△H. In an exothermic process, the potential energy of the products is lower
than the potential energy of the reactants. This means that the recoiling molecules
eventually have more kinetic energy than the reacting molecules.
In other words, the product molecules are “hotter.”When the potential energy
of the reactants is lower than that of the products, the reaction is endothermic
and the product molecules are “cooler.”An actual endothermic reaction
between N2 and O2 to form NO is represented in Figure 14-3.
Perhaps one of the most significant characteristics of any chemical reaction
is its activation energy. Consider, for example, the reactions of two nonmetals
with oxygen. An allotrope of phosphorus known as white phosphorus
reacts almost instantly with the oxygen in the air, even at room temperature,
producing an extremely hot flame. As a result, this form of phosphorus must
be stored under water to prevent exposure to the air. Hydrogen also reacts
with oxygen to form water. This is also a highly exothermic reaction that is
used in space rockets. A mixture of hydrogen and oxygen, however, does not
react at room temperature. In fact, no appreciable reaction occurs unless the
temperature is raised to at least 400oc or the mixture is ignited with a spark.
The difference in the rates of combustion of these two elements at room temperature
lies in the activation energies of the two reactions. Figure 14-4 illustrates
the activation energies for the combustion reactions of hydrogen and phosphorus.