Question

# Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for the redox change taking place in water.

Hint: Redox reaction is a type of chemical reaction in which change of oxidation states of atoms takes place. In such a reaction one atom is oxidized while another is reduced. By oxidation we mean loss of electron and by reduction we mean gain of electron.

Chlorine has the ability to kill pathogens such as bacteria and viruses. Due to which it possesses disinfectant properties. Chlorine is used in drinking water to kill germs as it breaks their bonds with the water molecules. When chlorine is added to water hypochlorous acid and hypochlorites are formed. When excess of chlorine is added concentration of hypochlorous acid and hypochlorites increases which is harmful for consumption. Due to which excess of chlorine is harmful in drinking water. This excess chlorine is removed by treating with sulphur dioxide. Reaction that will take place is:
$C{l_2} + S{O_2} + 2{H_2}O\xrightarrow{{}}2C{l^ - } + SO_4^{2 - } + 4{H^ + }$
This is the balanced redox reaction that will take place.
Steps for balancing this redox reaction are as follows
$C{l_2}(0) + S{O_2}( + 4)\xrightarrow{{}}C{l^ - }( - 1) + SO_4^{2 - }( + 6)$
Reduction reaction will be
$C{l_2} + 2{e^ - }\xrightarrow{{}}2C{l^ - }$
Oxidation reaction will be
$S{O_2}( + 4) + 2{H_2}O\xrightarrow{{}}SO_4^{2 - }( + 6) + 2{e^ - } + 4{H^ + }$
On adding these reactions we will obtain our final balanced reaction that is
$C{l_2} + S{O_2} + 2{H_2}O\xrightarrow{{}}2C{l^ - } + SO_4^{2 - } + 4{H^ + }$

Additional information: Sulphur dioxide is also used in production of sulphuric acid, sulphur trioxide and sulphides. It is used as a disinfectant, reducing agent, bleach and food preservative as well.

Note:
Other than purifying drinking water chlorine is also used as an antiseptic and in treating swimming pools. Its industrial uses include production of paper, plastic, dyes, textiles, medicines, antiseptics, insecticides etc.