well as by other means. In the last twenty years, for example, the use of isotopes (as “tracer” elements) has become a valuable tool for the study of reaction kinetics (in slow reactions). 3. CONCENTRATION DEPENDENCE OF REACTION RATES
Normally experimental data of kinetic investigations are records of concentrations of reactants and/or products as a function of time for constant temperatures (taken at various temperatures). Theoretical expressions for reaction rates (involving concentration changes) are differential equations of the general form: 1,cndc + f cm 2,co...
where (c) are concentration terms which have exponents that depend on details of the reaction. If we want to compare the theory with the experiment, it is therefore necessary to either integrate the theoretical laws or to differentiate experimental concentration vs time data. The rate laws are of importance since they provide analytical expressions for the course of individual reactions and enable us to calculate expected yields and optimum conditions for “economic” processes. In most instances the differential rate equation is integrated before it is applied to the experimental data. Only infrequently are slopes of concentration vs time curves taken to determine dc/dt directly (fig. 1). concentration [mol] time (s) the instantaneous reaction rate (initial rate) is given by the slope of the curve at t = 0, (Δc / Δt) Δc Δt 0 1 2 4 rate at t = 2 is given by the tangent to the curve (Δc / Δt) at t = 2 Rate of decay of reactant A Rate of formation of product B Figure 1 Variation with time of the concentration of a reactant (A) and of a product (B)
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