Electrochemical Series
Electrochemical Series , Many
half cell potential values can be calculated keeping hydrogen electrode as the
standard. These half cell potential values are standard values and are
represented as the standard reduction potential values as shown in the following
table which is also called “Electrochemical Series”. By measuring the
potentials of various electrodes versus standard hydrogen electrode (SHE), a
series of standard electrode potentials has been established called Electrochemical Series.
STANDARD REDUCTION POTENTIALS
Element
|
Electrode
Reaction (Reduction)
|
Standard Electrode Reduction
potential Eo,
volt
|
Li
|
Li+ +
e- → Li
|
-3.05
|
K
|
K+ +
e- → K
|
-2.925
|
Ca
|
Ca2+ +
2e- → Ca
|
-2.87
|
Na
|
Na+ +
e- → Na
|
-2.714
|
Mg
|
Mg2+ +
2e- → Mg
|
-2.37
|
Al
|
Al3+ +
3e- → Al
|
-1.66
|
Zn
|
Zn2+ +
2e- → Zn
|
-0.7628
|
Cr
|
Cr3+ +
3e- → Cr
|
-0.74
|
Fe
|
Fe2+ +
2e- → Fe
|
-0.44
|
Cd
|
Cd2+ +
2e- → Cd
|
-0.403
|
Ni
|
Ni2+ +
2e- → Ni
|
-0.25
|
Sn
|
Sn2+ +
2e- → Sn
|
-0.14
|
H2
|
2H+ +
2e- → H2
|
0.00
|
Cu
|
Cu2+ +
2e- → Cu
|
+0.337
|
I2
|
I2 +
2e- → 2I-
|
+0.535
|
Ag
|
Ag+ +
e- → Ag
|
+0.799
|
Hg
|
Hg2+ +
2e- → Hg
|
+0.885
|
Br2
|
Br2 +
2e- → 2Br-
|
+1.08
|
Cl2
|
Cl2 +
2e- → 2Cl-
|
+1.36
|
Au
|
Au3+ +
3e- → Au
|
+1.50
|
F2
|
F2 +
2e- → 2F-
|
+2.87
|
The negative sign of standard
reduction potential indicates that an electrode when joined with SHE acts as
anode and oxidation occurs on this electrode. Similarly, the positive sign
of standard reduction potential indicates that the electrode when joined with
SHE acts as cathode and reduction occurs on this electrode.
Application of Electrochemical series
Some
of the important applications of electrochemical series are discussed as
follows:
1. Relative Oxidising and Reducing Powers of Various Substances/ Reactivity of Metals.
The activity of the metal depends on
its tendency to lose electron or electrons, i.e., tendency to form cation
(M"+). This tendency depends on the magnitude of
standard reduction potential. The metal which has high negative value (or
smaller positive value) of standard reduction potential readily loses the
electron or electrons and is converted into cation. Such a metal is said to be
chemically active.
The
chemical reactivity of metals decreases from top to bottom in the series. The
metal higher in the series is more active than the metal lower in the series.
For example F2 has highest reduction potential which means it is
most easily reduced to F– ions. In other words, F2 is best oxidising
agent. Li+
ion, on the other hand, had lowest reduction potential. Hence Li+
is weakest reducing agent or conversely Li metal is best reducing agent. Thus,
it can be concluded that substances with higher reduction potentials are strong
oxidising agents while substances with lower reduction potentials are strong
reducing agents.
2. Calculation of Standard EMF of the cell
Standard
EMF of the cell can be calculated by applying the formula
E
cell = Eº (cathode) + Eº (anode)
The
electrode with higher electrode potential (Eº) act as cathode while that with
lower electrode potential will act as anode.
3. Predicting Feasibility of Redox Reaction.
Electrochemical
series helps to predict the feasibility of the redox reaction in a given
direction. If Ecell is positive then the redox reaction is feasible otherwise
it is non-feasible.
4. Predicting whether a Metal can Displace H2 Gas from Acid or not. The chemical reaction between metal M and acid to liberate H2 gas is represented by the reaction.
M (s) → Mn+ + ne
2H+ + 2e
→ H2
All
metals lying above hydrogen in electrochemical series can liberate H2 gas by
reaction with acids. On the other hand, the metal lying below hydrogen in the
electrochemical series cannot undergo such a reaction.