The one which is a uni-molecular reaction is - 

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HPSC Assistant Professor 2018 Chemistry (Official Paper)
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  1. \(\mathrm{N}_2\mathrm{O}_5\rightarrow \mathrm{N}_2\mathrm{O}_4+\frac{1}{2}\mathrm{O}_2\)
  2. H+ Cl→ 2HCl
  3. 2HI → H+ I2
  4. PCl+ Cl→ PCl5

Answer (Detailed Solution Below)

Option 1 : \(\mathrm{N}_2\mathrm{O}_5\rightarrow \mathrm{N}_2\mathrm{O}_4+\frac{1}{2}\mathrm{O}_2\)
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Detailed Solution

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CONCEPT:

Unimolecular Reactions

  • A unimolecular reaction is a type of reaction in which a single reactant molecule undergoes a transformation to form products.
  • These reactions typically involve the breaking of a bond within a single molecule or an internal rearrangement.
  • In kinetic terms, unimolecular reactions follow first-order kinetics.

EXPLANATION:

  • Option 1: \(N_2O_5 \rightarrow N_2O_4 + \frac{1}{2} O_2\) is a unimolecular reaction because a single molecule of \(N_2O_5\) decomposes into products.
  • Option 2: \(H_2 + Cl_2 \rightarrow 2HCl\) is a bimolecular reaction involving two reactant molecules.
  • Option 3: \(2HI \rightarrow H_2 + I_2\) is also a bimolecular reaction since it involves two molecules of HI reacting together.
  • Option 4: \(PCl_3 + Cl_2 \rightarrow PCl_5\) is a bimolecular reaction involving two different reactant molecules.

The correct answer is (1) \(N_2O_5 \rightarrow N_2O_4 + \frac{1}{2} O_2\).

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