Faraday's Laws of Electrolysis

Electrolysis is the use of electric current to stimulate a non-spontaneous chemical reaction. In electrolysis, an electric current is passed through an electrolytic solution to stimulate the flow of ions to bring about a chemical change. An electrolyte is a liquid (or generally salt solution of metal) that conducts electricity. 

What is an Electrode? 

An electrode can be defined as the point where current either enters or leaves the electrolyte or circuit. When the current leaves the electrode it is known as the cathode while when the current enters the electrode it is known as an anode. 

Electrodes are the main component of electrochemical cells. It is necessary that an electrode should be a good conductor of electricity. Although inert electrodes also exist which don’t take part in the reaction. The electrode can be of gold, platinum, carbon, graphite, metal etc. The electrode provides a surface for oxidation-reduction reactions in the cells. 

Electrodes are Mainly of two Types 

1. Reactive electrodes are those electrodes that take part in the reaction taking place in the cell and can dissolve in the electrolyte. 

Examples of reactive electrodes – copper electrodes, silver electrodes, zinc electrodes, copper electrodes etc. These are mainly used in potentiometric work.  

2. Inert electrodes are those electrodes that do not take part in the reaction. 

Examples of inert electrodes – Carbon electrodes, Platinum electrodes etc.

What is an Electrolytic Cell?

Electrolytic cells are those electrochemical cells that convert electrical energy into chemical potential energy. As we have discussed electrolysis above, you can relate that electrolytic cells work on the electrolysis process. Secondary cells or electrolytic cells are rechargeable; it means reversible chemical reactions occur in these cells. In these cells, the anode is always positive while the cathode is always negative. 

Faraday’s Laws of Electrolysis 

After having a clear understanding of electrolysis, electrodes, and electrolytic cells, now you are in the position to understand Faraday’s Laws of electrolysis. Faraday’s laws of electrolysis are based on the electrochemical research of Michael Faraday which he published in 1833. These show the quantitative relationship between the substance deposited at electrodes and the quantity of electric charge or electricity passed. 

Faraday’s First Law of Electrolysis 

Faraday’s First Law of Electrolysis states that “The mass of a substance deposited at any electrode is directly proportional to the amount of charge passed.” Mathematically it can be expressed as follows –

m ∝ Q ----------(1)

Where:

•  m = mass of a substance (in grams) deposited or liberated at an electrode.

• Q = amount of charge (in coulombs) or electricity passed through it

On removing the proportionality in equation (1) –

m=ZQ

Where Z is the proportionality constant. Its unit is grams per coulomb (g/C). It is also called the electrochemical equivalent. Z is the mass of a substance deposited at electrodes during electrolysis bypassing 1 coulomb of charge. 

Faraday’s Second Law of Electrolysis  

Faraday’s Second Law of Electrolysis states that “the mass of a substance deposited at any electrode on passing a certain amount of charge is directly proportional to its chemical equivalent weight.” Or “when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective chemical equivalent or equivalent weight”. Mathematically it can be represented as follows –

w ∝  E

Where w = mass of the substance 

E = equivalent weight of the substance

It can also be expressed as – w1/w2=E1/E2

The equivalent weight or chemical equivalent of a substance can be defined as the ratio of its atomic weight and valency. 

Equivalent weight=Atomic weight/Valency

Faraday’s Second Law of Electrolysis can be further explained by the following example –

Consider three different chemical reactions occurring in three separate electrolytic cells which are connected in series. Suppose in the 1st electrolytic cell sodium ion gains electrons and converts into sodium. 

Na+ + e− → Na

In 2nd electrolytic cell following reaction occurs –

Cu+2 + 2e− → Cu

In 3rd electrolytic cell following reaction occurs –

Al3++ 3e− → Al

When supposing y moles of electrons are passed through three cells, the mass of sodium, aluminium and copper liberated are 23y grams, 9y grams, 31.75y grams respectively. 

One mole of electrons is required for the reduction of one mole of ions. As we know, the Charge on one electron is equal to

1.6021×10-19

C and one mole of electrons is equal to

6.023×1023

electrons. So, charge on one mole of electrons is equal to –

(6.023×1023)×(1.6021×10-19C)=96500 C

This charge (96500 C) is called 1 Faraday. 

If we pass 1 Faraday of charge in an electrolytic cell, then 1gm of the equivalent weight of the substance will get deposited. So, we can write –

w=(Q / 96500)×E

On combining the 1st and 2nd laws we get – 

Z= E / 96500

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