Theory of Electrolysis

The phenomenon of the passage of the electric current through liquids and their decomposition into ions as a result of it is called the chemical effect of electric current.

Some important terms

1. Electrolysis: it is the process of decomposition of an electrolyte into its constituent ions by the passage of electric current through it.
2. Voltameter: The vessel in which electrolysis is carried out is called voltameter which is also called the electrolytic cell.
3. Electrodes:The metal rods or plates which are partially dipped in the electrolyte solution for passing the current through it are called electrodes.
4. Anode: The electrode connected to the positive terminal of an external battery is called anode.
5. Cathode: The electrode connected to the negative terminal of an external battery is called cathode.
6. Ionization: The process of decomposition of a compound into its constituent ions is called ionization.
7. Anions: The negatively charged ions which move towards the anode during electrolysis are called anions.
8. Cations: The ions which carry a positive charge and move towards cathode during electrolysis are called cations.
9. Chemical equivalent: It is the ratio of atomic mass and valency of a substance,
   $$ \text {chemical equivalent} = \frac {\text {atomic mass}}{\text {valency}} $$

Theory of Electrolysis

According to the ionic dissociation theory explained by Arrhenius, the molecules of an electrolyte exist in the form of ions even if the electrolyte is in the solid state. For example a common salt molecule (NaCl) exists in the form of ions even in the crystalline state i.e. \(NaCl \rightarrow Na^+ + Cl^- \). These ions are held together by the electrostatic force of attraction i.e. \( F = \frac {1}{4\pi \epsilon _0 \epsilon_r}\frac {Q_1Q_2}{r^2} \) where Q₁ and Q₂ are charged on two ions, r is the distance between them and ϵ_r be the relative permittivity of the medium between them.

When NaCl is dissolved in water (ϵ_r » 80), the force between them is lesser (1/80 times ) so they become free from each other’s attraction and, therefore, they get separated. When a potential difference is applied across the electrolyte, the cations, (Na⁺) moves towards cathode and anions (Cl^-) move towards the anode. One reaching to the respective electrodes, the ions get discharged and then appears as deposits on the electrodes or get liberated as a free gas. This explains the conduction of electric current through an electrolyte.

The electrolyte conduct electricity, but their conductivity is very low than that of good conductor due to following reasons:

1. Ions have very large mass compared to electrons. So they drift slowly under an electric field.
2. A number of ions per unit volume in an electrolyte is much less than the number of free electrons in a metallic conductor.
3. The electrolyte solution is dense and disordered and, therefore, ions have difficulty to drift through it.

Some Common Voltameters

Voltameter is the vessel in which electrolysis is carried out. Following are the most common voltameter:

1. Copper voltameter:

   A copper voltameter consists of a glass vessel containing an aqueous solution of copper sulphate as electrolyte and two copper plates as electrodes. A steady voltage is applied between the two electrodes.
   Reaction: CuSO₄ ionises in its aqueous solution as follows:
   $$ CuSO_4 \rightleftharpoons Cu^{++} + SO_4^{--} $$
   At the cathode:
   The Cu⁺⁺ ions drift towards the cathode and are neutralized by the electrons flowing in from the negative terminal of a battery
   $$ Cu^{++} + 2e^- \rightarrow Cu $$
   
   At the anode:
   Cu dissolves into the solution producing Cu⁺⁺ and 2e^-, the electrons flow to the positive terminal of the battery.
   $$ Cu \rightarrow Cu^{++} + 2e^- $$

2. Silver Voltameter:

   A silver voltameter consists of a glass vessel provided with silver electrodes. Silver electrodes are an aqueous solution of silver nitrate. A steady voltage is applied between the two electrodes.
   Reactions: Silver nitrate ionizes in its aqueous solution as follows:
   $$ AgNO_3 \rightarrow Ag^+ + NO_3^-$$
   At cathode: The Ag⁺ ions drift towards the cathode and the cathode is neutralized by the electrons flowing in from the negative terminal of the battery.
   $$ Ag^+ + e^- \rightarrow Ag $$
   
   At the anode: Ag dissolves into the solution producing Ag⁺ and e^-, the electrons being flowing to the positive terminal of the battery.
   $$Ag \rightarrow Ag^+ + e^- $$

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, et al.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.