Law of Electrolysis Chemistry Notes

Law of Electrolysis Chemistry Notes

→ Michael Faraday (1833-34) has done many experiments on electrolytic processes and obtained two important conclusions. The two main laws are given as below:

→ Faraday’s First Law of Electrolysis – “The amount of substance liberated at electrode is directly proportional to quantity of electric current passed in electrolytic solution.”

→ So, if mg substance is liberated on passing Q coulomb of electric current, then,
m ∝ Q
m = ZQ
Where, Z is proportionality constant and is called electro-chemical equivalent.
If I ampere electric current is passed through t second,
then,
Q = I × t
m = Z × Q
m = Z × I t
Now, if
Q = 1 C
or I = 1 A and t = 1 s
then,
m = Z × 1 × 1
or
M = Z

→ So, electrochemical equivalent of a substance is the mass of substance liberated, when one ampere current is passed in 1 second i.e. 1 coulomb quantity of electricity is passed.

Law of Electrolysis Chemistry Notes

Unit
Z = \(\frac{m}{I t}\) = g A-1 s-1

→ Faraday’s Second Law of Electrolysis According to this law, “When same quantity of electricity is passed throgh various electrolytic solutions arranged in series, then the weights of substances produced at electrodes are directly proportional to their chemical equivalent weights.” For exampleWhen same electric current is passed in two electrolytic solutions arranged in series i.e., copper sulphate (CuSO4) and silver nitrate (AgNO3), then the weight of copper and silver liberated will be as follows:

Law of Electrolysis Chemistry Notes 1

→ Although equivalent weight terms is not used in modern terms. Faraday’s law of electrolysis in terms of moles of electrons exchanged during electro-chemical change can be expressed as follows: “The moles of substance liberated i.e., quantity of chemical change is directly proportional to the number of moles of electrons exchanged during oxidation reduction reaction”.

Law of Electrolysis Chemistry Notes

→ When the same quantity of charge is passed through different electrolytes arranged in series then the quantities of substances deposited at electrodes of each electrolyte can be calculated easily. Consider the figure 3.3 in which electrolytic solutions of H2SO4, CuSO4 and AgNO3 are situated in equivalent amounts.

Law of Electrolysis Chemistry Notes 2

→ It is known to calculate the weight of H., Cu and Ag liberated at cathodes of three electrolytes, there weights are always obtained in the ratio of equivalent weights 1:13.78: 10.788
From Faraday’s second law

M ∝ E ……… (1)

→ If quantities of substances liberated from two electrolytic cells are m, and m, respectively, whose equivalent weights are E and E, respectively, then

Law of Electrolysis Chemistry Notes 3

→ from Faraday’s first law, m1 = Z1 It and m2= Z2 It in which w1 and w2 are chemical equivalents of both substances liberated.

Law of Electrolysis Chemistry Notes 4

→ So, electro-chemical equivalent of a substance is directly proportional to its equivalent weight. It is seen experimentally that if 1 ampere electric current is passed in 1 second i.e., on passing 1 coulomb electric charge, 0.0011180 g Ag, 0.0003296 g Cu and 0.00001036 × H2 are liberated. These quantities are called their electro-chemical equivalents respectively. So, that quantity (Q) of electricity which is necessary for liberation of one gram equivalent of Ag.

Law of Electrolysis Chemistry Notes 5

→ So, the quantity (charge) of electricity required for liberation of one gram equivalent weight of substance is equivalent to one Faraday. Hence, if I ampere electric current is passed in second for liberation of substance having E equivalent weight from electrolytic solution, then the quantity of substance liberated can be determined by following formula:

m = \(\frac{\text { I.t. } E}{F}\)

→ Both laws of Faraday are called laws of electrolysis. These laws are used in electroplating techniques. These laws are very helpful to understand behaviour of various electrolytes. 1 Faraday charge is equivalent to charge of one mole electrons.

Law of Electrolysis Chemistry Notes

Charge of one electron = 166 × 10-19 coulomb
So, charge of one mole electron
= 1.66 × 10-19 × 6.023 × 1023
= 96500 coulomb

Chemistry Notes

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