Overall cell reaction

It is the sum of electrode reactions.

2 Cl (aq) Cl 2 (g) + 2e
(oxidation at anode)

2 H 2 O (l) + 2e H 2 (g) + 2 OH (aq)
(reduction at cathode)

2 Cl^ (aq) + 2 H 2 O^ (l) Cl 2 (g) + H 2 (g)

• 2 OH (aq)

(overall cell reaction)

Results of electrolysis of aqueous NaCl

i. H 2 gas is liberated at cathode.

ii. Cl 2 gas is released at anode.

iii. Because Na⊕ ions remain unreacted and
OH ions are formed at cathode, NaCl
solution is converted to NaOH solution.

Do you know?

Sea water is the source of

300000 tones of Mg produced

every year by electrolysis.

Electrochemical art : Al, Cr and Sn

can be coloured by an electrochemical

process called anodizing. In this process

metal anode oxidizes to give metal oxide

coat. When an organic dye is added to the

electrolyte, dye molecules soak forming

spongy surface of coating and become

trapped with the hardening of the metal

oxide surface.

Q (C) = I (A) × t (s) (5.16)

ii. Calculation of moles of electrons passed

Total charge passed is Q(C). The charge

of one mole electrons is 96500 coulombs (C).

It is referred to as one faraday (1 F). Hence,

Moles of electrons actually passed

=

Q(C)

96500 (C/mol e)

(5.17)

iii. Calculation of moles of product formed

The balanced equation for the half

reaction occuring at the electrode is devised.

The stoichiometry of half reaction indicates

the moles of electrons passed and moles of

the product formed. For the reaction,

Cu^2 ⊕ (aq) + 2e Cu (s), two moles of

electrons are required for the production of

one mole of Cu. So we can calculate the moles

of product formed. The moles of electrons

actually passed are given by Eq. (5.16).

To simplify further we introduce the

entity mole ratio given by

Mole ratio =

(^) moles of product formed in the half reaction
moles of electrons required in the half reaction
For the reaction of Cu, mole ratio = 1
2
Therefore,
Moles of product formed
= moles of electrons actually passed × mole
ratio

Q(C)
96500 (C/mol e)
× mole ratio (5.18)

I (A) × t (s)
96500 (C/mol e)
× mole ratio (5.19)
iv. Calculation of mass of product :
Mass of product
W = moles of product × molar mass of product

I (A) × t (s)
96500 (C/mol e)
× mole ratio × molar mass
of product
(5.20)
5.5.3 Quantitative aspects of electrolysis :
a. The mass of reactant consumed or
the mass of product formed at an electrode
during electrolysis can be calculated by
knowing stoichiometry of the half reaction
at the electrode.
i. Calculation of quantity of electricity
passed : To calculate the quantity of
electricity (Q) passed during electrolysis, the
amount of current, I, passed through the cell
is measured. The time for which the current
is passed is noted.