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

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.

Electrolysis of copper sulphate

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.

Hcl is a strong electrolyte

NaCl is a strong electrolyte

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.

acetic acid is a weak electrolyte

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.

1 comment:

Rashid Mehmood said...

Very good post. keep it up..