many reactions. Of course, if n0, then HE,and the same amount of heat would
be absorbed or given off by the reaction whether it is carried out at constant pressure or
at constant volume.SPONTANEITY OF PHYSICAL AND CHEMICAL CHANGES
Another major concern of thermodynamics is predicting whethera particular process can
occur under specified conditions to give predominantly products. We may summarize this
concern in the question “Which would be more stable at the given conditions — the reac-
tants or the products?” A change for which the collection of products is thermodynamically
more stablethan the collection of reactants under the given conditions is said to be product-
favored,or spontaneous,under those conditions. A change for which the products are
thermodynamically less stablethan the reactants under the given conditions is described as
reactant-favored,or nonspontaneous,under those conditions. Some changes are spon-
taneous under all conditions; others are nonspontaneous under all conditions. The great
majority of changes, however, are spontaneous under some conditions but not under
others. We use thermodynamics to predict conditions for which the latter type of reac-
tions can occur to give predominantly products.
The concept of spontaneity has a very specific interpretation in thermodynamics. A
spontaneous chemical reaction or physical change is one that can happen without any
continuing outside influence. Any spontaneous change has a natural direction, like the
rusting of a piece of iron, the burning of a piece of paper, or the melting of ice at room
temperature. We can think of a spontaneous process as one for which products are favored
over reactants at the specified conditions. Although a spontaneous reaction mightoccur
rapidly, thermodynamic spontaneity is not related to speed. The fact that a process is spon-
taneous does not mean that it will occur at an observable rate. It may occur rapidly, at a
moderate rate, or very slowly. The rate at which a spontaneous reaction occurs is addressed
by kinetics (Chapter 16). We now study the factors that influence spontaneity of a phys-
ical or chemical change.THE TWO ASPECTS OF SPONTANEITY
Many product-favored reactions are exothermic. For instance, the combustion (burning)
reactions of hydrocarbons such as methane and octane are all exothermic and highly
product-favored (spontaneous). The enthalpy contents of the products are lower than
those of the reactants. Not all exothermic changes are spontaneous, however, nor are all
spontaneous changes exothermic. As an example, consider the freezing of water, which is
an exothermic process (heat is released). This process is spontaneous at temperatures below
0°C, but it certainly is not spontaneous at temperatures above 0°C. Likewise, we can find
conditions at which the melting of ice, an endothermic process, is spontaneous. Spon-
taneity is favoredbut not required when heat is released during a chemical reaction or a
physical change.
Another factor, related to the disorder of reactants and products, also plays a role in
determining spontaneity. The dissolution of ammonium nitrate, NH 4 NO 3 , in water is
spontaneous. Yet a beaker in which this process occurs becomes colder (see Figure 15-2).15-12
620 CHAPTER 15: Chemical Thermodynamics
See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.2, Product-Favored Systems,
and Screen 20.3, Directionality of
Reactions: Matter and Energy Dispersal.