# Lecture: Randomness

## The secret is avoid the “r” word.

When I taught equilibrium I never brought up anything about randomness or the role it played in the process of determining K values. I derived the mass action expression from the rate equations as follows.

Rate(forward) = Kf[A]a[B]b     Rate(reverse) = Kr[C]c[D]d

At equilibrium Rate(forward) =  Rate(reverse) Therefore:

Kf[A]a[B]= Kr[C]c[D]d   and Kf/Kr =  [C]c[D]d/ [A]a[B]b

You don’t find this very often in text books and for good reason. In fact some students will point out that something is wrong with all this when they see quite large K values for equilibriums where the forward reaction in endothermic. Based on energy alone this does not make much sense and there is also the problem in the fact that some reactions seem to go to completion. If the Kinetic energy distribution curve is in fact asymptotic there should always be at least some molecules with enough energy to make the reverse reaction occur. There has to be something else that is pushing these equilibriums away from what would make sense based on energy considerations alone. If this all comes out as you go along then you are in great shape and the need to introduce the drive towards maximum randomness as a new idea in the world is at hand.

A model of the interplay of Maximum Randomness and Minimum Energy