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Session 3-5: Chemical Equilibrium - Le Chatelier's principle

Le Chatelier's Principle: Part 1

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.

Le Chatelier's Principle: Part 2

In this lesson, you will learn about how a system at equilibrium responds to changes in temperature. LeChatelier's Principle states that a chemical system at equilibrium always works to restore equilibrium when it is stressed. To consider what happens to a system at equilibrium when temperature is changed, you must first consider the energetics of the reaction in question.

If the forward reaction is exothermic, then the reverse reaction must be endothermic. Let's examine this hypothetical reaction

 

A + B C + D ΔH = -75kJ This means that 75kJ of energy is released

when the forward reaction occurs, and 75kJ is absorbed when the reverse reaction occurs. So an increase in temperature would mean that the endothermic reaction would be favoured, to remove the excess heat, therefore counteracting the imposed stress.

Decreasing the temperature would cause the system to produce more energy; therefore the exothermic reaction would be favoured. An increase in temperature favours the endothermic reaction. A decrease in temperature favours the exothermic reaction. Challenge:

The dimerization of nitrogen dioxide to dinitrogen tetroxide is exothermic.

NO2(g) N2O4(g) ΔH = -57.2kJ brown colourless

 

Nitrogen dioxide is a brown gas, whereas dinitrogen tetroxide is colourless.

What observations do you think can be made when temperature is decreased?

How about when temperature is increased? Please pause the lesson to think about this,and resume once you are done.

A decrease in temperature favours the exothermic reaction, so more dinitrogen tetroxide is produced. Since it is a colourless gas, the mixture should appear paler.

An increase in temperature favours the endothermic reaction, so more nitrogen dioxide is produced. The mixture should therefore appear darker brown.

Addition of a catalyst does not affect the position of equilibrium as it increases the rate of both the forward and reverse reactions. It only quickens the attainment of equilibrium.

Let's return to the example of you digging a hole and your friend refilling it while you dig. Imagine that you are both given much larger shovels. The size of the hole still remains constant, but with each dig or fill, more soil is removed or filled.

In conclusion, when the temperature of a system at equilibrium is increased, the endothermic reaction is favoured. When the temperature of a system at equilibrium is decreased, the exothermic reaction is favoured. Adding a catalyst has no effect on the position of equilibrium.

 

 

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