Advanced Topic 493 of 1102 Kinetics & Equilibrium
Lindemann Mechanism
Explains unimolecular reaction pressure dependence through collisional activation.
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In-depth explanation of Lindemann Mechanism
The Lindemann mechanism explains why unimolecular gas phase reactions depend on pressure. A molecule must first be activated by collision with another molecule, A + M <-> A* + M, and the activated molecule then reacts unimolecularly, A* -> products, with rate constant k. At high pressure collisions are frequent, so activation is fast and the overall process is first order in A, with the rate limited by the unimolecular step. At low pressure activation is the slow step, so the reaction becomes second order, because the rate depends on the product of the concentrations of A and the collision partner M. The Lindemann mechanism is the basic model for falloff behaviour in gas phase kinetics.
Going deeper: Lindemann Mechanism
Key ideas: Steps: A + M -> A* + M (k1), A* + M -> A + M (k-1), A* -> P (k2). Steady state gives rate = k1 k2 [A][M] / (k-1[M] + k2). At high pressure rate = (k1 k2 / k-1)[A], first order; at low pressure rate = k1[A][M], second order. The crossover pressure is where k-1[M] equals k2.
Common mistakes and connections: Students overlook that M can be A itself or an inert gas, or claim that the activated molecule reacts immediately with no delay. The time lag in A* is what causes the pressure dependence. Another mistake is applying the high pressure limit at low pressure. Lindemann theory underestimates the rate of falloff, so RRK and RRKM theories refine it. Uses the steady state approximation.
Visualization of Lindemann Mechanism
At low pressure the rate falls because activation by collision is slow (second order); at high pressure k levels off and the reaction becomes first order. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The gas phase isomerization of cyclopropane shows this pressure dependence, with first order behaviour at high pressure.
Frequently asked questions about Lindemann Mechanism
What is Lindemann Mechanism?
Explains unimolecular reaction pressure dependence through collisional activation.
Is Lindemann Mechanism a beginner, intermediate or advanced topic?
Lindemann Mechanism is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Lindemann Mechanism?
Study these earlier topics in Kinetics & Equilibrium first: Kinetic vs Thermodynamic Product, Hammond Postulate, Eyring Equation.
What comes after Lindemann Mechanism?
The next topics in Kinetics & Equilibrium are Diffusion-Controlled Reactions and Relaxation Methods.
Can I practise Lindemann Mechanism with quizzes?
Yes. The interactive lesson includes a self-check question on Lindemann Mechanism, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Lindemann Mechanism fits in Kinetics & Equilibrium
Lindemann Mechanism is topic 74 of 91 in Kinetics & Equilibrium.