Faradays Laws of Electrolysis - First and Second Laws
Faraday’s Laws of Electrolysis
Before understanding Faraday’s laws of electrolysis, we must first understand the process of electrolysis of a metal sulfate.
Whenever an electrolyte like metal sulfate is diluted in water, its molecules split into positive and negative ions. The positive ions (or metal ions) move to the electrodes connected with the negative terminal of the battery where these positive ions take electrons from it, becoming a pure metal atom and getting deposited on the electrode.
Faraday’s laws of electrolysis are quantitative (mathematical) relationships that describe the above two phenomena.
Faraday’s First Law of Electrolysis
From the brief explanation above, it is clear that the flow of current through the external battery circuit fully depends upon how many electrons get transferred from negative electrode or cathode to positive metallic ion or cations. If the cations have valency of two like Cu++ then for every cation, there would be two electrons transferred from cathode to cation. We know that every electron has negative electrical charge – 1.602 × 10-19 Coulombs and say it is – e. So for disposition of every Cu atom on the cathode, there would be – 2.e charge transfers from cathode to cation.
Now say for t time there would be total n number of copper atoms deposited on the cathode, so total charge transferred, would be – 2.n.e Coulombs. Mass m of the deposited copper is obviously a function of the number of atoms deposited. So, it can be concluded that the mass of the deposited copper is directly proportional to the quantity of electrical charge that passes through the electrolyte. Hence mass of deposited copper m ∝ Q quantity of electrical charge passes through the electrolyte.
Faraday’s First Law of Electrolysis states that
The chemical deposition due to the flow of current through an electrolyte is directly proportional to the quantity of electricity (coulombs) passed through it.
i.e. mass of chemical deposition:
Where, Z is a constant of proportionality and is known as electro-chemical equivalent of the substance.
Faraday’s Second Law of Electrolysis
So far we have learned that the mass of the chemical, deposited due to electrolysis is proportional to the quantity of electricity that passes through the electrolyte. The mass of the chemical, deposited due to electrolysis is not only proportional to the quantity of electricity passes through the electrolyte, but it also depends upon some other factor. Every substance will have its own atomic weight. So for the same number of atoms, different substances will have different masses.
Again, how many atoms deposited on the electrodes also depends upon their number of valency. If valency is more, then for the same amount of electricity, the number of deposited atoms will be less whereas if valency is less, then for the same quantity of electricity, more number of atoms to be deposited.
So, for the same quantity of electricity or charge passes through different electrolytes, the mass of deposited chemical is directly proportional to its atomic weight and inversely proportional to its valency.
Faraday’s second law of electrolysis states that :
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.
Read article – Units of Resistivity
Visit NCERTplanet.com for NCERT solutions and Textbook downloads
