The Lindemann mechanism was one of the first attempts to understand unimolecular reactions. Lindemann mechanisms have been used to. – Free download as PDF File . pdf), Text File .txt) or read online for free. By Module No and Title 20 and Theories of unimolecular reactions- Lindemann Learn about Lindemann Mechanism for unimolecular gaseous reactions.
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Lindemann mechanism – Wikipedia
In chemical kineticsthe Lindemann mechanismsometimes called the Lindemann-Hinshelwood mechanismis a schematic reaction mechanism. Frederick Lindemann proposed the concept in and Cyril Hinshelwood developed it.
It breaks down an apparently unimolecular reaction into two elementary stepswith a rate constant for each elementary step. The rate law and rate equation for the entire reaction can be derived from the rate equations and rate constants for the two steps.
The Lindemann mechanism is used to model gas phase decomposition or isomerization reactions. Although the net formula for a decomposition or isomerization appears to be unimolecular and suggests first-order kinetics in the reactant, the Lindemann mechanism shows that the unimolecular reaction step is preceded by a bimolecular activation step so that the kinetics may actually be second-order in certain cases.
The activated intermediate is produced from the reactant only after a sufficient activation energy is acquired by collision with a second molecule M, which may or may not be similar to A. For each elementary step, the order of reaction is equal to the molecularity. The steady-state rate equation is of mixed order and predicts that a unimolecular reaction can be of either first or second order, depending on which of the two terms in the denominator is larger.
That is, the rate-determining step is the first, bimolecular activation step.
To account accurately for the pressure-dependence of rate constants for unimolecular reactions, more elaborate theories are required such as the RRKM theory. In the Lindemann mechanism for a true unimolecular reaction, the activation step is followed by a single step corresponding to the formation of products.
Whether this is actually true for any given reaction must be established from the evidence. This reaction was studied by Farrington Daniels and coworkers, and initially assumed to be a true unimolecular reaction.
However it is now known to be a multistep reaction whose mechanism was established by Ogg  llndemann. An analysis theorj the steady-state approximation shows that this mechanism can also explain the observed first-order kinetics and the fall-off of the rate constant at very low pressures.
29.6: The Lindemann Mechanism
From Wikipedia, the free encyclopedia. Journal of Chemical Education.