Disproportionation Reactions - UNSOLVED PRACTICE SET

Q7. Define disproportionation reaction. What are the necessary conditions for an element to undergo disproportionation?

Q8. In the reaction: 2Cu⁺ → Cu²⁺ + Cu, identify the oxidation and reduction processes. What is the oxidation number of copper in each species?

Q9. Explain why fluorine (F₂) does not undergo disproportionation, unlike other halogens.

Q10. Balance the following disproportionation reaction:

P₄ + NaOH + H₂O → PH₃ + NaH₂PO₂

Q11. In the reaction: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O, identify which manganese species is formed by oxidation and which by reduction.

Q12. A student claims that all elements in intermediate oxidation states can undergo disproportionation. Is the student correct? Give an example to support or refute this claim.

Q13. (a) Define disproportionation and comproportionation with one example each.

(b) Explain the conditions necessary for an element to undergo disproportionation using a Frost diagram or oxidation state diagram concept.

(c) Identify which of the following can undergo disproportionation and write balanced equations:

(i) Cl₂ in cold dilute NaOH

(ii) Cl₂ in hot concentrated NaOH

(iii) H₂O₂

(iv) Cu⁺ in aqueous solution

Q14. (a) Explain why the following species can or cannot undergo disproportionation:

(i) ClO⁻

(ii) ClO₂⁻

(iii) ClO₃⁻

(iv) ClO₄⁻

(b) For each species that can disproportionate, write the balanced equation in both acidic and basic medium.

(c) The standard reduction potentials are:

ClO₃⁻ + 2H⁺ + e⁻ → ClO₂ + H₂O; E° = 1.15 V

ClO₂ + H⁺ + e⁻ → HClO₂; E° = 1.27 V

Using these values, explain whether ClO₂ will disproportionate spontaneously.

Q15. (a) Define and explain the concept of a 'disproportionation reaction' with a clear example.

(b) The reaction of white phosphorus with hot NaOH is:

P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂

Analyse this reaction in terms of oxidation numbers and identify it as a disproportionation reaction. Calculate the oxidation number of phosphorus in each product.

(c) In the Indian context, bleaching powder (CaOCl₂) undergoes disproportionation when treated with dilute acids. Write the reaction and explain the oxidation number changes of chlorine.

Q16. Analyse each of the following reactions and determine whether it is a disproportionation reaction. If yes, identify the element that disproportionates and calculate the change in its oxidation number:

(a) 2H₂O₂ → 2H₂O + O₂

(b) Cl₂ + 2NaOH → NaCl + NaClO + H₂O (cold, dilute)

(c) 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (hot, concentrated)

(d) 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s)

(e) 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O

(f) 2NO₂ + H₂O → HNO₃ + HNO₂

For each disproportionation reaction, calculate:

(i) The number of electrons transferred in oxidation

(ii) The number of electrons transferred in reduction

(iii) Verify that electrons lost equal electrons gained

Q17. Consider the following set of chlorine species in different oxidation states:

(a) Identify which species can act as:

(i) Only oxidising agents

(ii) Only reducing agents

(iii) Both oxidising and reducing agents (disproportionation candidates)

(b) For each species identified in (a)(iii), write a balanced disproportionation reaction in basic medium.

(c) Calculate the volume of Cl₂ gas at STP produced when 10.0 g of ClO⁻ disproportionates completely. (M of ClO⁻ = 51.5 g/mol)

(d) A mixture contains equal moles of ClO⁻ and ClO₃⁻. Can they react with each other? If yes, write the reaction and identify the type.

Q18. In India, disproportionation reactions have important practical applications:

(a) Hydrogen peroxide (H₂O₂) is used as a disinfectant and bleaching agent. It undergoes disproportionation catalysed by MnO₂:

2H₂O₂ → 2H₂O + O₂

(i) Identify the oxidation and reduction half-reactions

(ii) Calculate the volume of O₂ produced at STP when 100 mL of 0.1 M H₂O₂ solution decomposes completely

(iii) Explain why MnO₂ acts as a catalyst and not a reactant

(b) In the production of chlorine from brine, the Deacon process involves:

4HCl + O₂ → 2Cl₂ + 2H₂O

However, an intermediate disproportionation occurs:

2CuCl → CuCl₂ + Cu

Analyse this as a disproportionation reaction and explain the role of copper in the catalytic cycle.

(c) In photography, the fixing process involves thiosulphate:

AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr

However, in acidic conditions, thiosulphate disproportionates:

S₂O₃²⁻ + 2H⁺ → S + SO₂ + H₂O

(i) Identify the oxidation number of sulphur in S₂O₃²⁻, S, and SO₂

(ii) Verify that this is a disproportionation reaction

(iii) Explain why photographic fixer solutions must be kept slightly basic

(d) A student prepares a solution of bleaching powder (CaOCl₂) for disinfecting water. The active component hypochlorite (ClO⁻) can disproportionate in sunlight:

3ClO⁻ → 2Cl⁻ + ClO₃⁻

Calculate the mass of ClO₃⁻ formed when 1.0 kg of bleaching powder (containing 35% available chlorine as ClO⁻) is exposed to sunlight and complete disproportionation occurs.