Something is missing

We mentioned in Chapter 1 that an early idea for understanding chemical reactions held that spontaneous reactions would always be accompanied by the loss of energy, because the reactants were at a higher energy level than the products, and they wanted to go “downhill.” This energy was usually thought to be in the form of heat, but this idea received a setback when it was found that some spontaneous reactions in fact absorb heat. Also, there are some reactions, such as the mixing of gases, where the energy change is virtually zero yet the processes proceed very strongly and are highly nonreversible. Obviously, something is missing. If the ball-in-valley analogy is right, that is, if reactions do proceed in the direction of decreasing chemical energy of some kind, something more than just heat is involved.

To learn more about chemical reactions, we have to become a bit more precise in our terminology and introduce some new concepts. In this chapter, we will define certain kinds of systems, because we need to be careful about what kinds of matter and energy transfers we are talking about; equilibrium states, the beginning and ending states for processes; state variables, the properties of systems that change during reactions; processes, the reactions themselves; and phases, the different types of matter within the systems. All these terms refer in fact to our models of natural systems, but they are also used to refer to things in real life.