Introduction

The first sentence of Gibbs' (1875) classic memoir “On the equilibrium of heterogeneous substances” is

Given the fact that virtually all of equilibrium thermodynamics can be derived by doing exactly that, as Gibbs did, this must rank as one of the world's great understatements. In this chapter, we begin to explore what Gibbs was referring to. By considering “the laws which govern any material system,” we should be able to find the answers to the questions we posed in Chapter 1.

The problem restated

Having taken a couple of chapters to get our terminology settled and to get used to discussing energy changes in systems, we must now get back to our main problem – what determines whether chemical processes will go or not go? Our method of determining this might be considered a bit simple-minded – we will simply determine the “chemical energy” differences between equilibrium states. Processes can take place spontaneously if they are in the direction of lowering the chemical energy. They cannot take place spontaneously in the opposite direction.

We have seen that the first great principle of energy transfers is that energy never disappears; it simply takes on different forms. It is the second principle or law that more directly addresses our main problem.