Chemical Equilbrium & Le Châtelier’s Principle

Le Châtelier’s Principle Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system adjusts in a way to reduce the change. Stress causes a change in a system at equilibrium

Types of Stress Three kinds of stress: When a stress is first applied to a system, equilibrium is disturbed and the rates of the forward and backward reactions are no longer equal. Three kinds of stress: changes in the concentrations of reactants or products changes in pressure changes in temperature

Changes in Concentration If more of a substance is added to a reaction at equilibrium, the system will shift to remove the excess amount of that substance If a substance is taken out of a reaction at equilibrium, the system will shift to produce more of the substance that was removed

Pencil’s down

Changes in Concentration Pencils down In a reaction of two colored complex ions: pale blue blue-purple When the reaction mixture in a beaker is pale blue, we know that chemical equilibrium favors the formation of reactants. **If ammonia is added, the system responds by forming more product and the solution becomes blue-purple.

Pencils down Predicting the Effect of a Change in Concentration on the Equil Position Which way would the following equilibria shift in order to re-establish equilibrium: 1. CO(g) + Cl2(g) COCl2(g) a. Remove COCl2 Answer: b. Add chlorine gas 2. 2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g) a. Remove O2 b. Add PbS c. Add SO2 3. 2H 2S (g) + O2(g) 2S(s) + 2H2O (g) a. Add sulfur Answer: b. Remove water vapor c. Remove H2S

Pressure and Volume Changes Reaction shifts to change the equilibrium position (concentrations), not the equilibrium constant. A (g) + B (g) C (g) Shifts the Equilibrium Change ↑ pressure Side with fewest moles of gas ↓ volume Side with fewest moles of gas ↓ pressure Side with most moles of gas ↑ volume Side with most moles of gas

Pressure Changes cont’d Pencils down N2(g) +3 H2(g) <=> 2 NH3(g) (a) A mixture of gaseous N2, H2, and NH3 at equilibrium. (b) Predict what happens when the pressure is increased. (c) Reaction occurs from left to right, decreasing the total number of gaseous molecules until equilibrium is re-established (d) Which molecule would end up with the highest concentration?

An equilibrium reaction that has the same # of moles of gas on both sides will not be affected by changes in pressure.

Predicting the Effect of a Change in Pressure on the Equilibrium Position Pencils down PROBLEM: How would you change the pressure of each of the following reactions to increase the yield of products? (a) CaCO3(s) CaO(s) + CO2(g) (b) S(s) + 3F2(g) SF6(g) (c) Cl2(g) + I2(g) 2ICl(g) PLAN: When gases are present a change in pressure will affect the concentration of the gas. If the pressure increases, the reaction will shift to fewer moles of gas and vice versa. SOLUTION:

***Temp—only factor that affects the value of Keq Temperature Changes H2(g) + I2(g) 2 HI(g) + heat Change Exothermic Rx Endothermic Rx ↑ temperature Keq ↓ (less products) Keq ↑ ↓ temperature Keq ↑ (more products) Keq ↓ ***Temp—only factor that affects the value of Keq

Predicting the Effect of a Change in Temperature on the Equilibrium Position Pencils down PROBLEM: How does an increase in temperature affect the concentration of the underlined substance and Keq for the following reactions? (a) CaO(s) + H2O(l) Ca(OH)2(aq) DH0 = -82kJ (b) SO2(g) S(s) + O2(g) DH0 = 297kJ PLAN: Express the heat of reaction as a reactant or a product. Then consider the increase in temperature and its effect on Keq. SOLUTION: (a) CaO(s) + H2O(l) Ca(OH)2(aq) (b) SO2(g) S(s) + O2(g)

Le Châtelier’s Principle Change Equilibrium Constant Change Shift Equilibrium Concentration yes no Pressure yes no Volume yes no Temperature yes yes