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Le Chatelier's principle

April 15, 2019

Le Chatelier's principle says that when there is a stress on a system that takes it away from equilibrium, the system will react in such a way as to get back to the favorable equilibrium position.

There are 4 stressors on the system. Lets list them and see what happens with each one.

Lets say our equation is

2A (aq) + B(s) <--->  C(g)

1. Concentration.

When concentration is increased, equilibrium shifts to the opposite side to make up for it. For example, adding more A shifts the equilibrium  to the right side, thus producing more C.

When concentration is decreased, equilibrium  shifts to the same side.

Changes in solids and liquids DO NOT shift the equilibrium.

2. Pressure and volume.

When volume is decreased, pressure is increased and equilibrium shifts to the side with less GAS moles. In the case of our equation, there are 0 moles of gas on the left and 1 on the right. Equilibrium will shift to the left.

When volume is increased, pressure is decreased and equilibrium shifts to the side with more gas moles.

3. Temperature.

In order to see the effect of changing temperature, we must first figure out on what side temperature is on. If the reaction is endothermic, heat is on the left side. If the reaction is exothermic, heat is on the right side.

Once this has been determined, we need to treat temperate as concentration and we can refer to point 1.

4. Catalyst

Catalyst speeds up the reaction in both forward and reverse directions. Catalyst does not effect the equilibrium and does not shift it.

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