REVERSIBLE AND IRREVERSIBLE CELLS
AND ELECTRODES
TYPES OF REVERSIBLE ELECTRODES
What are reversible conditions? In what manner these conditions are adhered
to a cell?
A cell is said to be thermodynamically reversible if it satisfies all tree
conditions mentioned below:
1. If External EMF < applied EMF of the cell, a small current should flow from the
cell and a small amount of chemical reaction occurs in the cell
2. If External EMF > applied EMF of the cell, an extremely small current will flow
through the cell in the opposite direction and a small amount 0f chemical reaction
occurs in the cell but in the reverse direction
3. If External EMF = applied EMF of the cell, no current flow, no cell reaction takes
place
A cell that satisfies these three conditions is called as reversible cell whereas
the one which doesn’t satisfy these conditions is said to be an irreversible
cell.
REVERSIBLE ELECTROCHEMICAL CELL:
Eg: Daniel cell is a reversible cell with a capacity of 1.1 V and gives a
general reaction as
Cu+2 + Zn Zn+2 + Cu
1. When an external voltage > 1.1V is applied, the reaction gets
reversed
backward reaction
2. When an external voltage < 1.1V is applied, forward reaction
ocZnrs (general manner)
forward reaction
3. When an external voltage = 1.1V, no reaction occurs in the cell
Thus, the reversible reaction can be demonstrated as:
Zn + Zn+2 Zn+2 + Zn
IRREVERSIBLE ELECTROCHEMICAL CELL:
If the cell reaction cannot be reversed back to the original state, it is said to
be irreversible.
Suppose a cell comprises of a Zn rod and an Ag rod dipped in dil. H2SO4.
Now when these two rods are joined by a wire, following reaction occurs,
Zn + H2SO4 ZnSO4 + H2 (g) --------------(1)
(Oxi): Zn Zn+2 + 2e
(Red): 2H+ H2 (gas)
Suppose when the cell is connected to an external EMF slightly greater than the
cell EMF, silver dissolves at one electrode and gas is evolved at the other
2Ag + H2SO4 Ag2SO4 + H2 (g) ------------------(2)
(Oxi): 2Ag 2Ag+ + 2e
(Red): 2H+ H2 (gas)
Zn + H2SO4 ZnSO4 + H2 (g) --------------(1)
2Ag + H2SO4 Ag2SO4 + H2 (g) ------------------(2)
Zn + 2Ag + 2 H2SO4 ZnSO4 + Ag2SO4 + 2 H2 (g)
The above cell is irreversible cell is represented as:
Zn | dil. H2SO4 | Ag
Another irreversible cell example is
Zn | dil. H2SO4 | Cu
REVERSIB
LE
ELECTROD
ES
Metal-
Metal-Metal
Insoluble Redox
Ion Gas Electrode
Metal Salt Electrodes
Electrodes
Electrodes
(1) Metal-Metal Ion Electrodes
(Electrodes of the first kind)
Metal rod dipped in a solution of its own ions , for example daniell cell
Zn (s)| Zn + 2(aq)
Cu (s)| Cu + 2(aq)
M (s)daniell
for example M + zcell: copper rod dipped in copper sulphate solution and zinc rod
dipped in zinc| sulphate solution
(aq)
If it acts as Anode, M (s) + ne- M +n(aq)
If it acts as Cathode, M +n(aq) + ne- M (s)
Electrode is reversible w.r.t. cation
(2)
Gas
Electrodes
Hydrogen Chlorine Oxygen
electrode electrode electrode
Since gases produced are nonconductors of electricity, an unattackable inert
metal is used for making electrical contact with the circuit over which the
gases are adsorbed.
Eg: Pt electrode
(a) Hydrogen electrode:
Hydrogen gas is bubbled at 1atm pressure in a 1M acid solution (say HCl)
and it forms the hydrogen electrode.
It is referred to as Standard Hydrogen Electrode (SHE)
2H+ (aq) + 2e - H2 (g)
Since H2 gas used is a non-conductors of electricity, an unattackable
inert metal is used for making electrical contact with the circuit such
as Pt and the electrode is represented as below
Pt | H2 (g) , H+ (aq)
Electrode is reversible w.r.t. H+
(b) Chlorine electrode:
In this electrode, chlorine gas is bubbled at a given pressure into a solution of
HCl and gives following reaction:
2Cl- (aq) Cl
2 (g) + 2e -
Pt | Cl2 (g) , Cl- (aq)
Electrode is reversible w.r.t. Cl- ions
(c) Oxygen electrode:
In this electrode, oxygen gas is bubbled at a given pressure into a solution
containing hydroxyl ions (OH-) and gives following reaction:
4OH- (aq) O2 (g) + 2H2O + 4e -
Since equilibrium is not quickly established between the hydroxyl ions of the
solution and the oxygen gas, thus this electrode does not behave truly as a
reversible electrode
(3) Metal-Insoluble Metal Salt Electrodes:
It consists of a metal +a sparingly soluble salt of the same metal
dipped in a solution of soluble salt of having same anion
Eg: Calomel electrode: Hg (s) , Hg2Cl2 (s) ; KCl(solution)
Calomel in literal terms is mercurous chloride
• If the electrode involves reduction; ions furnished by the sparingly soluble salt salt
gets discharged at the electrode. Thus, more amount of calomel would pass into the
solution whereas there is net increase in the concentration of chloride ions.
• If the electrode involves oxidation; 2 electrons get liberated and ions get into the
solution whereas (from the calomel) combine with ions (furnished by KCl) to form
sparingly soluble salt .
• Thus, general electrode reaction for calomel is given as under:
Electrode is reversible w.r.t. Cl- ions
(4) Redox Electrodes:
• The term REDOX is used for the electrode into which the electrode potential is
developed due to the presence of the same substance in two different oxidation states.
• The electrode is prepared by inserting an unattackable metal (eg: Pt) into an
appropriate solution such as the ones given below:
1. Pt wire inserted into a solution comprising of ions
2. Pt wire inserted into a solution comprising of ions
3. Pt wire inserted into a solution comprising of ions
• The potential at the electrode arises from the tendency of the ions in one oxidation
state to get converted to other more stable oxidation state.
Another important redox electrode includes QUINHYDRONE ELECTRODE which
comprises of a Pt wire inserted into a solution containing Hydroquinone and Quinone
in equimolar amounts.
