LeChatelier's Principle

Le Chatelier's Principle states that if an equilibrium system is disturbed, it will undergo a reaction that takes it back to equilibrium.  Changes in concentration cause a net reaction away from the added component toward the removed component. 

You can think of this as being a bit like a child's seesaw. Two children of equal weights might be able to hold the seesaw in perfect balance, but if a heavier child gets on one end, the lighter child might need to add a few rocks in order to restore the balance on the seesaw. If a reaction at equilibrium experiences a change in the moles of gas, pressure, or volume, a net reaction that partially reverses the change will occur in order to restore the equilibrium.  Although the concentration of the components changes as a result of the concentration and volume changes, K, ( the equilibrium constant) does not change. Nor does a catalyst affect the equilibrium position because it speeds the reaction equally in both the forward and reverse directions.

However,a temperature change does change K by causing a net reaction to replace removed heat (exothermic direction) or to absorb added heat (endothermic direction).  The following table summarizes these affects known as LeChatelier's Principle.
 
 

Change in Concentration	Change in Volume (Pressure)	Change in Temperature
K remains the same If concentration of product is increased, then concentration of reactant must increase to maintain the numerical value of K.  As a result, the equilibrium concentration of product is lower than it was when disturbance occured but concentration of reactants is higher. Conversely, if reactant concentration is increased, the equilibrium position moves to the product side and more product is  produced while the concentration of the reactant is decreased in order to reach equilibrium following the disturbance.	K remains the same Changing the volume of the container changes the concentration of the gases in the chamber.  When volume is reduced, pressure is increased and so the concentration increases. The equilibrium position will shift to relieve the pressure by reducing the number of moles of gas. If the number of moles of reactant and product are the same then there will be no effect on equilibrium	K does not remain the same when the temperature is changed The direction of the change depends on whether or not the reaction is exothermic or endothermic.  An increase in temperature will cause the equilibrium of an exothermic reaction to shift towards the reactants. A drop in temperature for the same system would cause an increase in the product concentration.
Example: The concentration of H2 is increased in the following system.  Will the conentration of HI increase or decrease when the new equilibrium is reached? H2 + I2    2HI Solution: The concentration of HI will increase.  Since K remains the same even after the concentration of hydrogen gas is increased, the concentration of the product must increase too.  The concentration of hydrogen gas at the new equilibrium will be lower than it was when more was added but higher than it was at the previous equilibrium.	Example: How would you change the volume of the following system if you wanted to increase the yield of products? S(s) + 3F2(g)       SF6(g) Solution: The solid sulfur does not enter into the calculation of K for this system.  That leaves 3 moles of reactant and 1 mole of product.  Increasing the pressure or reducing the volume of the container would favor the side with the smaller number of gas moles and consequently increase the yield.	Example: The system PCl3(g)  + Cl2(g)   PCl5(g) is exothermic in the forward direction (- delta Hrxn).  How would increasing the temperature affect the equilibrium concentraton of Cl2(g), and the K? Solution: The addition of heat to an exothermic reaction will favor the reverse reaction so  more reactants will be made.  The concentration of chlorine gas will increase.  Since the reactants will be increasing in concentration relative to the products, the numerical value of K will be smaller.