Faraday's Laws of Electrolysis

In this article, we will explore Faraday’s laws of electrolysis – the First Law of Electrolysis and the Second Law of Electrolysis. Michael Faraday, an English scientist, performed a series of experiments to know the factors that control the mass of an element deposited or liberated during the process of electrolysis. Michael Faraday was the first scientist who explains the quantitative aspects of electrolysis in the form of his two laws, which are known as Faraday’s laws of electrolysis.

Faraday’s First Law of Electrolysis

Faraday’s first law of electrolysis is the most fundamental law of electrolysis.

The statement of Faraday’s first law of electrolysis is as under:

During the process of electrolysis, the mass of an element deposited at an electrode is proportional to the quantity of electricity which passes through the electrolyte.

Therefore, if m is the mass of the element deposited at the electrode, and Q is the quantity of electricity, then according to Faraday’s first law of electrolysis, we have,

`\m∝Q`

Since Q = It, where I is the electric current and t is the time, then

`\m∝It`

`\⇒m=ZIt=ZQ`

Where, Z is a constant of proportionality, known as the electrochemical equivalent (E. C. E.) of the element deposited at the electrode during electrolysis. For different elements, Z has different values.

Definition of Electrochemical Equivalent:

In the above equation, if Q = 1 coulomb, then

`\m=Z`

Therefore, the Electrochemical Equivalent (ECE) of an element is defined as the mass of the element deposited at the electrode by the passage of 1 coulomb of electricity through the electrolyte during the process of electrolysis.

The unit of electrochemical equivalent is g/C or kg/C.

For example, the electrochemical equivalent of copper is 0.000304 g/C, which means that 0.000304 g mass of copper will be deposited on the cathode when 1 coulomb of electricity passes through an electrolytic solution of CuSO₄.

Faraday’s Second Law of Electrolysis

Faradays’ second law of electrolysis provides a relationship between the mass of an element and the chemical equivalent weight of that element.

Faraday’s second law is stated as under:

During the process of electrolysis, the mass of an element deposited on the electrode is proportional to the chemical equivalent weight of the element.

Therefore, if m is the mass of the element and E is the chemical equivalent weight of the component, then according to Faraday’s second law of electrolysis, we have,

`\m∝E`

Explanation:

The following figure illustrates Faraday’s second law of electrolysis.

In the shown figure, two voltameter namely, silver voltameter (with AgNO₃) and copper voltameter (with CuSO₄) are connected in series. Therefore, they share the same amount of electric current at the same time. Experimentally, it has been found that the masses of silver (Ag) and copper (Cu) deposited on the respective cathodes are in the ratio of 108:32, where 180 is the chemical equivalent weight of silver and 32 is the chemical equivalent weight of copper.

Relation between Chemical Equivalent and Electrochemical Equivalent

Consider the same quantity of electricity (Q) is passed through two electrolytes.

If the masses of substances deposited on the respective electrodes are m₁ and m₂, and their electrochemical equivalents are Z₁ and Z₂ respectively.

Then, according to Faraday’s first law of electrolysis, we have,

`\m_1=Z_1 Q`

`\m_2=Z_2 Q`

Therefore,

`\m_1/m_2 =Z_1/Z_2`

But, according to Faraday’s second law of electrolysis, we have,

`\m_1/m_2 =E_1/E_2`

Therefore, from the just above two equations, we get,

`\Z_1/Z_2 =E_1/E_2`

`\⇒E_1/Z_1 =E_2/Z_2`

In general,

`\E/Z="Constant"`

The ratio E/Z is constant for all substances and it is known as Faraday Constant (F).

Definition of Faraday Constant:

Experimentally, the value of the Faraday constant is found to be 96500 coulombs, thus it can be defined as under:

Faraday constant is defined as the quantity of electricity required to deposit 1 g of the chemical equivalent of a substance during the process of electrolysis. Its value is 96500 C.

For example, in a copper voltameter, 96500 C of electricity is to be passed through the electrolyte to deposit the mass of copper on the cathode equal to 31.75 g, which is the chemical equivalent of copper.

Hence, this is all about Faraday’s laws of electrolysis – the first and second laws of electrolysis.