Faraday’s First and Second Laws of Electrolysis

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Faraday’s Laws of Electrolysis

Before understanding Faraday’s laws of electrolysis, we have to recall 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 negative terminal of the battery where these positive ions take electrons from it, become pure metal atom and get deposited on the electrode. Whereas negative ions or sulphions move to the electrode connected with positive terminal of the battery where these negative ions give up their extra electrons and become SO4 radical. Since SO4 cannot exist in electrically neutral state, it will attack metallic positive electrode and form metallic sulfate which will again dissolve in the water. Faraday’s laws of electrolysis combine two laws and these are,

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 X 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 function of 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 only,

According to this law, the chemical deposition due to 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 electrochemical equivalent of the substance.

If we put Q = 1 coulombs in the above equation, we will get Z = m which implies that electrochemical equivalent of any substance is the amount of the substance deposited on passing of 1 coulomb through its solution. This constant of passing of electrochemical equivalent is generally expressed in terms of milligram per coulomb or kilogram per coulomb.

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 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 same amount of electricity, number of deposited atoms will be less whereas if valency is less, then for same quantity of electricity, more number of atoms to be deposited. So, for 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.

Chemical Equivalent or Equivalent Weight

The chemical equivalent or equivalent weight of a substance can be determined by Faraday’s laws of electrolysis and it is defined as the weight of that subtenancy which will combine with or displace unit weight of hydrogen. The chemical equivalent of hydrogen is, thus, unity. Since valency of a substance is equal to the number of hydrogen atoms, which it can replace or with which it can combine, the chemical equivalent of a substance, therefore may be defined as the ratio of its atomic weight to its valency.