Page 317 - Physical chemistry understanding our chemical world

284 ELECTROCHEMISTRY

Table 7.1 Faraday’s laws of electrolysis
Faraday’s first law
The number of moles of a species formed at an electrode during electrolysis is proportional to the
electrochemical charge passed: Q = I × t

Faraday’s second law
A given charge liberates different species in the ratio of their relative formula masses, divided by
the number of electrons in the electrode reaction

it only needs to deliver a tiny current of a micro-amp or so. Conversely, a car battery
(usually a ‘lead–acid cell’, as described on p. 347) is bulky and heavy because it
must deliver a massive amount of electrical energy, particularly when starting the car.
Other batteries generate currents of intermediate magnitude, such as those needed in
torches, mobile phones and portable cassette and CD players.
The amount of charge generated or consumed by a battery is in direct proportion
to the number of electrons involved, according to Faraday’s laws, which are given in
Table 7.1. Both electrons and ions possess charge. When a current is drawn through
a cell, the charged electrons move through the conductive electrodes (as defined on
p. 300) concurrently with charged ions moving through the electrolyte. The ions are
anions (which bear a negative charge) and cations (which are positive).
Underlying both of Faraday’s laws lies the fundamental truth that each electron
possesses the same charge.

Worked Example 7.1 What is the charge on 1 mol of electrons?

The charge e on a single electron is 1.6 × 10 −19 Cand there are
The coulomb, C, is the 23
6.022 × 10 electrons per mole (the Avogadro number L), so the
SI unit of charge.
charge on a mole of electrons is given by the simple expression
charge on one electron = L × e (7.6)

Inserting numbers into Equation (7.6), we obtain

Thechargeon1 molof charge on 1 mole of electrons
electrons is termed ‘a = 1.6 × 10 −19 C × 6.026 × 10 23 mol −1
faraday’ F.
−1
We see that 1 mol has a charge of 96 487 C mol . This quantity of
charge is known as the ‘Faraday’ F.

SAQ 7.1 An electrolysis needle (i.e. an electrode) delivers 1 nmol of
electrons to a hair root. How many faraday’s of charge are consumed, and
how many coulombs does it represent?