Le Chatelier's Principle

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  • Created on: 06-04-13 10:10
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Le Chantelier's Principle
If a dynamic equilibrium is disturbed by changing conditions, the reaction moves to counteract the
Things you to do to influence the reaction:
Increasing or decreasing the concentrations
Changing the pressure
Changing the temperature
Adding a catalyst (doesn't have an effect on the position of equilibrium
Adding and Removing Substances
If you add more A, the system responds by removing it. You might choose to
add more A if it is essential to convert as much of B as possible into products.
Alternatively, if you remove C the reaction will
respond by replacing it again by reacting more A and B. Removing a substance
formed is a useful way to move the equilibrium.
Changing the pressure
Only really applies to gas reactions and where the numbers of molecules on both sides of the
equilibrium are different.
If you increase the pressure, the reaction will respond by reducing it again. It can reduce the pressure
creating more C and D. Increasing the pressure will help the reaction go in the direction which
produces the smaller number of molecules.
Changing the temperature
If the forward reaction is exothermic, the back reaction would be endothermic by exactly the same
If you decrease the temperature of the equilibrium, it will respond in such a way as to increase the
temperature again.
If more C and D are produced, more heat is given out because of the exothermic change. The extra
heat with warm the mixture again.
Increasing the temperature has exactly the opposite effect. The reaction will move to get rid of the
extra heat by absorbing it in an endothermic change.
Adding a catalyst
Adding a catalyst speeds up the reaction. It speeds up the forward and back reaction by the same
The overall effect of this is that there is no change in the equilibrium position. The catalyst is just
added to increase the rate at which the equilibrium is reached.

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