Disproportionation reactions

Disproportionation reactions occur when a substance is simultaneously oxidised and reduced in the same chemical reaction.

Disproportionation occurs if the oxidation number of the same element both increases (oxidation) and decreases (reduction) within the same reaction.

For example, when chlorine gas reacts with water, it undergoes the following disproportionation reaction:

Cl_2(g) + H₂O_(l) ➔ HCl_(aq) + HClO_(aq)

Here, the oxidation number of chlorine increases from 0 to +1 in HClO and decreases to -1 in HCl, showing it has undergone disproportionation.

Using chlorine to disinfect drinking water

When chlorine gas is bubbled through water, disproportionation occurs:

Cl_2(g) + H₂O_(l) ➔ HCl_(aq) + HClO

The hypochlorous acid (HClO) then dissociates:

HClO ➔ H⁺_(aq) + ClO^-

These hypochlorite ions (ClO^-) act as a disinfectant, killing microorganisms in the water to make it safe to drink.

In sunlight, chlorine gas react with water to form hydrochloric acid and oxygen gas:

Cl_2(g) + 2H₂O_(l) ➔ 2HCl_(aq) + 1⁄2O_2(g)

This reaction depletes the hypochlorite ion disinfectant, meaning swimming pools require regular additions of chlorine to maintain effective disinfection.

Benefits and risks of using chlorine for water disinfection

Some benefits and risks associated with using chlorine to disinfect drinking water supplies are outlined below:

Benefits

• Kills disease-causing microorganisms.
• Prevents reinfection further down the supply.
• Removes bad tastes, smells and discolouration.

Risks

• Chlorine gas causes severe irritation if inhaled.
• Reacts with organics to form carcinogenic chlorinated hydrocarbons.

Due to some of the risks of using chlorine, alternatives such as ozone and ultraviolet light are sometimes used instead.

Ozone effectively kills microorganisms but is expensive and decays rapidly, while UV light damages microbial DNA but is less effective in cloudy water.

Making bleach through disproportionation

Mixing chlorine gas with cold dilute sodium hydroxide solution produces sodium chlorate(I) solution (NaClO), which is the key component of bleach:

• Cl_2(g) + 2NaOH_(aq) ➔ NaClO_(aq) + NaCl_(aq) + H₂O_(l) The oxidation number of chlorine increases from 0 to +1 in NaClO and decreases to -1 in NaCl, showing it has undergone disproportionation.

This bleach solution contains chlorate(I) ions (ClO^-) that act as oxidising agents to kill bacteria, making the bleach widely useful for disinfecting and cleaning purposes.