KINETICS - CAMBRIDGE UNIVERSITY - 1 Chemical kinetics...


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1 1. Chemical kinetics Kinetics is the study of the rates of chemical processes in an effort to understand what it is that influences these rates and to develop theories which can be used to predict them. A knowledge of reaction rates has many practical applications, for example in designing an industrial process, in understanding the complex dynamics of the atmosphere and in understanding the intricate interplay of the chemical reactions that are the basis of life. At a more fundamental level we want to understand what happens to the molecules in a chemical reaction – that is what happens in a single reactive encounter between two reagent molecules. By understanding this we may be able to develop theories that can be used to predict the outcome and rate of reactions. 1.1 Books Any general physical chemistry text (such as P W Atkins Physical Chemistry , OUP, any edition) will have several chapters on chemical kinetics. There is a nice small book about kinetics in the Oxford Chemistry Primers series: B G Cox Modern Liquid Phase Kinetics , OUP, 1994. For more detail (beyond the scope of this course), Reaction Kinetics by M J Pilling and P W Seakins (OUP 1995) is a good source. 2. Rates, rate laws and rate constants In this Section we will introduce the language and terms used to describe the rates of chemical reactions. At this stage we will not be concerned with the theory of reactions or mechanisms, but just stand back and describe the overall rates. 2.1 Rate of reaction The rate is defined as t t c in time , ion, concentrat in change We can talk about the rate of formation or loss of any species – reactant, intermediate or product. It is, however, important to specify which species we are talking about. The rate can be positive or negative: a positive rate means that the concentration is increasing with time e.g. a product; a negative rate means that the concentration is falling with time e.g. a reactant. The rate may vary with time (and concentration), so it is usual to define the rate over a very small time, t. We think of the rate as the derivative of concentration with respect to time [] t d ion concentrat d rate = This derivative is the slope of a graph of concentration against time, taken at a particular time. concentration, c time, t t c concentration time The rate is the instantaneous slope, and this varies with time.
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2 From its definition it is clear that the units of a rate are concentration per unit time, for example mol dm –3 s –1 . There are other measures of concentration, for example in the gas phase pressure is proportional to concentration, so a rate can be expressed in torr min –1 (1 torr = 1 mm Hg, a measure of pressure). It is also common to express concentration not in moles per unit volume but in molecules per unit volume, so the rate would be expressed in molecules dm –3 s –1 or molecules cm –3 s –1 .
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KINETICS - CAMBRIDGE UNIVERSITY - 1 Chemical kinetics...


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