The electrolytic process involves passing an electric current through an electrolyte between two conductive electrodes called the anode and the cathode. The changes occur at the electrodes.
When we connect the electrodes to a power source (direct current generator), the electrode connected to the positive terminal of the generator is the anode, and the electrode connected to the negative terminal of the generator is the cathode.
An electrolysis reaction can be considered as the combination of two half-reactions: an anodic oxidation and a cathodic reduction.
To explain the reactions at the electrodes, we will consider molten sodium chloride, since it contains only two types of ions. Inert electrodes are used; this means they do not react chemically with sodium and chloride ions.
Sodium ions (+), or cations, are attracted to the negative electrode (cathode). The cathode becomes negative due to the action of the power source, which supplies it with electrons.
The electrons at the cathode are in a state of high potential energy. The sodium ion has a positive charge, which means it attracts electrons, and an electron from a sodium atom would have lower potential energy than an electron at the cathode. Therefore, the electrons at the cathode move toward the cation due to the difference in potential energy. At the cathode, sodium ions become sodium atoms through the addition of an electron. This is a chemical change and can be represented by the following equation:
Na+ + e− → Na0
This chemical reaction represents an electron gain; therefore, the sodium was reduced and transitioned to a metallic state. Consequently, the chemical reaction that always occurs at the cathode is a reduction reaction.
The anode is positive because the power 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. When the chloride ions reach the anode, they supply electrons to it. The electrons move from a state of high potential energy to one of low potential energy. The reaction occurring 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.
Oxidation and reduction reactions occur simultaneously but separately, as they take place at different locations. The power source does not produce electrons; it merely transports them from one place to another. Thus, the electrons that the power source supplies to the cathode come from the anode. The function of the power source is to increase the potential energy of the electrons at the cathode.
These electrode reactions are called half-reactions, and the overall reaction for the electrolysis of sodium chloride is:
2Na+ + 2Cl− → 2NaO + Cl2
The nature of the reactions at the electrode depends on the applied potential difference or voltage.
Source: FES-CUAUTITLÁN Felipe Díaz del Castillo R., M.A.