Five ideas in chemical education that must die: Le Châtelier’s principle

The second idea in our series is the principle that ‘a system in equilibrium that is subjected to a stress will react in such a way as to counteract the stress’. This is a qualitative principle to predict the outcome of any change made in pressure, concentration or temperature to a system that is in a state of equilibrium. The problem is that it can fail to make the correct prediction in two out of the three cases that it is intended for, namely pressure and temperature changes.

To predict what happens when the pressure of a system is increased, a student might imagine a balloon filled with gas whose pressure is being increased by squeezing the balloon down to a smaller volume. The student might be justified in thinking that the system would react by increasing its volume to oppose the initial reduction in volume. This would of course be an incorrect prediction since it is known that systems proceed in the direction of volume decrease, following any increase in total pressure.

To educators, it might appear as a legitimate manoeuvre to rescue the wording in the supposedly guiding principle, and many inform students that what was intended was a pressure increase without a change in volume and that this has been the cause of the incorrect prediction. But this might only serve to confuse students further.

Inspired by the book This Idea Must Die Eric Scerri explores outdated chemical concepts, in this second article he discusses Le Châtelier’s principle.