The problem restated

We are looking for a chemical energy term that will always decrease in such spontaneous reactions and will enable us to systematize and predict what way reactions will proceed under given conditions. This may seem like a simple problem, but it is not. 

In this chapter, we will try to explain why this is so. As with the first law, there is no way of proving the second law. It is a principle that is distilled from our experience of how things happen. It can be stated in a lot of different ways, usually having something to do with the impossibility of perpetual motion or with the availability of energy, topics that seem to have little to do with the problem we have set for ourselves - that of finding an energy term that always decreases in spontaneous reactions. We wiU choose to state it in a way that emphasizes its role as a directionality parameter. This leads to the shortest possible path to the practical applications we wish to consider. 

So here s the plan. We will define entropy as a parameter in our model systems, having certain properties. This definition is not, of course, pulled out of the blue, but is based on many years of work by many scientists. It should be accepted at first on faith, as simply a usefiil parameter, because it wiU not have any intuitive meaning as do our other terms such as energy, work, and so on. Then we wiU show how the chemical energy term we have been looking for is related to entropy. Finally, we wiU discuss what entropy is (and what it is not), and in Chapter 5, we will discuss how to measure it. 

---