We now shift our discussion to chemical reactions in which bonds, primarily covalent bonds in reactant chemicals, are broken and new bonds are formed to generate reaction products. At any one time, several hundred different kinds of chemical reactions are occurring simultaneously in every cell, and many chemicals can, in principle, undergo multiple chemical reactions. Both the extent to which reactions can proceed and the rate at which they take place determine the chemical composition of cells. In this section, we discuss the concepts of equilibrium and steady state as well as dissociation constants and pH. These concepts will arise again and again throughout this text, so it is important for you to be familiar with them. In Section 2.4, we discuss how energy influences the extents and rates of chemical reactions.