First Law: It states that the mass (m) of substance deposited at the cathode during electrolysis is directly proportional to the quantity of electricity (total charge q) passed through the electrolyte i.e. m∝q or m= zq where the constant of proportionality z is called electrochemical equivalent (E.C.E.) of the substance.
Therefore we have m = zit. If q = 1 coulomb, then we have m = z * 1 or z = m
Hence, the electrochemical equivalent of substance may be defined as the mass of its substance deposited at the cathode, when one coulomb of charge passes through the electrolyte.
S.I. unit of electrochemical equivalent of a substance is kilogram coulomb-1 (kg-C-1).
E.C.E. for certain substances
|
Element
|
Atomic weight
|
Atomic number
|
Valence
|
E.C.E. (Z) in kg / C
|
|
Hydrogen
|
1.0008
|
1
|
1
|
10.4 ´ 10-9
|
|
Oxygen
|
15.999
|
8
|
2
|
82.9 ´ 10-9
|
|
Aluminium
|
26.982
|
13
|
3
|
93.6 ´ 10-9
|
|
Chromium
|
51.996
|
24
|
3
|
179.6 ´ 10-9
|
|
Nickel
|
58.710
|
28
|
2
|
304.0 ´ 10-9
|
|
Copper
|
63.546
|
29
|
2
|
329.4 ´ 10-9
|
|
Zinc
|
65.380
|
30
|
2
|
338.7 ´ 10-9
|
|
Silver
|
107.868
|
47
|
1
|
1118 ´ 10-9
|
|
Gold
|
196.966
|
79
|
3
|
681.2 ´ 10-9
|
Second Law: If same quantity of electricity is passed through different electrolytes, masses of the substance deposited at the respective cathodes are directly proportional to their chemical equivalents i.e. m∝E => m1/m2 = E1/E2
Let m be the mass of the ions of a substance liberated, whose chemical equivalent is E. Then, according to Faraday's second law of electrolysis,m∝E or m = constant * E or m/E =constant
Chemical equivalent E also known as equivalent weight in gm.
Relation between Electrochemical Equivalent and Chemical Equivalent
Suppose that on passing same amount of electricity q through two different electrolytes, masses of the two substances liberated are m1 and m2. If E1 and E2 are their chemical equivalents, then from Faraday's second law, we get m1/m2 = E1/E2.
Also from Faraday's first law m1/m2 = z1/z2 So z1/z2 = E1/E2 => z∝E
Faraday Constant: As we discussed above z∝E
.E=Fz => z = E/F = A/VF. 'F' is proportionality constant called Faraday's constant.
As Z=E/F and z = m/Q so E/F = m/Q hence if Q = 1 Faraday then E = m i.e. If electricity supplied to a voltameter is 1 Faraday then amount of substance liberated or deposited in (in gm) equal to the chemical equivalent.
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