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.
When reactants first mix together—before any products have been formed—their rate of reaction is determined in part by their initial concentrations. As the reaction products accumulate, the concentration of each reactant decreases and so does the reaction rate. Meanwhile, some of the product molecules begin to participate in the reverse reaction, which re-forms the reactants (microscopic reversibility). This reverse reaction is slow at first but speeds up as the concentration of product increases. Eventually, the rates of the forward and reverse reactions become equal, so that the concentrations of reactants and products stop changing. The system is then said to be in chemical equilibrium.
At equilibrium the ratio of products to reactants, called the equilibrium constant, is a fixed value that is independent of the rate at which the reaction occurs. The rate of a chemical reaction can be increased by a catalyst, which brings reactants together and accelerates their interactions, but is not permanently changed during a reaction. In this section, we discuss several aspects of chemical equilibria; in the next section, we examine energy changes during reactions and their relationship to equilibria.
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