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2.1.3Organic Chemistry

Redox and Disproportionation — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 3 of 42

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Part 1 of 5What Disproportionation Means

2.1.3 Redox and Disproportionation
Definition
  • Disproportionation is a reaction in which the same element is simultaneously oxidised and reduced.
Evidence
  • The element appears in one reactant but in two different oxidation states in the products.
Oxidation numbers
  • One product has a higher oxidation number than the reactant, the other a lower one.
Proof
  • Show the changes with oxidation numbers above the species in the equation.
Not every redox
  • A redox reaction is only a disproportionation if one single element is oxidised and reduced in the same reaction.

Part 2 of 5Chlorine with Alkali

Chlorine with Alkali
Cold dilute NaOH
  • Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l) forms chloride and chlorate(I).
Oxidation numbers
  • Chlorine goes from 0 to −1 in NaCl and from 0 to +1 in NaClO.
Hot concentrated NaOH
  • 3Cl₂(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO₃(aq) + 3H₂O(l) forms chloride and chlorate(V).
Use
  • Chlorate(I) solution is the active ingredient in bleach and disinfectant.
Uses of products
  • Sodium chlorate(V) can be used as a weedkiller, while chlorate(I) is used in bleach and water treatment.
ConditionsProducts containing ClOxidation numbers of Cl
Cold dilute NaOHNaCl and NaClO−1 and +1
Hot conc. NaOHNaCl and NaClO₃−1 and +5

Part 3 of 5Chlorine with Water

1
Chlorine dissolves in water and reacts reversibly: Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HClO(aq).
2
Chlorine goes from 0 to −1 in HCl and 0 to +1 in HClO, so this is disproportionation.
3
Chloric(I) acid, HClO, is the main disinfecting species and kills bacteria.
4
This reaction is used to treat drinking water and swimming pools.
5
The solution is acidic and may bleach damp litmus paper after turning it red.

Part 4 of 5Spotting Disproportionation

Step 1
  • Write oxidation numbers of the element in all species in the equation.
Step 2
  • Find the element that increases in oxidation number in one species.
Step 3
  • Check that the same element decreases in oxidation number in another species.
Step 4
  • If both changes involve the same element and reactant, it is a disproportionation reaction.

Part 5 of 5Exam Checklist & Tips

2.1.3 – Key Points for the Exam

  • Disproportionation Reaction where one element is both oxidised and reduced.
  • Chlorate(I) The ion ClO⁻ in which chlorine has oxidation number +1.
  • Chlorate(V) The ion ClO₃⁻ in which chlorine has oxidation number +5.
  • Bleach A solution containing chlorate(I) used to disinfect and whiten.

Common Mistakes

  • Saying disproportionation involves two different elements.
  • Writing chlorate(I) as ClO₃⁻ instead of ClO⁻.
  • Forgetting that the chlorine in the reactant has oxidation number 0.
  • Not stating the temperature and concentration of the alkali when asked for the product.

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