Electrolysis is a process of separating the bonded elements by passing electric current through them. It is very important to understand the principles on which electrolysis depends. The process uses a direct electric current (DC) to drive the non-spontaneous chemical reaction. It is widely used on many industries such as metallurgy and some chemical products where an electrolytic cell is used to separate closely bonded compounds. To perform the electrolysis, a specially-designed electrolytic cells are used. An electrolytic cell has a positive cathode and a negatively charged anode along with a solution of electrolyte. Electrolytic cells must be connected to an external source such as a battery that supplies electricity as electrolysis won't happen spontaneously. Once a specific amount of electricity is passed through the electrolyte solution, the charged electrodes react with the solution called oxidation-reduction reaction. The cathodes reduces positive ions by adding electrons into them, which creates neutral atoms. At the same time anode oxidizes negative ions to produce more free electrons. Hence, the half reactions can be written as:
Oxidation half of the reaction: X- → X + e-
Reduction half of the reaction: Y+ + e- → Y
Prerequisites to achieve electrolysis
Three important elements essential for electrolysis are the electrolyte, DC (direct current and 2 electrodes. Electrolyte must contain free ions that help in carrying electric current in the electrolyte. A direct current (DC) supply aids in providing the energy necessary to create or discharge the ions in the electrolyte. The 2 electrodes provides the physical interface between the electrical circuit providing the energy and the electrolyte.
Electrolysis of common compounds
Electrolysis of sodium chloride
Electrolysis of molten sodium chloride
Both sodium and chlorine are important elements. Sodium is mainly used in sodium vapor lamps and as the coolant in nuclear reactors. These two elements are produced when pure molten sodium chloride is electrolysed. Chlorine gas comes off at the anode and molten sodium deposited at the cathode. Sodium deposited remains in a liquid state due to its low melting point of 97.8℃. The electrolytic cell in which this process is carried out is known as the Down’s cell. The cell is uniquely designed in a manner that more freshly sodium chloride is added and be processed through electrolysis. The half reactions and overall cell reaction for this electrolysis are as follows:
NaCl → Na +(l) + Cl–(l)
At cathode: reduction of 2Na+(l) + e– → Na(l)
At anode: oxidation of 2Cl–(l) → Cl2(g) + 2e–
Net Reaction: 2Na +(l) + 2Cl–(l) → 2Na(l) + Cl2(g).
Electrolysis of Aqueous Sodium Chloride
Sodium chloride gets dissociated to release sodium and chloride ions in aqueous solution.On the other hand, water itself can undergo reduction and oxidation reactions at different potentials. Therefore, along with sodium and chloride, electrolysis of water also takes place . Water can be reduced to hydrogen gas and sodium ions reduced to sodium metal.
2H2O(l) + 2e– → H2(g) + 2OH– E° = -1.0 V
Na+(l) + e– → Na(l) E° = -2.71V
At anode: Oxidation reaction: at pH =7. Water can be oxidized to oxygen or chloride ion oxidized to chlorine molecule.
2H2O → O2(g) + 4H+ + 4e– E° =-1.42 V
2Cl– → Cl2 + 2e– E =- 1.36V
Hence the product of electrolysis of aqueous sodium chloride can be sodium metal, or hydrogen gas at the as well as cathode and chlorine or oxygen gas at the anode.
Electrolysis of Brine (Concentrated Sodium chloride)
If the electrolyte in aqueous solution is more easily oxidized or reduced than water, then the products of electrolysis will be substances other than hydrogen and oxygen. Electrolysis of concentrated sodium chloride is a good example which simultaneously produces sodium hydroxide, chlorine gas, and hydrogen gas. In this process, chloride ions are oxidized to produce chlorine gas at the anode. At the cathode, hydrogen gas is produced through reduction of water. Na+ ions are not reduced to sodium metal because water molecules are easily reduced than sodium ions. Other than hydrogen gas, hydroxide ions are also produced at the cathode. The half-reactions are:
At the anode: 2Cl_(l) ⟶ Cl2(g) + 2e-
At the cathode: 2H2O(l) +2e-⟶H2(g)+ 2OH-(aq)
Electrolysis of acidified water or disulphuric acid
The small amount of sulphuric acid is added to water to make water acidified or acidulated. The ions that can easily lose the electrons discharge their free electrons. The reactions can be written as: At cathode: H2O+2e−→H2+OH− At anode: 2H2O→O2+4H++4e− Net reaction: 2H2O→2H2+O2