Answer to Question #331481 in General Chemistry for Eva

Question

describe how chlorine gas is produced in electrolysis

Answer 1

Salt is widely used in the food industry as a preservative and flavour enhancer. It is vital for human health - we need sodium in our diet to allow our bodies to carry out essential functions. However, too much salt in the diet is associated with high blood pressure.

Sodium chloride is the raw material for the manufacture of hydrogen, chlorine and sodium hydroxide. Electrolysis is used. The electrolysis of sodium chloride solution (brine) is an important industrial process because hydrogen, chlorine and sodium hydroxide have many uses.

Electrolysis of sodium chloride solution

When an electric current is passed through concentrated sodium chloride solution, hydrogen gas forms at the negative electrode, chlorine gas forms at the positive electrode, and a solution of sodium hydroxide also forms.

You might have expected sodium metal to be deposited at the negative electrode. However, sodium is too reactive for this to happen so hydrogen is given off instead.

During electrolysis:

·        hydrogen ions H⁺(aq) (from the water) are discharged at the negative electrode as hydrogen gas, H₂(g)

·        chloride ions Cl^–(aq) (from the dissolved sodium chloride) are discharged at the positive electrode as chlorine gas, Cl₂(g)

·        sodium ions Na⁺(aq) (from the dissolved sodium chloride) and hydroxide ions OH^–(aq) (from the water) stay behind - they form sodium hydroxide solution, NaOH(aq)

Electrolysis of sodium chloride solution

 

The half-equations

A half-equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e^–. These are the half-equations for the electrolysis of sodium chloride solution:

At the cathode (negative electrode), the H⁺ cations are reduced when they gain electrons: 2H⁺(aq) + 2e^- → H₂(g)

At the anode (positive electrode), the Cl^- anions are oxidised when they lose electrons: 2Cl^-(aq) → Cl₂(g) + 2e^-

The chlor-alkali industry

The chlor-alkali process is the electrolysis of sodium chloride solution on an industrial scale. Concentrated sodium chloride solution is electrolysed in a diaphragm cell as shown in the diagram.

Products from the chlor-alkali industry

The three products of the electrolysis of concentrated sodium chloride solution have important uses in the chemical industry:

·        hydrogen is used as a fuel and for making ammonia

·        chlorine is used to sterilise water supplies, and to make bleach and hydrochloric acid

·        sodium hydroxide is used to make soap, paper and bleach