Changing Equilibrium

themometer.jpg (27084 bytes)
Do changes in concentration, temperature, pressure, catalysts and surface area change a system at equilibrium?

Since an equilibrium only occurs when the rates of the forward and reverse reactions are equal, anything that can change the reaction rate has the potential of changing equilibrium.  The following factors are all known to change the rates of some chemical reactions:

Do they all have effects on equilibrium?  They will only if they change the rate of the forward and reverse reactions unequally.

Remember that equilibrium is a lot like walking one step forward and one step back.   Obviously, you move a lot when you do this, but you get nowhere.  This is a dynamic process which has equal forward and reverse rates.  Try walking two steps forward and two steps back.  Do you get any farther?  Of course not, because although your rate of movement is different, it is still the same in the forward and reverse directions.

Chemical reactions are the same.  Once the reaction is at equilibrium, if you go equally faster in both the forward and reverse directions, the reaction will not appear to change at all.

An equilibrium can only change if the rates in the forward and reverse directions change differently.

Your goal in this chapter should be to understand the factors that can change a system at equilibrium, and be able to make predictions of change on equilibrium systems using le Châtelier's principle.

By the time you have completed this chapter you should be able to:

  • identify what factors can change a reaction rate
  • identify which of these factors will change the rate unequally in the forward and reverse directions
  • use le Châtelier's principle to predict what effect change will have on systems at equlibrium
  • carry out experimental procedures that demonstrate le Châtelier's principle
  • explain various technologies which have behaviours that demonstrate le Châtelier's principle