Le Chatelier’s Principle If an equilibrium system is stressed or disturbed, the system will change to partially relieve or undo the stress and a new equilibrium will eventually be established When the stress is applied, the reaction is no longer at equilibrium and will shift to regain equilibrium. Ex. if the concentration of a reactant in a system in equilibrium is decreased, products will be consumed to produce more of that reactant.

Le Chatelier’s Principle Heat is a reactant in endothermic reactions and a product in exothermic reactions Ex. Increasing temperature will shift an endothermic reaction to the right to regain equilibrium. Any endothermic chemical reaction is exothermic in the reverse direction.

Le Chatelier’s Principle Pressure is proportional to concentration for gases; therefore, for chemical reactions that have a gaseous product or reactant, pressure affects the system as a whole. Ex. Increased pressure shifts the equilibrium toward the smaller number of moles of gas Ex. If both sides of the equilibrium have an equal number of moles of gas, increasing pressure does not affect the equilibrium. Ex. Adding an inert gas, such as argon, to a reaction will not change the partial pressures of the reactant or product gases and therefore will have no effect on the equilibrium.

Le Chatelier Problems Consider the following equilibrium: N2O4(g) 2NO2(g) H° = +58.0 kJ Write down whether you think the following changes will shift equilibrium to the left, to the right, or whether no shift will occur: 1. Adding N2O4 2. Decreasing the temperature 3. Increasing the volume 4. Remove NO2 5. Adding N2, an inert gas 6. Adding a catalyst