CHEMISTRY TEXTBOOK

The standard conditions chosen are 1
M concentration of solution, 1 atm pressure
for gases, solids and liquids in pure form
and 25^0 C. The voltage measured under
these conditions is called standard potential
designated as E^0.

The standard cell potential is the
algebraic sum of the standard electrode
potentials similar to Eq. (5.22).

E^0 cell = E^0 oxi (anode) + E^0 red (cathode)
(5.23)

Here E^0 oxi is standard oxidation potential
and E^0 red is the standard reduction potential.

According to IUPAC convention,
standard potential of an electrode is taken
as the standard reduction potential.

It must be realised that standard oxidation
potential of any electrode is numerically
equal to its standard reduction potential with
the reversal of sign. For example standard
oxidation potential of Zn^2 ⊕ (1M) Zn electrode
is 0.76V. Its standard reduction potential will
be -0.76 V. Hereafter the standard reduction
potential will be called standard potential, the
voltage associated with a reduction reaction.

It follows that the standard cell potential
(emf) is written in terms of the standard
potentials of the electrodes. In Eq. (5.23),
E^0 oxi(anode) is replaced by - E^0 red (anode).
We then write,

E^0 cell = - E^0 red (anode) + E^0 red (cathode)

Omitting the subscript red , we have

E^0 cell = E^0 (cathode, +) - E^0 (anode, -)
(5.24)

Remember...

• While constructing a galvanic
  cell from two electrodes, the
  electrode with higher standard
  potential is cathode (+) and that with
  lower standard potential is anode (-).


• The difference in electrical potential
  between anode and cathode is cell
  voltage.

5.7.2 Dependence of cell potential on

concentration (Nernst equation) : The

standard cell potential tells us whether or

not the reactants in their standard states

form the products in their standard states

spontaneously. To predict the spontaneity of

reactions for anything other than standard

concentration conditions we need to know

how voltage of galvanic cell varies with

concentration.

Dependence of cell voltage on

concentrations is given by Nernst equation.

For any general reaction,

aA + bB cC + dD

The cell voltage is given by

Ecell = E^0 cell - RT

nF

1n

[C]c [D]d

[A]a [B]b

= E^0 cell -

2.303RT

nF

log 10

[C]c [D]d

[A]a [B]b

(5.25)

where n = moles of electrons used in

the reaction, F = Faraday = 96500 C, T =

temperature in kelvin, R = gas constant =

8.314 J K-1mol-1

At 25^0 C, 2.303RT

F

= 0.0592 V

Therefore at 25^0 C, eq. (5.24) becomes

Ecell = E^0 cell -

0.0592V

n

log 10

[C]c [D]d

[A]a [B]b

(5.26)

The Eq. (5.25) or Eq. (5.26) is the

Nernst equation. The first term on the right

hand of Nernst equation represents standard

state electrochemical conditions. The second

term is the correction for non standard state

conditions. The cell potential equals standard

potential if the concentrations of reactants

and products are 1 M each. Thus,