Sorry, I couldn’t resist. Great class on Wednesday folks! I really appreciate how prepared all of you were, and thought you had some great discussions. Be sure that you understand the parts of the activity we didn’t get to during class.

The first two questions are, I think related:

1. “Can we actually define what compressibility is?”
2. “Is the assumption from KMT that gases have negligible volume valid?”

The compressibility factor, Z is

As you proved in class, Z = 1 for an ideal gas. If Z < 1, the (real) gas exerts less pressure than an ideal gas, and if Z > 1, the (real) gas exerts more pressure than an ideal gas (assuming the same temperature and molar volume).

Great, so what does this mean? Well, if the gas is exerting less pressure than an ideal gas, it must be more compressible (less effort on your part to reduce the volume) than an ideal gas. If the gas is exerting more pressure than an ideal gas, it is less compressible. So the compressibility factor is a measure of how compressible the gas is compared to an ideal gas.

I don’t know if that is really useful, but it is quite useful for thinking about deviations from ideality (which is a large part of what pchem is all about). We also found out from the activity that when Z < 1, attractive forces between the real gas molecules are more important, and when Z > 1, the non-zero volume of the gas molecules is the important non-ideality.

This should match what you discovered in class:

• The pressure of a non ideal gas increases relative to ideal when the non-zero volume of the gas particles is taken into account (the volume occupied by the gas decreases).
• The pressure of a non ideal gas is decreased relative to an ideal gas when attractive forces are included.

So it looks like the assumptions of KMT are not exactly valid for real gases.

The last question from yesterday’s class was “Why do we subtract a term for the attractive forces to reduce the pressure, rather than just divide?” You’ll have to wait for tomorrow’s class to get the answer to that one!