Page 106 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

90 Chapter 7

Faraday’s Laws of Electrolysis

Faruday’s First Law of Electrolysis states that the mass of a
substance (element) deposited at the cathode (‘CROA’) during
electrolysis is directly proportional to the quantity of charge
(measured in coulombs) passing through the solution.
Faraday’s Second Law of Efectrofysis states that the number of
moles of electrons needed to discharge one mole of an ion at an
electrode is equal to the number of charges on that ion.

In summary : m oc Q
m oc It (since Q = It from physics)
+ m= kZt

(since a proportionality sign can always be replaced by = k’)
+ m =zIt
where z is de_fined as the electrochemical equivalent of the element.

The electrochemical equivalent of an element is the mass of that
element produced (deposited, in the case of a solid, or evolved, in the
case of a gas) at the cathode when 1 coulomb of charge passes through
the electrolyte solution.
Note the correct units: Z = current, measured in ampires, A; t =
time, measured in seconds, s; Q = charge, measured in coulombs, C;
rn = mass, measured in kilograms, kg (the unit of mass is the kilo-
gram, kg, not the gram, g). Hence, since m = zlt, z = m/(It) and so
kg A- s- is the unit of the electrochemical equivalent.
I
The quantity of electricity can then be measured in terms of the
number of moles of electrons passing through the electrolytic cell. The
amount of substance undergoing the chemical change is related to the
number of electrons involved in the respective half-reaction, and can
be expressed in terms of the number of moles of substance or the
number of reactive species, i.e. the number of chemical equivalents.
The concept of a redox reaction and the number of reactive species is
used to determine the amount of substance deposited or the volume of
gas evolved during electrolysis, e.g.

cu2+(aq) + 2e -, CUO(~)

to
=+ 2 electrons are needed to reduce Cu2+(aq) metallic copper, CU’(~).