Electrolysis of some common electrolytes

Let us study various characteristics of electrolysis of some common electrolytes.

Sodium Chloride (Fused)

Lead Bromide (Fused)

Water Acidified with Sulphuric Acid

1. The electrolytic cell is set up as shown in the figure 4.10.

2. Pure water is a non-electrolyte and it ionises to a negligible extent. Out of about 10 million molecules of water, just about one molecule ionises as follows to establish an equilibrium as follows:

If these two ions i.e., H⁺ and OH^- recombine to form a molecule of water, then another molecule will ionise. Even if the H⁺ and OH^- are removed from the electrolyte, then another molecule of water will ionise.

Thus though water is a non-electrolyte, it can be electrolytically decomposed by removing the H⁺ and OH^- ions continuously. This is done with the help of sulphuric acid, which dissociates as follows:

A mixture of sulphuric acid and water together yield the following ions:

Cations: H⁺ only.

Anions: OH^- and

• As there is only one kind of cation i.e., H⁺, hydrogen is formed at the cathode.

• At the anode, there are two kinds of ions i.e., and OH^-. The hydroxyl ions being much lower than sulphate ions in the electrochemical series will get discharged yielding water and oxygen. The sulphate ion though it may migrate to the anode, remains un-affected, hence is called spectator ion.

As per the equations given above, at the cathode 4H⁺ ions are required and at the anode 4OH^- are needed. In other words, four molecules of water are needed for the reactions. But during the reaction at the anode two molecules of water are also produced. Hence the total quantity of water used is only 2 molecules. It may be represented as shown:

For every one molecule of oxygen formed, two molecule of hydrogen are formed. Thus the volume of hydrogen formed at cathode is double than that of the oxygen formed at the anode.

Aqueous Copper Sulphate Between Copper Electrodes

In the electrolysis of aqueous copper sulphate between copper electrodes the anode is made up of a thick block of impure copper, and the cathode is made up of a thin sheet of pure copper (Fig.4.11).

The reactions that take place are:

Though H⁺ and Cu²⁺ are present as cations, the copper ions, being lower in the electrochemical series, get discharged first. Hydrogen ions remain as spectator ions.

The situation at the anode is entirely different. The two anions migrate to the anode, but neither of them gets discharged. Instead, the copper of the anode dissolves in the solution producing copper ions.

Electrolysis by this type of process is used for refining impure copper.

For every copper ion discharged at the cathode, a copper ion is released into the solution at the anode. The sulphate and the hydroxyl ions remain as spectator ions. The total number of copper and sulphate ions remain constant throughout. Assuming that no evaporation takes place, the blue color of the copper sulphate is maintained throughout.

Aqueous Copper Sulphate Between Copper Cathode and Platinum Anode

During the electrolysis of aqueous copper sulphate using copper anode, copper ions are liberated there. But if instead of copper the anode is made of platinum, the anode does not take part in the reaction. Instead the hydroxyl ions get discharged.

The reactions that take place are:

Of the four kinds of ions present, Cu²⁺ and OH^- get discharged. As more and more OH^- ions are lost, the H⁺ ion concentration increases. The ions do not get discharged at all. So the net total effect is that H⁺ ions and ions are left behind as spectator ions. In other words, as the reaction proceeds more and more sulphuric acid is being formed. So the blue color of the copper sulphate becomes lighter and lighter, and finally the solution becomes colorless.