Energy changes always accompany chemical reactions. For various reasons it is useful and instructive to know -- qualitatively or quantitatively -- how the potential energy of a chemical system changes over the course of the entire reaction, that is, on the way from reactants to products. Was the process fast or slow? Was more than one step involved? If so, which was the slowest step? This page shows how reaction energy diagrams provide, at a glance, the answers to these and many other questions.
Energy changes accompany chemical reactions. Energy diagrams are quite useful in illustrating these changes on a continuous basis as the reaction proceeds. Terms such as "activation energy" (Ea), "transition state" (*), and "enthalpy change" are easy to define by referring to a graph such as Figure 1. Endothermic and exothermic reactions are just as easily defined, according to the sign of DHrxn.
Energy diagrams are used frequently, because they contain lots of useful information -- if you know how to interpret them! Specifically, energy diagrams tell you about the relative rate of a reaction, or of any step in a reaction, and they tell you about thethermodynamic favorability of a reaction, or of any step in a reaction. Take a few minutes to review the four basic types of single-step processes that can occur, shown in Fig.2:
Some reactions occur via several steps, in any conceivable combination of the basic four shown above. Regardless of how many steps there are, though, certain facts are always true:
Self-test question #1
Draw an energy diagram for a two-step reaction that is exothermic overall, and consists of a fast but endothermic first step, and a slow but exothermic second step. Indicate DGrxn, as well as DG1* and DG2* for the first and second activation energies, respectively. Label the positions corresponding to the transition states with an asterisk.
Self-test question #2
Calculate the equilibrium constant (at 25°C) for a reaction having DGrxn= -6.4 kcal/mol.