75 Introduction

The lizard in the photograph below (Figure 75-1) is not simply enjoying the sunshine or working on its tan. The heat from the sun’s rays is critical to the lizard’s survival. A warm lizard can move faster than a cold one because the chemical reactions that allow its muscles to move occur more rapidly at higher temperatures. A cold lizard is a slower lizard and an easier meal for predators.

From baking a cake to determining the useful lifespan of a bridge, rates of chemical reactions play important roles in our understanding of processes that involve chemical changes. Two questions are typically posed when planning to carry out a chemical reaction. The first is: “Will the reaction produce the desired products in useful quantities?” The second question is: “How rapidly will the reaction occur?” A third question is often asked when investigating reactions in greater detail: “What specific molecular-level processes take place as the reaction occurs?” Knowing the answer to this question is of practical importance when the yield or rate of a reaction needs to be manipulated.

In the previous chapter, we began to explore the first question:  predicting the spontaneity of a process (e.g. a phase change or a chemical reaction) based on the change in enthalpy and entropy of the system. However, knowing whether or not a process is spontaneous does not allow us to predict the RATE at which that process will occur.  The study of chemical kinetics allows us to explore the rate at which a reaction yields products. This information can be used to help to understand the molecular basis by which a reaction occurs. This chapter examines the factors that influence the rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to describe the rates at which reactions occur.