Last month I ran over the basics of equilibria. The rationale for this was that I felt that it was a part of the course that had been poorly answered in the recent IB May examinations. I do need to stress that this is my own personal opinion and not that of the IB.

This month I will run over the more difficult concepts associated with equilibrium, Le Chatelier’s principle.

Le Chateliers principle allows you to determine what happens to a system at equilibrium when you impose a change on it. It is possible to impose the following four changes to a system:

1. A change in concentration

2. A change in pressure

3. A change in temperature

The effect of each of these changes will be considered separately, but in order to determine what happens we use an important principle that was first put forwards by Henry Louis Le Chatelier in 1884. Le Chatelier carried out work on reactions in equilibria and realised that whatever you did to a reaction, it responded and did the opposite.

By Le Chatelier, Public Domain

The principle is fondly referred to as Le Chatelier’s principle, but it is worth remembering that it is a law of opposites. Whatever you do, the reaction does the opposite.


There are actually four changes that you could make with the concentration. You could increase the concentration or reactants or decrease them OR increase the concentration of products or decrease them.

So, if, for example, the concentration of reactants are increased, the equilibrium responds by doing the opposite – by decreasing them. The only way it can do this is by making more products. So the equilibrium will shift to the left.

The other changes can be summarised:

Reactant concentration decrease – equilibrium increases the concentration of reactants – shifts to the left to make more reactants (and less products).

Product concentration increase – equilibrium decreases the concentration of products – shifts to the left to make more reactants (and less products).

Product concentration decrease – equilibrium increases concentration of products – shifts to the right to make more products (and less reactants).

The same idea works for pressure, except for pressure it is important to understand that all of the reactants and products generate pressure. So if the pressure is increased, the reaction tries to decrease the pressure and does this by shifting the equilibrium to the side with the fewest moles of gas, as this will have the lowest pressure. A corresponding decrease in pressure has the opposite effect – the reaction tries to increase the pressure and does this by shifting to the side of more moles of gas (ie, the side that generates the most pressure).

Temperature is similar to pressure, except in order to decide the effect of temperature, one needs to know if the reaction is exothermic or endothermic. An endothermic reaction is only endothermic from left to right. It will be exothermic in the right to left direction. The opposite applies for an exothermic reaction.

An increase in temperature (an exothermic change) will shift the reaction in the endothermic direction and a decrease in temperature (an endothermic change) shifts the reaction in the exothermic direction.

How do you find the topic of equilibria? Do you have any questions or any helpful ways of remembering these different ideas? If you do, please post them below as I would love to hear them.