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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?

  • 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.

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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