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Researchers should cite this work as follows:

• Vailance, C. (2023). 2008-Claire_Vailance-Reaction_Kinetics-Course_Notes. SIMIODE, QUBES Educational Resources. doi:10.25334/Y7ZK-BH66

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2008. Vailance, Claire. Reaction Kinetics. Course Notes. 31 pages.

Source: http://vallance.chem.ox.ac.uk/CVteaching.html . Accessed 13 March 2023.

This is but one of several set of notes which may interest the reader on chemical kinetics.

Introduction
Chemical reaction kinetics deals with the rates of chemical processes. Any chemical process may be broken down into a sequence of one or more single-step processes known either as elementary processes, elementary reactions, or elementary steps. Elementary reactions usually involve either a single reactive collision between two molecules, which we refer to as a a bimolecular step, or dissociation/isomerisation of a single reactant molecule, which we refer to as a unimolecular step. Very rarely, under conditions of extremely high pressure, a termolecular step may occur, which involves simultaneous collision of three reactant molecules. An important point to recognise is that many reactions that are written as a single reaction equation in actual fact consist of a series of elementary steps. This will become extremely important as we learn more about the theory of chemical reaction rates.

As a general rule, elementary processes involve a transition between two atomic or molecular states separated by a potential barrier. The potential barrier constitutes the activation energy of the process, and determines the rate at which it occurs. When the barrier is low, the thermal energy of the reactants will generally be high enough to surmount the barrier and move over to products, and the reaction will be fast. However, when the barrier is high, only a few reactants will have sufficient energy, and the reaction will be much slower. The presence of a potential barrier to reaction is also the source of the temperature dependence of reaction rates, which we will cover in more detail in Section 19.

The huge variety of chemical species, types of reaction, and the accompanying potential energy surfaces involved means that the timescale over which chemical reactions occur covers many orders of magnitude, from very slow reactions, such as iron rusting, to extremely fast reactions, such as the electron transfer processes involved in many biological systems or the combustion reactions occurring in flames.

Keywords: reaction, kinetics, bimolecular, isomerization, termolecular, electron transfer, combustion