Video Transcript
What equation shows the reaction at
the cathode during the electrolysis of potassium chloride solution using inert
electrodes?
In this question, we need to
predict the products of the electrolysis of a salt solution, specifically potassium
chloride, which is KCl.
Electrolysis is the process of
using electricity to break chemical bonds and separate a substance into its
constituent parts. Here is a picture of an
electrolytic cell. It consists of a power source,
which is a direct current power supply. And it has two different
electrodes, the anode and the cathode. When the dc power supply is
switched on, the anode will become the positive terminal and the cathode the
negative terminal. These terminals will attract the
positive and negative ions found in salt solutions, or in molten salts.
Potassium chloride is an ionic
compound that can dissociate into its constituent ions, K+ and Cl−, as shown. In addition, water will ionize into
H+ ions and OH− ions. The anions, which are negatively
charged ions, are attracted to the positively charged anode and will migrate towards
the anode, where they are oxidized. In other words, the anions lose
electrons to the positive anode. The cations, which are positively
charged ions, are attracted to the negatively charged cathode and will migrate
towards the cathode, where they are reduced. In other words, the cations gain
electrons from the cathode.
We are asked what reaction occurs
at the cathode. We may be tempted to think that
solid potassium forms at the cathode. But remember, there are hydrogen
ions in solution too. And these cations compete with the
potassium cations at the cathode.
In order to work out which of these
positive ions are reduced, we can use the reactivity series. We need to look at where potassium
lies in relation to hydrogen. The elements above hydrogen in the
list are more reactive than hydrogen. And we can see that potassium is
more reactive than hydrogen. The more reactive an element is,
and thus the higher the element is on the list, the more easily it is oxidized. And the less reactive an element
is, and the lower it is on the list, the less easily it is oxidized and the more
readily it is reduced.
So, when potassium ions and
hydrogen ions compete for electrons at the cathode, hydrogen ions will be reduced
more readily than potassium ions. So hydrogen ions will be reduced in
preference to potassium ions. Hydrogen ions will gain electrons
and be converted into hydrogen gas. So the correct reaction at the
cathode will be the production of hydrogen gas from its ions. Two H+ plus two electrons yields
H2.
