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Chemical Kinetics in Constant-Pressure Systems

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P. L. Corio

University of Kentucky, Department of Chemistry, Lexington, KY 40506-0055

J. Chem. Educ., 1999, 76 (10), p 1452

DOI: 10.1021/ed076p1452

Publication Date (Web): October 1, 1999

Abstract

A theory is developed for treating chemical kinetics in nonflow, constant-pressure systems. The procedure is straightforward and applicable to systems in which several reactions occur, subject to any given rate laws. A significant difference from kinetics at constant volume is that the varying volume precludes the use of concentrations to express the conservation conditions. Instead the conservation conditions must be expressed in terms of the moles of components and converted to concentrations when the relation between volume and concentration is established. The theory is applied to reactions in ideal systems for zero-, first-, and second-order reactions, permitting an immediate comparison with the corresponding rate laws in constant-volume kinetics. In each of these constant pressure systems, a stationary state is possible in which the concentration of a reactant does not appear to exhibit a change of concentration; that is, the decrease in its concentration is exactly compensated by the decrease in volume. A similar stationary state is also possible for the concentration of a product. Both cases can be experimentally realized by specialization of the initial conditions, though both stationary states are unstable. Finally, for rate laws of the form r = k[A]^a[B]^b[C]^c, there is an apparent linear variation of the rate constant with concentration.