The Course of a Reaction

Figure 13.4 plots the course of a reaction. The initial average energy of the reactants is indicated at the left side of each graph. If molecules are to collide effectively, they must have more than the average energy. They must have enough to overcome the repulsive forces between molecules. This added amount of energy, the activation energy, is the difference between the initial energy and the energy at the peak of each graph. Molecules having sufficient energy can collide, and, if they are correctly oriented at collision, their bonds may break and the bonds of the products form. As the new bonds form, energy is released, leaving the product molecules with the average energy shown at the right of the graph.

If the energy released is less than the activation energy - a net absorption of energy - the reaction is endothermic (Figure 13.4a). If the energy released is greater than the activation energy - a net release of energy - the reaction is exothermic (Figure 13.4b). Remember that of all the molecules present, only some will collide; of those collisions, only some are effective and result in reactions.

This picture of a reaction is analogous to riding a bicycle over a mountain pass. The activation energy of the reaction is comparable to the energy needed to pedal to the top of the pass. The energy released by the rearrangement of bonds is comparable to that gained in coasting down from the top of the pass to the floor of the next valley. If this second valley is higher than the one you started from, the energy gained in coasting down is less than the energy expended in pedaling up. This situation corresponds to an endothermic reaction, which has a net absorption of energy. If the second valley is lower than the one you started from, you gain more energy coasting down than was used pedaling up. This situation corresponds to an exothermic reaction, which results in a net release of energy.