Laidler, Meiser, Sanctuary textbook – Support Portal

Equilibrium is certainly one of the most interesting topics for a chemist! However, it seems that to study it is neither easy (nor enjoyable) for our students. This is probably due to the inherent difficulty in handling a system *not* using a *state function* corresponding to an *observable;* using rather, this strange *equilibrium constant*, a “practical” interpretation of which may seem unclear.

In the Physical Chemistry textbook, Chemical kinetics follow (Chapters 9 and 10) the equilibrium (Chapter 4), however I believe that they can give some useful insight to understanding *equilibrium* itself – or, to more specific, **dynamic equilibrium** (see Page 4-28, to find an easy and clear correlation between kinetics and equilibrium).

Given a reversible reaction, as the direct route (Reactants → Products) proceeds [R] decreases and [P] increases. Consequently, velocity (R→P) decreases and v (P→R) increases. At a certain state, the two velocities will become equal: that is, the same quantity of reactants which are consumed by the direct reaction is gained, at the same time, by the inverse reaction.

From a certain point of view, we can imagine this process like two cities full of cars, a certain number of which are moving from one to the other. If the number of cars leaving City 1 is equal to the number of cars leaving City 2, at any moment we will count so many cars in each city. The amount of cars in City 1 is not necessarily equal to the amount of cars in City 2; simply, their number in each city is the same, at any time.

The same can be said about reactions (the animation at Page 4-28, see screen grab above, is inspiring in this context)! Do not believe equilibrium to mean [R] = [P], but rest assured that, *given enough time*, the concentration (or the pressure, for gases) of the species in the reactor will find a fixed value, to be kept indefinitely.

So, what can the main problem for the student trying to understand chemical equilibrium be? Probably, the difference between chemical and *mechanical* equilibria: the latter is defined by a situation where “all is fixed”, while the former is a situation where “all MACROscopic variables are *fixed*”, but nothing is said about MICROscopic ones, which in turn can (and, probably, *have to*) be in *constant motion*.

Does it sound so difficult? I hope not, otherwise… well, you need to find *equilibrium* between your hate for this topic and your need to understand it! 🙂

Thanks again Gaetano. Indeed equilibrium is a concept we teach and try to define as you say.

When I teach chemical kinetics, I tell my students that we live in a dynamic world and equilibrium is a condition of no noticeable change. However I think it important to tell them that most reactions are constantly fed (We put food in our mouths and gas in our cars). We do not want equilibrium in manycases: our hearts and cars will stop!