 On the basis of many observations, the French chemist Le Chatelier in 1884 proposed a simple rule which could be used to predict how an equilibrium system will shift when it is subjected to a stress, such as a change in concentration, change in temperature, etc. Le Chatelier’s Principle (LCP)  When a stress is applied to a system at equilibrium, the system adjusts so as to offset the effect of the stress. (Think of a system as preferring its original equilibrium condition. If it is disturbed and is not at equilibrium, it will adjust or shift so as to get back to an equilibrium condition.)

Examples:Predicting Equilibrium Shifts Reaction:N 2(g) + 2O 2(g) + 34kJ heat 2NO 2(g) More N 2 is added to the container  If more N 2 is added, [N 2 ] increases. This stress disrupts the equilibrium condition.  To reduce the stress, the system will try to use up the extra N 2 molecules by means of the forward reaction. Thus [N 2 ] and [O 2 ] will decrease since they are being used up in the forward reaction, and [NO 2 ] increases.  The equilibrium thus shifts to the RIGHT and reaches a new equilibrium condition.

Increase the temperature by adding heat N 2 + 2O kJ (heat) 2NO 2  The added heat is a stress, which can be reduced by using up the “extra” heat in the forward, endothermic reaction.  Thus [N 2 ] and [O 2 ] decrease, [NO 2 ] increases and the equilibrium shifts to the right.

The Volume of the container is reduced. Reaction:N 2(g) + 3H 2(g) 2NH 3(g)  Reducing the volume stresses the molecules by crowding them together in the container. The stress can be reduced by stimulating the forward reaction, which will produce fewer molecules in the container ( since for every 4 molecules which react, 2 are produced).  This gives a lower [N 2 ], [H 2 ] and a greater [NH 3 ]; the new equilibrium position is farther to the right.

1.Which way will the equilibrium shift if more H 2 is added to this reaction at equilibrium? N 2 + 3H 2 2NH 3 2. Using the same reaction, which way will the equilibrium shift if some NH 3 is removed from the reaction when it is at equilibrium?

3.Which way will the equilibrium shift if the system temperature goes up? 2SO 2 + O 2 2SO 3 + heat 4. Using the same reaction, which way will the equilibrium shift if heat is removed?

5. The container holding the following reaction (already at equilibrium) has its volume suddenly reduced by half. Which way will the equilibrium shift to compensate? PCl 3 + Cl 2 PCl 5

6. The container holding the following reaction (already) at equilibrium has its volume suddenly increased. Which way will the equilibrium shift to compensate? H 2 + Cl 2 2HCl

7. The system below is already at equilibrium when a catalyst is added to the system. What happens to the position of the equilibrium? PCl 3 + Cl 2 PCl 5