The electrolytic process consists of passing an electric current through an electrolyte, between two conductive electrodes called anode and cathode. Where the changes occur at the electrodes.
When we connect the electrodes to a power source (direct current generator), the electrode that is attached to the positive pole of the generator is the anode and the electrode that is attached to the negative pole of the generator is the cathode.
An electrolysis reaction can be considered as the set of two half-reactions, an anodic oxidation and a cathodic reduction.
To explain the reactions at the electrodes, molten sodium chloride will be considered, because it contains only two types of ions. Inert electrodes are used, which means that they do not react chemically with the sodium and chloride ions.
The sodium (+) ions, or cations, are attracted to the negative electrode (cathode). The cathode is made negative by the action of the source pumping electrons to it.
The cathode electrons are in a high potential energy state. The sodium ion is positively charged, this means that it attracts electrons and that an electron from a sodium atom would have a lower potential energy than an electron from the cathode. Therefore the cathode electrons move towards the cation, due to the potential energy difference. At the cathode the sodium ions become sodium atoms by addition of an electron. This is a chemical change and can be represented by the following equation:
Na+ +e- →Na0
This chemical change represents a gain of electrons, so the sodium was reduced and went to the metallic state, consequently, the chemical change that always occurs at the cathode is a reduction.
The anode is positive because the source pumps electrons out of it and also attracts chloride (-) ions or anions. At the anode the electrons have low potential energy. In contrast, the outer electrons of the chloride ion are in a high potential state.
high potential state. When the chloride ions arrive at the anode, they provide electrons to the anode. The electrons move from a high potential energy state to a low potential energy state. The change at the anode can be represented by another equation:
2Cl- →Cl2 +2e-
Chloride ions lose electrons and are transformed into chlorine atoms, which in turn form chlorine gas molecules. The anodic reaction is always an oxidation reaction.
The oxidation and reduction reactions occur simultaneously, but separately, since they occur at different points. The source does not produce electrons, it only transports them from one place to another, so the electrons that the source supplies to the cathode come from the anode. The function of the source is to raise the potential energy of the electrons at the cathode.
These electrode reactions are called semi-reactions, and the overall reaction of sodium chloride electrolysis is:
2Na+ +2Cl- →2Na0 +Cl2
The nature of the electrode reactions depends on the applied potential difference or voltage.
Source: FES-CUAUTITLÁN Mtro. FELIPE DÍAZ DEL CASTILLO R.