When electricity is passed through an ionic solution, often a reaction happens. This type of reaction is called electrolysis and is an example of a redox reaction. These reactions are important because they are used in the production of many chemicals and it is also the way metals are plated.
Electric Current
In order to understand what is going on we need to understand what an electric current is. In an electric circuit current flows when there is a movement of electrons (tiny negatively charge particles) in the wires. If we say that electric current flows when there is a movement of charged particles, of which electrons are an example, we will understand what happens in electrolysis.
Ionic Solution
Electrolysis is carried out by putting two electrodes, attached to a power source, into a solution. An ionic solution contains two types of charged particles, positive and negative. For example, in a solution of Sodium Chloride (table salt) there are positive sodium ions and negative chloride ions.
Anode and Cathode
The power source attached to the electrodes can be thought of as an electron pump, causing electrons to move along the wires. This means that one of the electrodes will be positive because electrons are being pumped away from it; this is called the Anode. The other electrode has electrons being pumped into it so it is negatively charged; this is called the Cathode.
Attraction of Ions
Now we can understand that when these electrodes are placed in a solution the ions will be attracted to one electrode or the other. The negatively charged particles will move towards the anode (positive electrode) and the positively charged ions will be attracted to the cathode (negative electrode).
Reaction at Cathode
What happens when the ions arrive at the electrodes? There is an exchange of electrons, so there will be either reduction or oxidation taking place. At the cathode there are extra electrons on the surface. These are taken up by the positive ions and they are reduced, often to the element. So, for example, when a solution containing copper (2+) ions is electrolysed, copper metal is deposited on the cathode, plating in with copper.
Reaction at Anode
At the anode there is a different reaction. The anode has a lack of electrons, so it pulls the electrons off the negative ions, so oxidising the negative ion to its neutral state. So, for example, chloride (1-) ions are oxidised to chlorine gas which bubble off the anode.
Unusual Case
Things are not quite as simple as this, however, because when sodium chloride is electrolysed chlorine is produced at the anode, as expected. At the cathode, however, another gas is given off, and there is no sign of sodium metal as one would predict. This is because in a solution we have to take into account the presence of water. Some water molecules in a solution will have broken down into hydrogen ions (H+) and hydroxide ions (OH-). So at the cathode the sodium ions have to compete with hydrogen ions for the electrons. Hydrogen ions always take the available electrons when they compete with sodium, forming hydrogen gas at the cathode, but copper ions win the day in a copper ion solution, due to differences in reactivity.
An understanding of electrochemistry will help us when we consider such things as metal extraction and how batteries work.
