Le Châtelier’s Principle If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change If a component (reactant or product) is added to a reaction system at equilibrium (constant T and P or constant T and V), the equilibrium position will shift in the direction that lowers the concentration of that component. If a component is removed, the opposite effect occurs.

As4O6(s) + 6C(s) <-> As4(g) + 6CO(g) Example As4O6(s) + 6C(s) <-> As4(g) + 6CO(g) Predict the direction of the shift of the equilibrium position in response to each of the following changes in conditions: Addition of carbon monoxide Addition or removal of carbon or tetraarsenic hexoxide (As4O6) Removal of gaseous arsenic (As4)

Changing Pressure Add or remove a gaseous reactant or product Add an inert gas (one not involved in the reaction) - won’t affect equilibrium Change the volume of the container**

N2(g) + 3H2(g) <-> 2NH3(g) Changing Volume When the volume of a container changes, the system also must change its volume (by changing the number of molecules present) Products/Reactants will decrease/increase based on what will have the larger affect (how many molecules are present) N2(g) + 3H2(g) <-> 2NH3(g) Decreasing volume -> shifts right Increasing volume -> shifts left

Example If the volume is reduced, what will happen to the equilibrium? P4(s) + 6Cl2(g) <-> 4PCl3(l) Answer: Shifts RIGHT PCl3(g) + Cl2(g) <-> PCl5(g) PCl3(g) + 3NH3(g) <-> P(NH2)3(g) + 3HCl(g) Answer: No Shift

Changing Temperature This will affect the equilibrium constant (K) Depending on whether the reaction is endothermic (heat is a reactant) or exothermic (heat is a product) will determine which way the reaction shifts N2(g) + 3H2(g) <-> 2NH3(g) + 92kJ Energy Added (increase temp) = Eq shift L, K decreases 556kJ + CaCO3(s) <-> CaO(s) + CO2(g) Energy Added (increase temp) = Eq shift R, K increases

Predict how K will change as temperature is increased: Example Predict how K will change as temperature is increased: N2(g) + O2(g) <-> 2NO(g) ∆H° = 181kJ Answer: shifts right, K increases b. 2SO2(g) + O2(g) <-> 2SO3(g) ∆H° = -198kJ Answer: shifts left, K decreases

Summary - Know Concepts For Test 58 kJ + N2O4(g) <-> 2NO2(g) CHANGE SHIFT Add N2O4(g) Right Add NO2(g) Left Remove N2O4(g) Remove NO2(g) Add He(g) None Decrease Container Volume Increase Container Volume Increase Temperature Decrease Temperature