ABSTRACT
Any kinetic study using isothermal calorimetry utilizes that the reaction rate, v, may be expressed as
v dQ dt HVr sam
=
∆ (20.1)
where: ΔrH is the enthalpy change of the reaction at the conditions in the calorimetric cell Vsam is the volume of the sample dQ/dt is the heat ow (in J/s) required to keep the sample isothermal (detected by the
instrument)
One important consequence of Equation 20.1 is that calorimetry can in principle be applied to any reaction with ΔrH ≠ 0, and hence does not rely on the selection of substrate or conditions where spectral or other properties change as the reaction progresses. Moreover, the method does not need any labeling or post-experiment procedures and it provides realtime data, which readily elucidate the time-course of the reaction. Another implication of Equation 20.1 is that the rate of reaction is proportional to the primary observable of the method (dQ/dt), and this is unique to calorimetry. Other approaches detect a concentration (typically of a product), and the rate must be calculated from changes occurring over two or more measurements (or the slope of a continuous curve). e concept of a direct rate measurement oers special advantages, particularly when kinetic measurements must be made against a background of high product concentration. Examples of this include analyses of product inhibition or kinetic studies late in the course of the reaction, where signicant conversion of the substrate has occurred. Under such conditions, it is hard if not impossible to derive reaction rates from concentration measurements because the changes over limited time intervals are small compared with the accumulated (or added) background of the product. e calorimetric signal, on the other hand, is almost independent of the product concentration and calorimetry may therefore be particularly benecial for this type of work.
