Tuesday, April 17, 2012

Why temperature and pressure?

Before introducing the Gibbs free energy in my thermodynamics class I asked the students to say which variables they thought were generally the "easiest" to control in chemistry and physics experiments: volume and temperature, volume and energy, pressure and temperature, ...., or all?

Many students thought volume and temperature. (Maybe because what they mostly learn about is gases!).
I think the "correct" answer is pressure and temperature because
* these are environmental not system variables
* it is very hard to control the volume of a solid.

Am I right?
I now realise this is a rather subtle point and worth getting students to think about.
Appreciating it makes students think about experiments rather than mathematics and helps motivate why the Gibbs free energy is actually the most useful thermodynamic function.

1 comment:

  1. One point is that our world is most naturally described as being one of constant pressure - not constant volume. In other words a majority of chemical processes that occur daily are open to the constant pressure of the atmosphere.

    Another point is, since any type of free energy is measuring the useful work that can be extracted from a system, we should think about the most common form of that work. Engines are rarely based on constant volume processes, because if there is no PV work, then you can only get out heat, rater than mechanical work.

    So, I'd agree that the Gibbs function is probably the most useful in the sense that it is appropriate for a majority of common applications. However, there are important constant volume measurements in chemistry like bomb calorimetry.