3.1.3(e)-(f) - Disproportionation and chlorine in water treatment

Meaning of Disproportionation

Redox reactions are tracked by oxidation numbers. Oxidation is an increase in oxidation number, and reduction is a decrease in oxidation number. In most redox reactions, one element is oxidised and a different element is reduced. In disproportionation, the same element does both jobs.

The key is that the same element undergoes both oxidation-number changes. Saying only that the same species reacts does not demonstrate this; a correct explanation must show its oxidation and reduction. Equivalent wording is valid when the two changes are unambiguous. Chlorine is a good example because chlorine starts in Cl₂ with oxidation number 0, then one chlorine atom can end up in a compound where chlorine is -1 while another ends up where chlorine is +1.

Worked example: decide whether this reaction is disproportionation.

Assign oxidation numbers to chlorine:

The same element, chlorine, has been reduced from 0 to -1 and oxidised from 0 to +1. Therefore this is a disproportionation reaction.

Chlorine with Water

When chlorine dissolves in water, a small amount reacts to form hydrochloric acid and chloric(I) acid, also called hypochlorous acid.

In water, HCl produces chloride ions, Cl⁻, and HClO can be thought of as the +1 chlorine product. For this lesson, the key chemistry is the redox pattern: chlorine in Cl₂ starts at 0, then forms a -1 chlorine product and a +1 chlorine product.

The product HClO is important in water treatment because chlorine-containing species in water kill bacteria and other microorganisms. You do not need a biochemical mechanism here; the required point is the treatment purpose and the redox equation that produces the chlorine-containing disinfecting species.

Worked reasoning: explain why the equation matters in water treatment.

1. Chlorine is added to water.
2. Chlorine reacts with water to form HCl and HClO.
3. This reaction is disproportionation because chlorine is both reduced and oxidised.
4. The disinfecting benefit is that microorganisms are killed.

A careful answer says "kills bacteria" or "kills microorganisms", not just "removes bacteria". Removal sounds like filtration; chlorination is a chemical disinfection step.

Chlorine with Cold Dilute Sodium Hydroxide

Chlorine also disproportionates when it reacts with cold, dilute aqueous sodium hydroxide. This reaction forms sodium chloride, sodium chlorate(I), and water. Sodium chlorate(I), NaClO, is also called sodium hypochlorite and is the active chemical in bleach.

The ionic equation shows the redox pattern more clearly because sodium ions are spectators:

Now assign chlorine oxidation numbers:

The condition "cold, dilute" is part of the required OCR boundary because it gives the chlorate(I) product used as bleach. Do not replace this with hot concentrated alkali chemistry unless a question explicitly gives you the information.

Worked example: complete the explanation for bleach formation.

Chlorine in Cl₂ is 0. In NaCl, chlorine is -1, so that chlorine has been reduced. In NaClO, oxygen is -2 and sodium is +1, so chlorine must be +1; that chlorine has been oxidised. The reaction is disproportionation because chlorine is both oxidised and reduced.