Instead of an ammeter that simply measures the current, one can place a device that is powered by
electric current so as to extract the potential energy of the electrons. That is, after all, why galvanic
cells are useful. The common dry cell battery and the lead-acid storage battery found in cars are
examples of galvanic cells.
ELECTROLYTIC CELLS
A redox reaction occurring in an electrolytic cell has a positive ∆G and is therefore nonspontaneous.
In electrolysis, electrical energy is required to induce reaction; i.e., instead of extracting work from a
spontaneous redox reaction, we supply energy to force a nonspontaneous redox reaction to occur.
The oxidation and reduction half-reactions are usually placed in one container. Where the ammeter
or electrical device used to be for the galvanic cell, we need to place a source of electrical power, like
a battery, instead (see figure on the next page).
Michael Faraday was the first to define certain quantitative principles governing the behavior of
electrolytic cells. He theorized that the amount of chemical change induced in an electrolytic cell is
directly proportional to the number of moles of electrons that are exchanged during a redox
reaction. The number of moles exchanged can be determined from the balanced half-reaction. In
general, for a reaction that involves the transfer of n electrons per atom:
Mn+ + ne− → M(s)one mole of M(s) will be produced if n moles of electrons are supplied.
BASIC CONCEPT
One Faraday is equivalent to the amount of charge contained in one mole of electrons.The number of moles of electrons needed to produce a certain amount of M(s) can now be related
to a measurable electrical property. One electron carries a charge of 1.6 × 10−19 coulombs (C). The
charge carried by one mole of electrons can be calculated by multiplying this number by Avogadro’s
number, as follows: