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• 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?

• 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.

In this lesson, you will learn about Le Chatelier's Principle, which explains what a system at

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B2 US equilibrium system pressure stressed reaction concentration

# Le Chatelier's Principle Part 1 | The Chemistry Journey | The Fuse School

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Wayne Lin posted on 2015/06/27
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