ELECTROCHEMISTRY

 (a)  The process of chemical decomposition of an electrolyte on the passing of electric current through its aqueous solution or in its molten state is known as electrolysis.
(b)  It involves oxidation process at the anode and reduction process at the cathode.
(c)  On electrolysis, If more than one type of cations are obtained then that cation is liberated at cathode which has higher reduction potential.
(d)  On electrolysis, If more than one type of anions is obtained then that anion is liberated at anode which has low reduction potential.
(e)  Electrolysis process appears only at electrodes at the time for which electricity is passed.

For Example (i)  Electrolysis of aq. NaCl
NaCl (aq) ¾®  Na+ + Cl¯
H2O  ¾® H+ + OH¯
At cathode 2H+ + 2e– ¾® H2
At Anode 2Cl¯ ¾® Cl2 + 2e–
For Example (ii) Electrolysis of aq. CuSO4                   
CuSO4   ¾® Cu++ + SO4¯ ¯
H2O   ¾® H+ + OH¯
At cathode Cu++ + 2e– ¾® Cu
At Anode 4OH¯  ¾® 2H2O + O2 + 4e–                       

(a) Ohm’s Law:
According to this law, the current (I) flowing through a conductor at a given standard temperature is directly proportional to the potential difference (V) and inversely proportional to the resistance (R). i.e.
I  =    or  V =  IR

(b) Specific resistance (r): The resistance between two opposite faces of one cm cube of metal is called as specific resistance (r). It is also called resistivity

r  ==
= ohm-cm.

(c) Specific conductance (k): The reciprocal of specific resistance is called specific conductance (k). It is also called as conductivity.
k  =         or k =
Q  = Cell constant
\    k =  

(d) Electrical conductance (C) :

(i) It is the reciprocal of resistance (R), which is expressed in ohm–1 or mho.

C =

(ii)  The unit of electrical conductance is called semen (s)

(e) Equivalent conductance ():
(i) It is defined as the conductance of all the ions produced by the ionisation of one gram equivalent of an electrolyte in a given solution.
Þ   = k ×
Or   = k ×
where N = normality C = concentration of the solution

(ii) The unit of equivalent conductance is ohm-1 cm2 equivalent-1 or S cm2 eq-1.

(f)  Molar conductance (): It is defined as the conductance of all the ions produced by the ionisation of 1 mole of an electrolyte present in V ml of solution.
= k  ×
= k ×

Its unit is ohm-1 cm2 mol-1 or S cm2 mol-1.
Where M = Molarity and C = concentration of the solution.

Note : - On dilution, the value of equivalent and molar conductance increases whereas the value of specific conductivity decreases because of no. of ions in per ml. of the solution decreases.

Faraday’s First Law - The amount of substance deposited during the passes of electric current is directly proportional to the quantity of electric charge passed through the electrolyte.

W  µ Q
or                      W  µ I t         Q  Q = I × t
or                      W  = Z. i. t

where  Q = total charge passed through the solution
i = current in ampere
t = time for which current flows in seconds.
Z = Electrochemical equivalent (constant)

7.2 Faraday’s Second Law - According to this law, when the same quantity of electric current is passed through different electrolytes, then the amount of deposited substance at the respective electrodes, are in the ratio of their equivalent masses then suppose that W1 and W2  are the amounts of the elements deposited by passing of electric current through their electrolytic solution, and E1 and E2 are the respective equivalent weights.                                               

or                                  (Q W = Z i t)
\                             
Thus , the electrochemical equivalent (Z) of an element is directly proportional to its equivalent weight (E), Means

E  a Z
or                        E  = FZ

where F = proportionality constant (Faraday)
If  F = 96540 C

\                        E  = 96540  Z
or                        E  = 96500 Z