Electrical Engineering ⇒ Topic : Faradays Laws of Electrolysis

First law:- It states that the mass of ion liberated at an electrode is directly proportional to the quantity of electricity, i.e., charge which passes through the electrolyte

If  m = mass of ions liberated

   Q = quantity of electricity

       = I x t where jis the current and t is the time

    Z = Constant, known as electrochemical equivalent (I.C.E.) of the substance then m = ZIt

Second law:- It states that the masses of ions of different substances liberated by the same quantity of electricity are proportional to their chemical equivalent weights Electroplating is the application of the principles of electrolysis.

Faraday's Laws of Electrolysis

Faraday performed a series of experiments to determine the factors which govern the mass of an element deposited or liberated during electrolysis. He summed up his conclusions into two laws, known as Faraday's laws of electrolysis.

First law. The mass of an element deposited or liberated at an electrode is directly proportional to the quantity of electricity that passes through the electrolyte.

If m is the mass of an element deposited or liberated due to the passage of I amperes for t seconds, then according to first law,

                                                                           m  ∝  Q

          or                                                               m ∝ It                                                                          (Q = It)

          or                                                               m = ZIt or m = ZQ

where Z is a constant known as electro-chemical equivalent (E.C. E.) of the element. It has the same value for one element but different for other elements.

                                             If Q= 1 coulomb, then, m = Z.

Hence electro-chemical equivalent (E.C.E.) of an element is equal to the mass of element deposited or liberated by the passage of 1 coulomb of electricity through the electrolyte. Its unit is gm/C or kg/C.

For example, E.C.E. of copper is 0.000304 gm/C. It means that if 1 coulomb of electricity is passed through a solution of CuSO₄, then mass of copper deposited on the cathode will be 0.000304 gm.

The validity of first law is explained by the fact that current inside the electrolyte is earned by the ions themselves. Hence the masses of the chemical substances reaching the anode and cathode are proportional to the quantity of electricity earned by the ions i.e., mass of an ion liberated at any electrode is proportional to the quantity of electricity passed through the electrolyte.

Second law. The mass of an element deposited or liberated during electrolysis is directly proportional to the chemical equivalent weight of that element i.e.

                                                   m ∝ Chemical equivalent weight of the element (E)

Faraday's second law is illustrated in Fig. (a) where silver and copper voltameters are connected in series. When the same cun-ent is passed for the same time through the two voltameters,it will be seen that the masses of silver (Ag) and copper (Cu) deposited on the respective cathodes are in the ratio of 108 : 32. These values of 108 and 32 are respectively the equivalent weights of the silver and copper.

                                                                                                                   figure (a)

Faraday's second law can be explained as follows. The negative ions (i.e. NO₃^- and SO₄^{- -}) from the solutions give up their respective extra electrons to the anodes. These electrons come to cathodes via the external circuit and are taken up by the positive ions (Ag⁺ and Cu⁺⁺) to become metallic atoms and get deposited on the respective cathodes. Suppose 10 electrons are flowing in the external circuit. Since silver is monovalent (i.e., its valency is 1), 10 silver ions must be liberated at the cathode of silver voltameter. Again copper is bivalent (i.e., its valency is 2) and hence 5 copper ions must be liberated at the cathode of copper voltameter. This means that mass of an element (silver or copper) liberated is directly proportional to the atomic weight and inversely proportional to the valency of that element i.e.