Disproportionation is an essential redox concept where the same element is both oxidised and reduced in a single chemical reaction. This blog post will help you fully understand disproportionation reactions, with clear definitions, oxidation state explanations, key examples, and real-world applications such as water treatment and bleach production.

A disproportionation reaction is a redox reaction in which one element, in a single oxidation state, reacts to form two different compounds: one where it has a higher oxidation state (oxidation) and one with a lower oxidation state (reduction).

Key Definition: Disproportionation is the simultaneous oxidation and reduction of atoms of the same element in a chemical reaction.

Look for:

• An element appearing in the reactants and again in two different forms in the products.
• Changes in oxidation number: one atom goes up (oxidised), one goes down (reduced).

Equation: Cl₂+H₂O→HCl+HOCl

• Cl₂ is in oxidation state 0.
• In HCl, Cl is –1 (reduction).
• In HOCl, Cl is +1 (oxidation).

Use:
This reaction is used in water treatment to kill bacteria. The hypochlorous acid (HOCl) formed is a powerful disinfectant.

Benefits

• Kills harmful bacteria and viruses in water.
• Makes drinking water safe, preventing diseases like cholera and typhoid.

Risks

• Chlorine gas is toxic and dangerous to handle.
• Can react with organic compounds in water to form chlorinated hydrocarbons (e.g. THMs), which are possibly harmful or carcinogenic.

Chlorine and Cold, Dilute Sodium Hydroxide (Bleach Production)

Equation: Cl_2​+2NaOH→NaCl+NaClO+H₂​O

When chlorine is added to sodium hydroxide, chlorine is disproportionated and forms sodium chloride (NaCl) and sodium chlorate (I), also commonly named hypochlorite.

• Cl in Cl₂ has oxidation state 0
• In NaCl, Cl has oxidation state –1
• In NaClO, Cl has oxidation state +1

Use:
This reaction produces sodium chlorate(I), the active ingredient in household bleach.

Over time, bleach can decompose with hypochlorite breaking down via another disproportionation reaction forming sodium chloride and sodium chlorate (V).

Reaction: 3NaClO→2NaCl+NaClO₃

Here:

• Cl in NaClO has oxidation state +1
• In NaCl, Cl has oxidation state –1
• In NaClO, Cl has oxidation state +5

Balancing a disproportionation reaction can feel tricky, but there is a clear method to do it systematically using half-reactions.

Let’s practice with the following disproportionation reaction. In acidic solution, nitrous acid (HNO₂) decomposes to nitric acid (HNO₃) and nitrogen monoxide (NO).

Step 1: Identify the oxidation states

• N in HNO₂ has oxidation state +3
• N in HNO₃ has oxidation state +5
• N in NO has oxidation state +2

Step 2: Write the half-equations.

• HNO₂ → HNO₃; more oxygen atoms on RHS so add water on LHS
• HNO₂ + H₂O → HNO₃ ; 3H on LHS so complete with H⁺ on RHS
• HNO₂ + H₂O → HNO₃ + 2H⁺ ; 2 positive charges on RHS, but 0 on LHS so balance the half-equation with electrons
• HNO₂ + H₂O → HNO₃ + 2H⁺ + 2e^– (eq. 1)
• Write the half-equation for HNO₂/NO
• HNO₂ + H⁺ + e^– → NO + H₂O (eq. 2)

Step 3: Add both equations balancing the charges: eq. 1 + 2x eq.2

HNO₂ + H₂O + 2HNO₂ + 2H⁺ + 2e^– → HNO₃ + 2H⁺ + 2e^– + 2NO + 2H₂O

Step 4: Simplify on both sides

3HNO₂ → HNO₃ + 2NO + H₂O

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When calcium hypochlorite Ca(ClO)₂ is heated, the Ca(ClO)₂ decomposes to form CaCl₂ and Ca(ClO₃)₂. This is a disproportionation reaction. Write an equation for this decomposition and explain, using oxidation numbers, why this is a disproportionation reaction.

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