The concept of reversing the direction of the spontaneous reaction in a
galvanic cell
through the input of electricity is at the heart of the idea of
electrolysis. See
for a comparison of galvanic and electrolytic
cells. If you
would like to review your knowledge of galvanic cells (which I strongly
suggest) before
learning about electrolytic cells, click
here.
Figure %: Comparison of Galvanic and Electrolytic Cells
Electrolytic cells, like galvanic cells, are composed of two
half-cells--one is a
reduction half-cell, the other is an oxidation half-cell. Though
the direction of
electron flow in electrolytic cells may be reversed from the direction of
spontaneous
electron flow in galvanic cells, the definition of both cathode and
anode remain
the same--reduction takes place at the cathode and oxidation occurs at the
anode. When
comparing a galvanic cell to its electrolytic counterpart, as is done in
, occurs on the right-hand half-cell. Because the
directions
of both half-reactions have been reversed, the sign, but not the magnitude,
of the cell
potential has been reversed. Note that copper is spontaneously plated
onto the copper
cathode in the galvanic cell whereas it requires a voltage greater than
0.78 V from the
battery to plate iron on its cathode in the electrolytic cell.
You should be asking yourself at this point how it is possible to make a
non-spontaneous
reaction proceed. The answer is that the electrolytic cell reaction is not
the only one
occurring in the system-the battery is a spontaneous redox reaction. By
Hess's
Law, we can sum the ΔG of the battery and the
electrolytic cell to arrive at the ΔG for the
overall
process. As long as that ΔG for the overall
reaction is
negative, the system of the battery and the electrolytic cell will continue
to function. The
condition for ΔG being negative for the
system (you
should prove this for yourself) is that Ebattery is greater than -
Ecell.
Electrolysis of Water
During the early history of the earth, hydrogen and oxygen gasses
spontaneously reacted to
form the water in the oceans, lakes, and rivers we have today. That
spontaneous direction
of reaction can be used to create water and electricity in a galvanic cell
(as it does on the
space shuttle). However, by using an electrolytic cell composed of water,
two electrodes
and an external source emf one can reverse the direction of the process
and create
hydrogen and oxygen from water and electricity.
shows a setup
for the electrolysis of water.
Figure %: Setup for the Electrolysis of Water
The reaction at the anode is the oxidation of water to O2 and
acid while the
cathode reduces water into H2 and hydroxide ion. That reaction
has a
potential of -2.06 V at standard conditions. However, this process is
usually performed
with [H+] = 10-7 M and [OH-] = 10-
7 M, the concentrations of hydronium and hydroxide in pure water.
Applying the
Nernst Equation to calculate the potentials of each half-reaction, we find
that the potential
for the electrolysis of pure water is -1.23 V. To make the electrolysis of
water occur, one
must apply an external potential (usually from a battery of some sort) of
greater than or
equal to 1.23 V. In practice, however, it is necessary to use a slightly
larger voltage to get
the electrolysis to occur on a reasonable time scale.
Pure water is impractical to use in this process because it is an
electrical insulator. That
problem is circumvented by the addition of a minor amount of soluble salts
that turn the
water into a good conductor (as noted in ). Such
salts have
subtle effects on the electrolytic potential of water due to their ability
to change the pH of
water. Such effects from the salts are generally so small that they are
usually ignored.
