Le Châtelier’s Principle When a system is at equilibrium: If any stress is placed on the system, equilibrium will shift to overcome the stress. Stressors can be: Adding products Adding reactants Changing temperature Etc.

Le Châtelier’s Principle To predict the shift, look at which side has gained something from the initial stress. The reaction shifts to the side opposite the side that gained from the stress.

Le Châtelier’s Principle How do you predict changes? After the change, determine which side has more substance. The reaction shifts to the opposite side.

Le Châtelier’s Principle To shift a reaction to the reactant side (): Add products Remove reactants If exothermic: Raise temperature If endothermic: Lower temperature

Le Châtelier’s Principle To shift a reaction to the product side (): Add reactants Remove products If endothermic: Raise temperature If exothermic: Lower temperature

Le Châtelier’s Principle 2SO2(g) + O2(g) ⇄ 2SO3(g) ∆H = -198 kJ Predict how this reaction shifts when the following variables are changed: Add O2 Add SO3 Raise the temperature Shifts to the products Shifts to the reactants

Special Note about Temperature Changes Temperature changes also affect Keq values. If a temperature change shifts the equilibrium to the right (), then Keq will INCREASE. If a temperature change shifts the equilibrium to the left (), then Keq will DECREASE.

Le Châtelier’s Principle Special Scenario for gases: If pressure increases, the equilibrium shifts to the side with less total moles of gas. If pressure decreases, the equilibrium shifts to the side with more total moles of gas.

Adding a Catalyst No affect on equilibrium

Le Châtelier’s Principle 2SO2(g) + O2(g) ⇄ 2SO3(g) ∆H = -198 kJ Predict how this reaction shifts when the following variables are changed: Cool down the system Increase the pressure Remove SO3 Shifts to the products Shifts to the products Shifts to the products

Review 2SO2(g) + O2(g) ⇄ 2SO3(g) ∆H = -198 kJ How does the reaction shift when: SO2 is added The temperature is raised The volume is increased A catalyst is added Shifts to the products Shifts to the reactants Shifts to the reactants No effect

Predicting Shift At Non-Equilibrium Conditions If concentrations are not at equilibrium, the equilibrium constant is labeled as Q. Compare the Kc value to the Q value to predict shift: If Kc > Q, the equilibrium shifts to the product side. If Kc < Q, the equilibrium shifts to the reactant side. If Kc = Q, the system is at equilibrium.

Q & A The reaction 2A + B ⇄ 3C has an equilibrium constant value of Kc = 5.00. If you add the following solutions to a flask, which way will the reaction shift to reach equilibrium? [A] = 0.200 M [B] = 0.100 M [C] = 0.200 M [C]3 Q = [A]2 [B] [0.200 M]3 Q = [0.100 M]2 [0.200 M] Q = 4.00 Because Kc > Q, the reaction shifts to the products side.

More Q &A H2(g) + F2(g) ⇄ 2 HF(g) Kc = 115 What is the value of Kc for the reverse reaction? If the equation was written as 2H2(g) + 2F2(g) ⇄ 4 HF(g) what would the value of Kc be? [HF]2 [H2] [F2] 1 If 115 = Then = = 8.70  10-3 [H2] [F2] [HF]2 115 [HF]2 [HF]4 If 115 = Then = (115)2 = 13200 [H2] [F2] [H2]2 [F2]2