Electrode Potential of Some Metals with respect to SHE (or) Electrochemical Series (or) EMF Series

ELECTRODE POTENTIAL OF SOME METALS WITH RESPECT TO SHE OR ELECTROCHEMICAL SERIES OR EMF SERIES

The standard electrode potential (reduction) of a number of electrodes are given in table 3.2. This values are determined potentiometrically by combining the electrode with the another standard electrodes, whose electrode potential is zero.

Table 3.2 Electrochemical series

Definition

When various electrodes (metals) are arranged the order of their increasing values of standard reduction potential on the hydrogen scale, then the arrangement is called electrochemical series.

Significance of emf series (or) Applications of electrochemical series (or) Applications of Nernst equation (or) Importance of electrode potential

The emf series (electrode potential) provide valuable information as given below.

1. Calculation of standard emf of the cell

The standard emf of a cell (E°) can be calculated if the standard electrode potential values are known using the following relation.

E°_cell = E°_R.H.E ‒ E°_L.H.E

2. Relative ease of oxidation (or) reduction

Higher the value of standard reduction potential (+ve value) greater is the tendency to get reduced. (i.e. Metals on the top (‒ve value) are more easily ionised) (oxidised).

(a) The fluorine has higher positive value of standard reduction potential (+2.87 V), and shows higher tendency towards reduction.

(b) The lithium has highest negative value (‒3.01 V) and shows higher tendency towards oxidation.

3. Displacement of one element by the other

Metals which lie higher in the emf series can displace those elements which lie below them in the series.

For example, we may know whether Cu will displace Zn from the solution or vice‒versa. We know that standard reduction potential of Cu & Zn.

i.e., E°Cu²⁺/Cu = +0.34 V and E°zn²⁺/Zn = ‒0.76 V.

So, Cu²⁺ has a great tendency to acquire Cu form, than Zn²⁺ has for acquiring Zn form.

4. Determination of standard free energy change (ΔG) and equilibrium constant for the reaction

Standard electrode potential can also be used to determine the standard free energy change (ΔG) and equilibrium constant (K) for the reaction. We know that

‒ ΔG° = RT ln K = 2.303 RT log K

log K = [ ‒ ΔG° ] /  [ 12.303 RT ]

 = nFE° / 2.303RT

 [∴ ΔG° = nFE° ]

From the value of E°, the equilibrium constant for the cell reaction can be calculated.

5. Hydrogen displacement behaviour

Metals with negative reduction potential (i.e., the metals placed above H₂ in the emf series) will displace the hydrogen from an acid solution.

Example

Zinc reacts with dil H₂SO₄ to give H₂ but Ag does not why?

Zn + H₂SO₄ → ZnSO₄ + H₂ ↑

E°_Zn = ‒ 0.76 volt

The metal with positive reduction potential (ie., the metals placed below H₂ in the emf series) will not displace the hydrogen from an acid solution.

 Ag + H₂SO₄ → No reaction

E°_Ag = +0.80 volt

6. Predicting Spontaneity (or) feasibility of Redox Reactions

Spontaneity of redox reaction can be predicted from the emf (E°) value of the complete cell reaction (ΔG = ‒nFE°).

(i) If E° of the cell is positive, the reaction is spontaneous (or) feasible (ΔG = ‒ve).

(ii) If E° of the cell is negative, the reaction is not spontaneous (or) not feasible

(AG = +ve).

In general, an element having lower reduction potential can displace another metal having higher reduction potential from its salt solution spontaneously.