Electrode potential is a measure of the tendency of an electrode to gain or lose electrons in a half-cell reaction. Standard electrode potentials (E°) are measured under standard conditions. E° values are used to determine the feasibility of redox reactions, calculate free energy change, and understand the oxidizing or reducing properties of substances. The electrochemical series arranges E° values from strongest reducing agent to strongest oxidizing agent. E° data can also be used to calculate non-standard electrode potentials and cell potentials using Nernst equations.

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❖ Oxidation : Loss of electrons / increase in oxidation number
❖ Reduction : Gain of electron / decrease in oxidation number
❖ Oxidizing agent (O.A.) : electron acceptor/oxidation number decreases
❖ Reducing agent (R.A.) :electron donor / oxidation number increases.

4. Electrode potential (E):
Electrode potential is a measure of the tendency of an electrode in a half-cell reaction to gain or
lose electrons.
Standard electrode potential (E°): When potential measured at standard conditions ;
concentration is 1M, Pressure is 1 atm at 25°C temperature.
Types od Standard Electrode Potential
1) Oxidation electrode potential (E°ox): It is measure of the tendency of the electrode in a half-
cell to get oxidized or to lose electrons.
Zn(s) →Zn2+ (aq) + 2e–, E°ox or E°Zn/Zn2+ = +0.76 V
2) Reduction electrode potential (E°red): It is measure of the tendency of the electrode in a half-
cell to get reduced or to gain electrons.
e.g. – Cu2+(aq) + 2e- → Cu(s), E°red or E°Cu2+/ Cu = +0.34 V

5. for standard electrode potentials
• An important convention
According to the present convention, the value of electrode potential is expressed as
reduction electrode potential. Thus, if we say that the value of standardelectrode
potential of zinc electrode is –0.76 volt, this means that the given value is the value of
the standard reduction electrode potential of Zn electrode
i.e., EZn
2+
/ Zn = - 0.76 volt
(E°ox) = – (E°red)
–(E°ox) = (E°red)

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Electrochemical Series
The values of standard reduction
potential of many electrodes have
been determined At 25°C
or (25 + 273) = 298K. These values
have been arranged in their
Increasing order, in the form
of a series, which is called as
electrochemical series.

7. USE OF
ELECTRODE
POTENTIAL DATA

8. 1) To know the feasibility of a chemical reaction
With the help of potential data, we can determine the feasibility of a
redox reaction. The electric pressure that makes current flow in a circuit
is called as electromotive force.
Electromotive force (E.M.F.) = E°cathode – E°anode
= E°more – E°less
= E°right – E°left
Suppose, we want to know feasibility of the following redox reaction:
Sn2+ + Cu→ Sn + Cu2+ (E°Sn
2+
/Sn = –0.136 v and E°Cu
2+
/Cu = 0.34)
As reduction potential of E°Cu
2+
/Cu is higher as compared to E°Sn
2+
/Sn
reduction will takes place at copper half cell.

9. 2e– + Sn2+ → Sn (E°Sn
2+
/Sn = –0.136 v) Cathode
Cu → Cu2+ + 2e– (E°Cu/Cu
2+ = –0.34) Anode
E.M.F. = Ecathode – Eanode
= –0.136 – (+0.34)
= –0.476V
Due to negative value of E.M.F. of the cell, it is not a feasible cell.
Whereas if the cell reaction is
Cu2+ + Sn → Sn2+ + Cu
Then, it will be feasible to occur, because now E.M.F. will be positive.
E°cell = E°cathode – E°anode
Note:- If we know E°cathode and E°anode we can easily calculateK. If K is very high, the
reaction will proceed towards forward direction.

10. 2) To calculate the redox potential and free energy change
There is the following relationship between standard Eletrode Potential (E° )and
free energy change(ΔG°):
ΔG° = –nFE°
If E° is positive, the value of ΔG° will be negative and a reaction is generally
thermodynamically feasible if value of ΔG° is negative.

11. 3) To calculate the value of reduction electrode potential of a given
electrode in non-standardconditions
The value of (Ered)elec. can be calculated by using the following Nernst Equation:

12. 4) To calculate the value of potential (e.m.f.) of a given cell in standard conditions
The value of E°cell can be calculated by using the equation.
E°cell = (E°red) cathode
– (E°red) anode
5) To calculate the value of Ecell by using Nernst equation

13. 6) To know oxidizing and reducing property of substancesin aqueous solution
• The substance which has a (+)ve value of E°red, has a tendency to gain electrons
to undergo reduction and hence is capable of acting as an oxidizing agent in
aqueous solution and hence higher the oxidizing power of the substance.
• The substance which has a (–)ve value of E°red, has a tendency to lose electrons
to undergo oxidation and hence is capable of acting as an reducing agent in
aqueous solution and hence lower is the reducing power of the substance.
• Since, E°red value increases down the electrochemical series (– → O → +), the
oxidizing power of oxidizing agents also increases.
• Since, E°red value increases down the electro-chemical series, the reducing
power of the reducing agents decreases in the series.

15. 7) To calculate redox potential and equilibrium constant
As redox reactions are reversible in nature, therefore, there is equilibrium between
oxidizing and reducing agent. If we know the redox potential, we can calculate
equilibrium constant for the reaction which can further decide the direction of the
reaction forward / backward
Zn(s) + Cu2+(aq) ⇌ Zn2+(aq) + Cu(s)
(Equilibrium constant) K = [Zn2+][Cu(s)] / [Zn(s)][Cu2+]
As concentration of solids remains constant, [Cu(s)]/[Zn(s)] =1 (constant)
So, K= [Zn2+] / [Cu2+]
K is related to E.M.F. of a cell by the following relation :
E°cell = 2.303RT/nF Log K
= 0.059/n log K (at 25°C )

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