Fundamental Principles of Chemical Kinetics

Thermodynamic data give us a means of quantitatively expressing stability. Now we need to explore the relationship between structure and reactivity. The quantitative description of reactivity is called chemical kinetics. A fundamental thermodynamic equation relates the equilibrium constant for a reaction to the free-energy change associated with the reaction  

The free energy contains both enthalpy and entropy terms  

Thus we see that thermodynamic stability, as measured by free energy, places a limit on the extent of a chemical reaction. However, it does not directly determine the rate of the reaction. 

The nature of the rate constants for individual steps in a chemical reaction can be discussed in terms of transition state theory, which is a general approach for analyzing the energetic and entropic components of a reaction process. In transition state theory, a reaction is assumed to involve the attainment of an activated complex that goes on to product at an extremely rapid rate. The rate of decomposition of the activated complex has been calculated from the assumptions of the theory to be about 6-10 s at room temperature. The observed rate constant k is given by the expression  

Although the terms transition state and transition structure are often used interchangeably, if the transition state is taken as defined by transition state theory, it may differ in structure from the maximum energy obtained by computation K. N. Houk, Y. Li, and J. D. Evanseck, Angew. Chem. Int. Ed. Engl, 31, 682 (1992). 

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The fundamental principles of chemical kinetics, rate constant estimation and chemical reactor design are the basis of the analysis and study of microkinetics. 

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