Keszei, Ernő. 2016. Chemical Kinetics for Beginners. Course material supported by the Higher Education Restructuring Fund allocated to ELTE by the Hungarian Government.
Table of Contents
1. Introduction 4
2. Formal kinetic description of elementary reactions 7
2.1. Solution of rate equations of integer order reactions 9
2.1.1. Zeroth-order reactions 13
2.1.2. First-order reactions 14
2.1.3. Second-order reactions 16
2.1.4. Third-order ractions 20
2.2. Generalisation and extension of the order of reaction; pseudo-order 23
Suggested literature 26
Reaction kinetics is the branch of physical chemistry which deals with the temporal evolution of chemical reactions. It provides explanation as to why some reactions do not take place, although their products would be much more stable thermodynamically than the reactants are; it accounts for the fact that some reactions go very fast while some other rather slowly. Furthermore, it also provides several useful tools to calculate the actual rate of reactions within a wide variety of circumstances, including e. g. the dependence of the rate on temperature, pressure, the solvent applied, etc.
The most important goal of kinetic research is the identification and characterisation of elementary molecular events which make possible the transformation of reactants into products. At the time of the beginning of reaction kinetic studies – in the second half of the 18th century when mechanical models dominated natural sciences – the scheme of these simple molecular events that constitute the overall reaction has been called as the mechanism of the reaction, which term survived and is still widely used. This means that the elementary steps of the reaction – the most simple events when typically two molecules directly encounter – are identified, and the complex reaction mechanism is constructed from these elementary steps. Most real-life chemical reactions comprise quite a number of such elementary steps; the number of them e. g., in gas reactions can be as much as a few hundreds. The overall rate of these reactions can successfully be calculated over a wide range of circumstances if we can calculate the rate of all the constituent elementary reactions within the given conditions, and also know the way of their connection within the reaction mechanism. This is the reason why the theory of elementary reactions has a paramount importance in chemical kinetics.
However, to explore the precise mechanism of composite reactions is not an easy task. To be able to construct a reliable mechanism, we have to identify all the components which take part in the process of the reaction, even if they are short-lived and have rather low concentration. We also have to keep track of the temporal evolution of possibly all these components, although this is not always possible. In many reactions, there are so called intermediates (substances formed from the reactants and readily removed in consecutive reactions leading to the products) that are rather short-lived and present only in very low concentrations, thus they easily remain unnoticed for the experimentalist. After having traced the temporal evolution of as many components of the reacting mixture as possible, the task of the kineticist is to construct a suitable mechanism that can explain all the concentration changes as a function of time. Consequently, theories of composite reactions also constitute an important topic of reaction kinetics.
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