Electrolysis of sodium chloride
i) Molten sodium chlorideThe only ions present are Na+ and Cl-
ii) Concentrated aqueous sodium chlorideThe products are not the same because water is also involved.
Probable reactions at the cathode
The reduction potential of water is greater so H2 is evolved.
Probable reactions at the anode
The reduction potentials are nearly the same.Cl2 is preferentially evolved because of
iii) Higher concentration of Cl- ions.iv) Over voltage of hydrogen. (Extra voltage required for that reaction because it is kinetically slow process)
Actual reactions taking place are:
v) Electrolysis of aqueous CuSO4 using platinum electrode.
Comparing the reduction potentials, the actual reactions are
vi) Electrolysis of CuSO4 using copper electrodes
The actual reactions are
If more than one substance is present in an electrolytic cell the substance liberated at the cathode has highest reduction potential and that at the anode will have the lowest reduction potential.Therefore,
i) Na+, Ca2+ and Al3+ have lower reduction potential than water so H2 gas is evolved at the cathode.ii) Cu2+, Ag+ have much higher reduction potential than that of water. So these ions are deposited at the cathode.