In this lesson, you will learn about Le Chatelier’s
Principle, which explains what a system at equilibrium does in response to “stresses”.
Let’s return to our original example of you digging a hole and your friend refilling it
simultaneously. If you start digging at a rate faster than
refilling, the hole gets larger. In order to maintain a constant size of the
hole, your friend must work harder to fill it faster.
Following on the same idea, when a chemical system at equilibrium is stressed, the system
works to restore equilibrium. This is Le Chatelier’s Principle.
The stresses are Changes to the concentration of either the
reactants or products Changes to the pressure, though this is only
applicable to gaseous systems Changes to the temperature
Let’s examine a hypothetical reaction at equilibrium. If we added more A and B, the system becomes
stressed and is no longer at equilibrium. To counteract the stress, the system forms
more C and D, in order to remove the excess A and B.
The equilibrium, therefore, “shifts” to the right.
As you can see, equilibrium has now been restored. If we added more C and D, the system becomes
stressed and is also no longer at equilibrium. To counteract the stress, the system forms
more A and B. Therefore, equilibrium shifts to the left.
What happens if we remove C and D as they are being produced, or in other words, if
the concentration of C and D is decreased? Please pause the lesson to think about this,
and resume when you are done. The system is now stressed and no longer at
equilibrium. To counteract the stress, more C and D are
produced, so equilibrium shifts to the right. When concentration increases, equilibrium
shifts to the opposite side of the reaction. When concentration decreases, equilibrium
shifts to the same side of the reaction. This stress to a system at equilibrium is
only applicable to gaseous systems. For this stress, we will examine another hypothetical
reaction at equilibrium: An increase in pressure means that there is
a decrease in volume, so there is less space. Equilibrium will shift to the side of the
reaction with fewer moles. In our example, an increase in pressure will
cause equilibrium to shift to the right, since there are fewer moles — 2 moles compared
to 3 moles on the left. A decrease in pressure means that there is
an increase in volume, so there is more space. Equilibrium shifts to the side with more moles,
so in our example, equilibrium shifts to the left.
So an increase in pressure favours the side with fewer moles, and a decrease in pressure
favours the side with more moles. In our next lesson, you will learn about how
a system works to restore equilibrium in response to changes in temperature.
In summary, LeChatelier’s principle states that when a system at equilibrium is stressed,
the system works to restore equilibrium.