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Chemistry · Glossary

What is Lindemann mechanism, fall-off region?

Also known as: Lindemann mechanism · fall-off region

Definition 13.6 University Chemistry — Year 3 · Chapter 13 — Complex Kinetics: Chains, Enzymes and Oscillations

The Lindemann mechanism of a unimolecular reaction is the sequence A+M⇌A∗+M\mathrm{A + M \rightleftharpoons A^* + M} (k1k_1, k−1k_{-1}), activation and deactivation by collision with any molecule M, followed by A∗→P\mathrm{A^* \to P} (k2k_2). The fall-off region is the range of pressures where the observed first-order rate constant falls from its high-pressure value towards second-order behaviour.

The Lindemann fall-off curve (model constants): first order with the limiting constant k_∈fty at high pressure, second order (k_1[ M]) at low pressure, and half of k_∈fty at [ M]_1/2 = k_2/k_-1. Real fall-off curves are broader: the rate constant k_2 of an energised molecule grows with its energy.
The Lindemann fall-off curve (model constants): first order with the limiting constant k∞k_\infty at high pressure, second order (k1[M]k_1[\mathrm M]) at low pressure, and half of k∞k_\infty at [M]1/2=k2/k−1[\mathrm M]_{1/2} = k_2/k_{-1}. Real fall-off curves are broader: the rate constant k2k_2 of an energised molecule grows with its energy.
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