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Pressure Dependence of Gas-Phase Reaction Rates

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Stéphanie de Persis , Alain Dollet and Francis Teyssandier

Institut de Science et de Génie des Matériaux et Procédés, CNRS UPR8521, TECNOSUD, Rambla de la Thermodynamique, F-66100, Perpignan, France

J. Chem. Educ., 2004, 81 (6), p 832

DOI: 10.1021/ed081p832

Publication Date (Web): June 1, 2004

Section:

History, Education, and Documentation

Abstract

The rate of some gas-phase reactions depends only on temperature, but the kinetics of many gas-phase reactions is also influenced by pressure, sometimes to a very large extent. However, despite its importance, this phenomenon remains generally unknown to most students. Sophisticated Rice–Ramsperger–Kassel calculations are commonly performed to predict the value of unimolecular and bimolecular rate coefficients at any temperature or pressure. This article is intended to show that only simple concepts are required to qualitatively explain and describe the pressure dependence of gas-phase reaction rates. Collisional energy transfer between reactant and third-body gas molecules (usually the bath-gas), on the one hand, and intramolecular rearrangements, on the other hand, are the competing processes that are responsible for the influence of pressure on gas-phase reaction kinetics. Depending on the relative weight of the two above processes, three domains of pressure can be distinguished that allow a simple mathematical description of the variations of reaction rate coefficients with pressure. The supplemental material available at JCE Online includes a full review of this topic.