO4). Then we supply electricity from an external source. Due to electrostatic attraction, Cu++ ions come to the negative electrode. Then each Cu++ ion receives two electrons from that negative electrode. Therefore Cu++ ions become neutral copper atoms. Then they get deposited on the negative electrode. As a result, the weight of the negative electrode increases. At the same time, sulfur ions (SO4) come to the positive electrode. Here they give up their two electrons. Thereafter they form copper sulfate (CuSO4). But copper sulfate cannot exist in its molecular form in the water solution. Therefore copper sulfates get dissolved in the electrolyte as Cu++ and SO4 ions. Therefore the positive electrode losses it’s copper. As a result, the weight of the positive electrode decreases.

Explanation of Faraday’s First Law of Electrolysis

Here we have seen in our example of electrolysis, each Cu++ ion takes two electrons from the negative electrode. Again electrons mean the negative charge. So, there is a direct relation between the weight of the copper gained by the electrode with the transfer of negative charge.

Statement of Faraday’s First Law of Electrolysis

The mass of ions liberated at an electrode is directly proportional to the electric charge passing during the electrolysis.

Expression of the First Law

Suppose m grams of ions liberated for q coulomb charge passing through the electrolyte during electrolysis. So as per Faraday’s First Law of electrolysis

Faraday's first law

Here, mathematically Z is the constant of proportionality. We call this constant as the electrochemical equivalent of the substance. Here the substance means the product liberated during electrolysis. Now if q is 1 coulomb then

Faraday's first law 1

Definition of Electrochemical Equivalent

The electrochemical equivalent of substance is nothing but the mass of the substance which liberated during electrolysis for 1 coulomb of electric charge.

Again, the mass of a substance for a certain number of atoms is directly proportional to the atomic weight. Again the number of atoms of the substance liberated for a certain amount of electric charge is inversely proportional to its valency. For example, the number of atoms of copper will be half of that of the silver for the same amount of electric charge used during electrolysis. Because copper is bivalent and silver is monovalent. For example, for n number of electrons transferred at the electrode, there is n number of silver atoms liberated. Whereas for the same n number of electrons transferred at the electrode there is n/2 number of copper atoms liberated.

To conclude we can write

Faraday's law equation¬†We call this F as Faraday’s constant.

Value of Faraday’s Constant

We can find out the value of Faraday’s constant as follows.

The value of the electrochemical equivalent of silver is 0.001118. That means 1 coulomb of electric charge liberates 0.001118g of silver during electrolysis. Again we know the atomic weight of silver is 107.88. Its valency is 1. So we can write

value of Faraday's constant

Faraday’s Second Law of Electrolysis

This Faraday’s second law of electrolysis tells that the quantity or mass liberated during electrolysis has also a relation with its chemical equivalent weight.

Statement of Faraday’s Second Law of Electrolysis

The masses of the different substances liberated for the same quantity of electric charge passing through electrolytes during electrolysis are proportional to their chemical equivalent weight. Suppose a certain current passing through dilute CuSO4 and dilute AgNO3 for a certain time. Then we will find that the ratio of the masses of liberated silver and copper is 107.88: 31.54. Because the chemical equivalent weight of silver is 107.88g and that of copper is 31.54g.

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