With the exceptions of Chapters 2 and 3, our discussions have largely focused on equilibrium conditions, and chiefly on the equilibrium of systems in which the only kind of mechanical work that takes place is expansion work. Even with these severe restrictions thermodynamics can provide a wealth of information about the nature and evolution of planetary bodies. But these are by no means the limits of thermodynamics. In fact, one could argue that thermodynamics only begins to get interesting when these restrictions are lifted. An in-depth discussion of the possibilities that open up would demand an entire book, at least as long as this one. That is a fight for another day. The goal of this chapter is to lift a corner of the proverbial veil. I will introduce some of the principles of linear non-equilibrium thermodynamics and use them to examine chemical diffusion and chemical reaction mechanisms and rates. These are two classes of processes that are responsible for displacing systems towards equilibrium in a wide range of situations.

Non-equilibrium thermodynamics

By definition, equilibrium thermodynamics concerns itself with static systems and with “quasi-static” transformations between equilibrium states. These are abstractions, but if our discussions so far are any indication, they are very useful ones. Throughout the previous chapters we have also come across non-equilibrium processes, however. Examples include catastrophic planetesimal collisions (Chapter 2), heat transfer (Chapter 3) and non-isentropic melting (Chapter 10).