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.