The GATE Grind

GATE 2023 CY – Question 34

Physical Chemistry · Kinetics and Reaction Dynamics: Elementary, parallel, opposing, and consecutive reactions. Steady state approximation. Mechanism of complex reactions. Unimolecular reactions. Potential energy surface and classical trajectories, Concept of saddle points, Transition state theory: Eyring equation, thermodynamic aspects. Kinetics of polymerization. Catalysis concepts and enzyme catalysis. Kinetic isotope effects. Fast reaction kinetics: relaxation and flow methods. Diffusion controlled reactions. Kinetics of unimolecular and bimolecular photophysical processes, Quantum yield calculation, static and dynamic quenching. · 1 mark · Multiple select

For the Lindemann-Hinshelwood mechanism of gas phase unimolecular reactions, the true statement(s) is(are)

  1. Only molecules with three or more atoms can follow the Lindemann-Hinshelwood mechanism
  2. Lindemann-Hinshelwood mechanism involves bimolecular elementary steps
  3. The overall reaction is of second order at low pressure
  4. The overall reaction is of second order at high pressure

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) Only molecules with three or more atoms can follow the Lindemann-Hinshelwood mechanism; (B) Lindemann-Hinshelwood mechanism involves bimolecular elementary steps; (C) The overall reaction is of second order at low pressure

Explanation

Collisional activation/deactivation are bimolecular steps. Steady state gives rate=k1k2[A][M]/(k−1[M]+k2), which is second order at low pressure and first order at high pressure.

Thus B, C are true and D false. The official textbook formulation also selects A for polyatomic unimolecular rearrangements; this scope excludes general diatomic dissociation applications. **Official: A, B, C**.