Background The chemical reaction will stay at chemical equilibrium until some other factor affect
its condition. Regardless of how the system changes, there are only one of these two
1. The forward reaction speeds up to produce more product.
2. The reverse reaction speeds up to produce more reactant.
After some time, the reaction will reach a new equilibrium point again.
What is the Le Chatelier's principle?
During the 1888, a French chemist Henri Louis Le Chatelier determined that the direction
in which chemical equilibrium shifts can be figured out using a general principle.
Le Chatelier's principle - A change of conditions applied to a system in
equilibrium will result a shift in the direction that best reduces the stress and a new
equilibrium position will be reached.
Let's look at some of the types of changes that affect equilibrium.
1. Change in Concentration
If the concentration increases in the product or the reactant, the reaction will react
faster for the side that got the increase. Example: If we add some N2 to the above reaction at equilibrium point,
the forward reaction will increase in speed until a new equilibrium is reached.
If the concentration decreases in the product or the reactant, the reaction will react
faster on the side that did not get the decrease in the concentration. Example: If we remove some 2NH3 from the above reaction, the forward
reaction will increase until a new equilibrium is reached.
2. Change in Volume If the volume increases, the reaction will shift in the direction that produces more
moles of gas. Example: In this case, the reverse reaction will occur because the reactants have
four total moles of gas vs. the two moles of gas in the product.
If the volume decreases, the reaction will shift in the direction that produces fewer
moles of gas. Example: In this case, the forward reaction will occur because the product has
fewer moles of gas than the reactant.
If the number of moles in the product and the reactant are the same, then nothing will
If there is no gas in the reaction, then nothing will happen.
3. Change in Temperature If the temperature increases: Example: We can look at the reaction the same as . We can treat the Heat the same as
another product, so if we increase the temperature it would be the same as increasing the
concentration of the product. So the reaction will move in the reverse direction.
A few other things will also effect the equilibrium point, such as the pressure change.
However, it is very important to note that the addition of a catalyst will NOT
change the equilibrium point.