Electrolysis; strong and weak electrolyte

Electrolysis

Electrolysis is the dissociation of a electrolyte into ions at the electrodes by the passage of electric current.

That is, it is the conduction of electric current through an electrolyte together with the resulting chemical changes.

Electrolysis is carried out in an apparatus called voltameter or electrolytic cell. It consists of a glass vessel containing an electrolyte and 2 metal plates called electrodes, connected to a battery. The electrode connected to the positive terminal of the battery is called anode, and the one connected to the negative terminal is called cathode.

Electrolysis of Copper Sulphate solution

Electrolysis of copper sulphate solution

It is found that copper is removed from the anode and is deposited on the cathode. Due to ionization, the CuSO₄ solution is dissociated.

When the source of emf is connected, a steady current flows in the circuit. Then, the following happen

1. Electrons flow from the negative terminal of the battery via the wire to the cathode C.

2. Electrode C is at a lower potential than electrode A. Therefore, the Cu²⁺ ions move towards C, while the ions move towards A.

3. At the cathode C, the following reduction reaction occurs

These Cu atoms get deposited on the cathode.

4. At the anode A, the following oxidation reaction takes place. The Cu atoms are from the anode.

5. The Cu²⁺ ions go into the solution. The released electrons flow back to the positive terminal of the battery via the wire.

Thus, copper gets deposited at the cathode, while the anode loses an equivalent amount of copper. The concentration of CuSO₄ in the solution remains unchanged.

Electrolysis of Silver Nitrate Solution

Here, the electrolyte is AgNO₃ and the electrodes are silver plates. The process of electrolysis is identical to that of CuSO₄ solution, except for one important difference. Copper has valency two, while silver has valency one. The reactions at the electrodes are

So, silver gets deposited at the cathode, while the anode loses an equal amount of Ag. The concentration of AgNO₃ in the solution stays the same.
In the electrode position of silver, one electron circulated for depositing one silver atom on the cathode; but in the case of copper, two electrons circulate for the deposition of one copper atom.

Types of electrolytes

There are three types of electrolytes: Strong, weak and non electrolytes.

Strong electrolytes

A strong electrolyte is one which undergoes complete ionization when dissolved in water. The solution contains only the ions and not molecules.

Examples:

HCl, HNO₃, H₂SO₄, NaOH, Ca(OH)₂, NaCl, KCl, CH₃COONa etc.

Weak electrolytes

A weak electrolyte is one which undergoes partial ionization or dissociation. Here, in solution the ions and the dissociated molecules will be in equilibrium with each other. When such a solution is diluted, the degree of ionization increases. It becomes complete at infinite dilution.

Examples:

HCOOH, CH₃COOH, NH₄OH, CH₃NH₂, CH₃COONH₄, H₃PO₄ etc.

Non-electrolytes

A substance which doesn't allow the electric current to pass through is called a non electrolyte. These substances have no ions. Therefore they do not ionize in water. (Covalent compounds).

Examples: C₆H₆, toluene, sugar, urea, CH₄, C₂H₆ etc.