3 Energetics
A full treatment of this important—and indeed exciting—area of
chemistry belongs to physical chemistry. Here, we are chiefly
concerned with two fundamental questions about a chemical
reaction—why does it proceed, and why does it give one product
rather than another? There are many processes, both physical and
chemical which proceed spontaneously. Consider first two flasks,
one containing only oxygen and the other only nitrogen, which
are connected by opening a tap. The two gases mix spontaneously
and the mixture is eventually uniform in both flasks—there has
been no chemical reaction but spontaneous mixing has occurred.
When anhydrous aluminium chloride is added to water the reaction
described on p. 45 occurs with the evolution of a great deal of heat—
a strongly exothermic spontaneous reaction. Addition of solid
ammonium nitrate to water leads to solution with the absorption
of heat—a spontaneous endothermic reaction. These reactions are
all spontaneous, but clearly there are wide differences in the apparent
energy changes involved.
CHEMICAL STABILITY
Before we proceed to discuss energy changes in detail it is first
necessary to be clear that two factors determine the stability of a
chemical system—stability here meaning not undergoing any
chemical change. These two factors are the energy factor and the
kinetic factor.
62