Quick Revision of Equilibria


Students learn to explain the effect of changing the following in an equilibrium reaction: pressure, volume, concentration, temperature.

A sealed vessel with a liquid-vapour equilibrium

Part1:A sealed vessel largely filled with water and with some space at the top (which can contain some air, it doesn't matter - the curious can look up Dalton's Law of Partial Pressures), if kept between 0 and 100 °C, is an example of an equilibrium system. There is liquid water present in the lower half and in the top part is some water vapour (the vessel must be sealed to prevent this vapour escaping). The interesting part is that the molecules in the liquid phase and the gas phase are always exchanging. Another way of saying this is that there is a dynamic equilibrium between the two phases.

We use this to illustrate Le Châtelier's Principle.

Le Châtelier's Principle

This may be formally stated as "if a system at equilibrium is disturbed by an external change, then the equilibrium adjusts to oppose the external change". This may be expressed as Nature Is A Real Swine (there are several less polite ways of expressing this).

Let's illustrate the effect of a temperature change. We know that going from liquid to vapour is an endothermic reaction: heat has to be put in to convert liquid water to water vapour, and conversely that if water evaporates, the system cools (one cools down when one sweats if the sweat can evaporate). Le Châtelier's principle is then illustrated by realising that if we heat the vessel containing liquid water and water vapour, this external change is opposed by the reaction taking in this heat to form more water vapour.

The effect of a pressure change is illustrated by realising that as liquid goes to vapour, the pressure is increased; hence if the vessel is compressed (note a volume change in this system is the same as a pressure change), this external change is opposed by vapour condensing (hence reducing the pressure).

The effect of changing concentration, i.e. adding a substance to one half of the reaction, is illustrated by realising that if we add water vapour (i.e. add something to the product side of the reaction), this external change is opposed by water vapour condensing to liquid water, that is the reaction goes to the left (i.e. forms more reactant). You can see that this is the case by considering what happens if you have a glass of water in a (sealed) room where the humidity is 100% (like Darwin in the Wet Season!). Adding a second glass of water has no effect: there is no net evaporation or condensation.