3.2.5 Group 7(17), the Halogens - Uses of chlorine and chlorate(I)

Specification

The reaction of chlorine with water

Chlorine is a pale green toxic gas that dissolves in water giving a pale green solution. However, chloride is very reactive and undergoes reaction with the water molecules forming a mixture of hydrochloric acid and hypochlorous acid (chlorate(I) acid). This solution is usually called chlorine water:

Cl₂ + H₂O HCl + HOCl

This is an example of a disproportionation reaction in which the chlorine gets simultaneously oxidised and reduced.

Chlorine as an element is in the zero oxidation state. In hydrogen chloride it is in the -1 oxidation state and in chlorate(I) acid it is in the +1 oxidation state.

1 1/2Cl₂ + 1e Cl^-

21/2Cl₂ + H₂O HOCl + H⁺ + 1e

The first equation shows chlorine being reduced and the second shows chlorine being oxidised:

If chlorine water is tested with indicator paper, it first registers the red colour of the hydrochloric acid, but then is rapidly decolourised by the action of the hypochlorous acid, which is a good bleaching agent.

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Water treatment

The poisonous nature of chlorine is taken advantage of in water treatment. Humans are tolerant to small doses of chlorine, while small microorganisms are killed at even low chlorine concentrations.

This allows water treatment plants to add chlorine to the process, killing microorganisms and making the water fit for human consumption.

Ozone gas is also used for water treatment, but it has the disadvantage of being difficult to store and more expensive. It does however have the advantage of not adding taste to the water.

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The reaction of chlorine with cold, dilute, aqueous NaOH

As a solution of chlorine water is a mixture of acids it is logical that it should react with bases. Sodium hydroxide neutralises chlorine water forming salts from the two acids present:

2NaOH + HOCl + HCl NaCl + NaOCl + 2H₂O

If chlorine is bubbled through cold sodium hydroxide solution a different reaction occurs:

3Cl₂ + 6NaOH --> NaClO₃ + 5NaCl + 3H₂O

This is an example of a disproportionation reaction in which the chlorine is simultaneously oxidised and reduced.

Chlorine as an element is in the zero oxidation state. In sodium chlorate(V) it is in the +5 oxidation state and in sodium chloride it is in the -1 oxidation state.

The two half equations representing each process are:

1/2Cl₂ + 1e Cl^-

1/2Cl₂ + 6OH^- ClO₃^- + 3H₂O + 5e

To add the two half-equations together the electrons must first be equalised by multiplying the first equation by five:

5/2Cl₂ + 5e 5Cl^-

1/2Cl₂ + 6OH^- ClO₃^- + 3H₂O + 5e

---------------------------------------------------- add

3Cl₂ + 6OH^- ClO₃^- + 3H₂O + 5Cl^-

Sodium chlorate(V) is a useful weedkiller and oxidising agent.

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