Net cell reaction
The net cell reaction is the sum of two
electrode reactions.
2 Cl (l) Cl 2 (g) + 2e
(oxidation half reaction)
2 Na⊕ (l) + 2e 2 Na (l)
(reduction half reaction)
2 Na⊕^ (l) + 2 Cl^ (l) 2 Na (l) + Cl 2 (g)
(overall cell reaction)
Results of electrolysis of molten NaCl
i. A pale green Cl 2 gas is released at anode.
ii. A molten silvery-white sodium is formed
at cathode.
Decomposition of NaCl into metallic
sodium and Cl 2 (g) is nonspontaneous. The
electrical energy supplied by the battery
forces the reaction to occur.
Do you know?
Refining of metal and
electroplating are achieved by
electrolysis.The other is the reduction of water to
hydrogen gas.
ii. 2 H 2 O (l) + 2e H 2 (g) + 2 OH(aq),
E^0 = - 0.83 V
The standard potential (section 5.7.1) for
the reduction of water is higher than that for
reduction of Na⊕. This implies that water has
much greater tendency to get reduced than the
Na⊕ ion. Hence reaction (ii), that is, reduction
of water is the cathode reaction when the
aqueous NaCl is electrolysed.
Oxidation half reaction at anode : At anode
there will be competition between oxidation
of Cl ion to Cl 2 gas as in case of molten
NaCl and the oxidation of water to O 2 gas.
i. 2 Cl (aq) Cl 2 (g) +2e,E^0 oxi = - 1.36 V
ii. 2H 2 O (l) O 2 (g) + 4H⊕ (aq) + 2e
E^0 oxi = - 0.4 V
Standard electrode potential for the
oxidation of water is greater than that of Cl
ion or water has greater tendency to undergo
oxidation. It is, therefore, expected that anode
half reaction would be oxidation of water.
The experiments have shown, however, that
the gas produced at the anode is Cl 2 and
not O 2. This suggests that anode reaction is
oxidation of Cl to Cl 2 gas. This is because
of the overvoltage, discussion of which is
beyond the scope of the present book.Remember...
When molten ionic compound
is electrolysed, a metal is formed
at the negative electrode and a nonmetal
at the positive electrode.5.5.2 Electrolysis of aqueous NaCl :
Electrolysis of an aqueous NaCl can be
carried out in the cell used for the electrolysis
of molten NaCl using inert electrodes shown
in Fig. 5.4. The fused NaCl is replaced by
moderately concentrated aqueous solution of
NaCl. The water involved in electrolysis of
aqueous NaCl, leads to electrode reactions
that differ from electrolysis of molten NaCl.
Reduction half reaction at cathode : At
cathode, two reduction reactions compete.
One is the reduction of Na⊕^ ions as in case
of molten NaCl.
i. Na⊕^ (aq) + e Na (s), E^0 = -2.71 V
It has been found experimentally
that the actual voltage required for
electrolysis is greater than that calculated
using standard potentials. This additional
voltage required is the overpotential.