Definition
LeChatlier's Principle - when a stress is applied to a system at equilibrium, the system will shift to relieve that stress and reestablish equilibrium
Endothermic reaction - requires energy (commonly heat) input for reaction to take place
Therefore:
Heat + A + B <---> AB
If you take away heat, reaction favors the reactants (left)
If you add heat, reaction favors the products (right)
Exothermic reaction- gives off energy in the process of chemical reaction
C + D <---> CD + Heat
If you take away heat, the reaction favors the products (right)
If you add heat, the reaction favors the reactants (left)
Now for the little bit more complex stuff: gas equilibrium
PV= nRT
Pressure and Volume are inversely related according to this universal equation
2A g + B g <---> AB g
If pressure is increased in this equation, volume will decrease and therefore system will shift to the side with the least number of moles of gas (right)
If pressure is decreased, volume will increase, and therefore the system will shift to the side with the most number of moles of gas (left)
Remember that equilibrium constant is specific for a system at a given temperature. No matter how much you change the volume, pressure, or concentration of a system, it will not change the equilibrium constant Keq (or Kc as some books call it). Only a change in temperature can change the value of Keq (Kc).
Whereas a change in pressure, volume, or concentration only changes the position of equilibrium (either left of right).
Hope that helps TeefRcool, if not let me know. If it still doesn't make sense, then consider investing in Schaum's outline of College Chemistry...it teaches you the basics of equilibrium from the beginning. Or you can invest in a set of cds called AudioOsmosis (put out by ExamKrackers), they do an excellent job of teaching chemistry basics and providing intuitive ways of understanding the matter (the price is steep though, $200 for the cds, consider borrowing them from a pre-med friend).
Peace,
IndDent