le chatelier's principle

Le Chatelier's principle states that when a system in chemical equilibrium is disturbed by a change of temperature, pressure, or a concentration, the system shifts in equilibrium composition in a way that tends to counteract this change of variable. The three ways that Le Chatelier's principle says you can affect the outcome of the equilibrium are as follows:
  • Changing concentrations by adding or removing products or reactants to the reaction vessel.
  • Changing partial pressure of gaseous reactants and products.
  • Changing the temperature.

3 ways that Le Chatelier's principle says you can affect the outcome of equilibrium


These actions change each equilibrium differently, therefore you must determine what needs to happen for the reaction to get back in equilibrium.

example involving change of concentration:

In the equation:
2NO(g) + O2(g) <--> 2NO2(g)

change of concentration


If you add more NO(g) the equilibrium shifts to the right producing more NO2(g) If you add more O2(g) the equilibrium shifts to the right producing more NO2(g) If you add more NO2(g) the equilibrium shifts to the left producing more NO(g) and O2(g)

ex. involving pressure change:

In the equation:
2SO2(g) + O2(g) <--> 2SO3(g)

ex. involving pressure change


an increase in pressure will cause the reaction to shift in the direction that reduces pressure, that is the side with the fewer number of gas molecules. Therefore an increase in pressure will cause a shift to the right, producing more product. (A decrease in volume is one way of increasing pressure.)

example involving temperature change:

In the equation:
N2(g) + 3H2(g) <--> 2NH3 + 91.8 kJ

ex. involving temperature change


an increase in temperature will cause a shift to the left because the reverse reaction uses the excess heat. An increase in forward reaction would produce even more heat since the forward reaction is exothermic. Therefore the shift caused by a change in temperature depends upon whether the reaction is exothermic or endothermic.