Halogen element - Production and use

Authors Avatar

from halogen element 

Production and use

Rock salt deposits are usually mined; occasionally water is pumped down and , containing about 25 percent sodium chloride, is brought to the surface. When the brine is evaporated, impurities separate first and can be removed. In warm climates salt is obtained by evaporation of shallow seawater by the sun, to give bay salt.

Chlorine is produced on a large scale by any of a number of different methods:

1. By electrolysis of a concentrated solution of sodium chloride in water. Hydrogen is evolved at the cathode, and chlorine at the anode. At the same time, an alkali metal hydroxide is produced in the electrolyte, and hence this process is often referred to as chlorine-alkali-electrolysis.

The chemical reactions that take place at each electrode and the overall cell process are given in the following equations:

in which the symbol e- represents a single electron. In the reaction vessel, free chlorine and hydroxide ions must not come in contact with each other, because chlorine would be consumed according to the reaction

To accomplish the separation of chlorine gas and the hydroxide ion, a porous wall is inserted between the electrodes (diaphragm process), or the iron cathode is replaced by a cathode consisting of liquid mercury (mercury cathode process) which avoids the production of hydroxide ions at the electrode. Instead, free sodium is discharged at the cathode, and this metal is readily dissolved in the mercury forming an amalgam, as follows:

Join now!

The amalgam is allowed to react with water outside the cell:

The overall process is equivalent to the cell process given above.

2. By electrolysis of fused sodium chloride, which also produces metallic sodium; chlorine is again evolved at the anode.

3. By electrolysis of fused magnesium chloride, in which chlorine is formed as a by-product in the manufacture of metallic magnesium.

4. By oxidation of hydrogen chloride. In this process gaseous hydrogen chloride mixed with air or oxygen is passed over pumice in contact with cupric chloride as a catalyst, as shown in the following equation:


This is a preview of the whole essay