1.2.3 States

A serious problem in chemistry back in the olden days (the term olden days meaning any time at least one year before the author was born) was that of defining the state of a system.

The term state of a system is itself somewhat ambiguous. In plain English it would refer to the general condition of a system. And to an extent that is what is meant. But the technical term goes a bit further, it implies enough knowledge of the general condition of a system to allow that system to be exactly duplicated. A consequence of that is that the system must be at equilibrium. At equilibrium the properties of the system do not change with time.^1.2

In order to repeat an experiment (and that is the heart of an experimental science) one must be able to start a repetition with the system in exactly the same condition it was in before the experiment was done the first time. And one needs to be able to write those conditions down because, in fact, it could be someone else repeating your experiment.

How much do you have to write in order for your system to be duplicated exactly?

You could begin by telling the other experimenter about the properties of your system. Useful properties turn out to be things like the temperature, pressure, volume, number of moles of each constituent, the density, the total mass, etc. etc.

It was quickly found out that only a few properties had to be communicated. Once they were set, all the other properties of the two systems became identical. And how many properties did have to be communicated? It turned out experimentally that for a single component system no more than three are ever needed, as long as one is extensive. (Don't worry, I'll define that in a minute or two.) For more complex systems more properties are needed. The actual number depends on the number of components and the number of phases (like solid, liquid, etc.) present. There's a simple formula for it, called the Gibbs Phase Rule which we will derive later in this course.

Thermodynamic properties are classed into one of two groups, extensive and intensive. It is easy to tell the difference. All you have to do is to take your system and make an imaginary copy of it. Put the two systems in contact with each other and remove any barrier between them.

One of two things will happen. Either your property will remain the same, in which case it is intensive or it will double, in which case it is extensive. All thermodynamic properties are required to be in one of these two groups.

Take the pressure as an example. The pressure in each of the two systems must be the same since they are identical. Putting them together will result in no change in pressure. Thus the pressure is an intensive property.

On the other hand the volume will clearly double. Volume is an extensive property.

Intensive properties include things such as pressure, temperature, and density. Extensive ones include thing such as volume, number of moles, and energy.

One last note: it is interesting that chemical systems can have their state defined by specifying only a relative few variables. Imagine the difficulty of specifying the exact state of a kitchen in a typical house. The location and orientation of every pot, pan, dish, towel, etc., would have to be given and that is only for starters...