The relationship between the quantity of electricity passed through the electrolyte and the quantity of a substance liberated at the electrode is given in the form of Faraday’s laws of electrolysis.
FARADAY’S FIRST LAW OF ELECTROLYSIS
This law states that the mass of a substance liberated at the electrode is directly proportional to the quantity of electricity passed through the electrolyte.
Here, Q = quantity of electricity
I = current in amperes
t = time in seconds
Z = constant of proportionality called electrochemical equivalent (ECE)
If I = 1 ampere and t = 1 second, then m = Z
Thus, electrochemical equivalent of a substance is the amount of substance liberated at the electrode when current of one ampere is passed through the electrolyte for one second.
FARADAY’S SECOND LAW OF ELECTROLYSIS
This law states that the amounts of different substances liberated by the same quantity of electricity passing through their electrolytic solution are directly proportional to their chemical equivalent masses (chemical equivalent mass of metal can be obtained by dividing its atomic mass with number of electrons required to reduce its cation). The law can also be stated as follows; when same quantity of electricity is passed through different electrolytes connected in series then the masses of the substances liberated at the electrodes are in the ratio of their chemical equivalent masses (atomic mass+ Number of electrons required to form the product) or the ratio of their electrochemical equivalents.
For example, if the two electrolytic cells A (containing AgNO3 solution) and B (containing CuSO4 solution) are connected in series and same quantity of electricity is passed through the cells. Then the ratio of the mass of copper deposited at cathode in electrolytic cell B (x g) to that of silver deposited in cell A (y g) is equal to the ratio of their chemical equivalent masses.
Mass of cu (x) / Mass of Ag (y)
= Chemical Equivalent Mass of Cu / Chemical Equivalent Mass of Ag = Z Cu / Z Ag
Now, each copper (Cu2+) ion requires 2 electrons to form Cu and each Ag+ needs 1 electron to form Ag. Thus, chemical equivalent mass of Cu is 63.5/2 and that of Ag is 108/1.
Thus, the ratio x / y = 63.5 / 2 x 108.