How does electrolysis work?

May 26th, 2007
There are several misconceptions about electrolysis, such as the idea that electricity flows across the electrolysis cell.
The conditions required for electrolysis are an electrolyte with ions that are free to move. They may be in solution or in the form of a molten salt. There must then be an electrical potential applied by means of electrodes across the electrolyte. This electrical potential creates a large electrostatic force field that attracts the charged ions.
The actual charges on the ions are what attracts them to the electrodes (that are charged electrically) the positive ions migrate to the negative electrode (the cathode) and the negative ions migrate to the positive electrode (the anode).
When a negative ion arrives at the positive electrode it has its extra electron stripped off by the electrode. This electron continues on to the battery just like normal electricity (all electrons are, of course, indistinguishable)
Cl- –> Cl + 1e
When a positive ion arrives at the negative electrode it finds billions of electrons just waiting to fill up its positive ‘hole’. One of these extra electrons jumps onto the positive ion cancelling it out.
Na+ + 1e –> Na
The overall effect of these two processes, or electrode reactions, is that one electron has left the cathode and one electron has arrived at the anode. From the point of view of the battery and external circuit one electron has left the negative side and has arrived at the positive side. i.e. a current has flowed around the circuit.
Nothing actually flows across the electrolysis cell, the electrons are absorbed by one electrode and produced at the other by two different reactions. For these reactions to be able to occur the ions in the cell must be able to move, i.e. in solution or molten.
The net result for the electrolyte is that the sum of the two electrode reactions has happened in the cell.
2Cl- –> Cl₂ + 2e
2Na+ + 2e –> 2Na
———————————————–
2Cl- + 2Na+ –> 2Na + Cl₂
or
NaCl(l) —-> 2Na(l) + Cl₂(g)
The chlorine reaction was doubled up to show that chlorine is released as Cl₂ molecules. The sodium is double up to equalise the charge. Under the conditions of electrolysis of molten sodium chloride the sodium is released as a liquid.