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.

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.

If chlorine is reacted with hot concentrated sodium hydroxide solution, sodium chlorate(V) solution (NaClO₃) is produced instead:

• 3Cl_2(g) + 6NaOH_(aq) ➔ NaClO_3(aq) + 5NaCl_(aq) + 3H₂O_(l) The oxidation number of chlorine increases from 0 to +5 in NaClO₃ and decreases to -1 in NaCl, showing it has undergone disproportionation.

The sodium chlorate(V) solution produced contains chlorate(V) ions (ClO₃^-) that are powerful oxidising agents used for bleaching paper and textiles.