In Chapter 1, we discussed the equilibrium states of a thermochemical system. We have shown that by knowing the initial state of a reactive mixture, we can determine its final state after chemical and thermal equilibria have been established. However, the equilibrium calculations are not able to answer such relevant questions as: how does the mixture get from the initial state to the final state, and how long does it take to do so? Obviously if a particular reaction proceeds exceedingly slowly compared to other physical or chemical processes of interest, it is likely that this reaction could be either irrelevant to the system's behavior or not of controlling importance in being the rate-limiting step in the system's evolution.
In this chapter, we first present the phenomenological law describing the general dependence of reaction rates on reactant concentrations and temperature. We then discuss multistep reactions and some approximation techniques used to simplify the representation of these reactions.
In Section 2.2, we present the specific functional dependence of the reaction rate on temperature—the Arrhenius law. We then derive and discuss three theories of reaction rates. The collision theory is based on the kinetic theory of gases, counting the frequency of molecular collisions that are energetic enough to cause the colliding molecules to react. The transition state theory examines the activated state of molecules and derives the reaction rate by considering the characteristic times and energies associated with their transition from activated to reacted states.