Le Châtelier

Standard Know how to use Le Châtelier’s principle to predict the effect of changes in concentration, pressure, and temperature.

18.2 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

II.A. 3 Types of Stress Three kinds of stress: changes in the concentrations of reactants or products changes in pressure changes in temperature 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.

II.A.1. 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

II.A.1. 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: Shift to RIGHT b. Add chlorine gas Answer: Shift to RIGHT 2. 2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g) a. Remove O2 Answer: Shift to LEFT b. Add PbS Answer: NO Change c. Add SO2 Answer: Shift to LEFT 3. 2H 2S (g) + O2(g) 2S(s) + 2H2O (g) a. Add sulfur Answer: No Change b. Remove water vapor Answer: Shift to RIGHT c. Remove H2S Answer: Shift to LEFT

II.A.2. Pressure and Volume Changes Reaction shifts to change the equilibrium position (concentrations), not the equilibrium constant. A (g) + B (g) C (g) Change Shifts the Equilibrium ↑ 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

II.A.2. 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.

II.A.2. 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: (a) CO2 is the only gas present. To increase its yield, we should decrease the pressure. (b) There are more moles of gaseous reactants than products, so we should increase the pressure to shift the reaction to the right. (c) There are an equal number of moles of gases on both sides of the reaction, therefore a change in volume will have no effect.

II.A.3. 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

II.A.3. 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) heat An increase in temperature will shift the reaction to the left, decrease [Ca(OH)2], and decrease Keq. (b) SO2(g) + heat S(s) + O2(g) The reaction will shift right resulting in an decrease in [SO2] and increase in Keq.

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

QUIZ 3-5 Which way will the equilibrium shift with the following changes? 2PbS(s)+3O2(g)↔2PbO(s)+2SO2(g) a. Remove O2 b. Add PbS c. Add SO2

S (s) + 3F2(g) ↔ SF6(g) ΔH = -35 kJ QUIZ 3-6 S (s) + 3F2(g) ↔ SF6(g) ΔH = -35 kJ Which way would the equilibrium shift if: Increase pressure. Increase temperature Increase the amount of S Increase the amount of F2 Increase the amount of SF6

Cl2 (g) + I2 (g) ↔ 2ICl (g) ΔH = +35 kJ QUIZ 3-7 Cl2 (g) + I2 (g) ↔ 2ICl (g) ΔH = +35 kJ Which way would the equilibrium shift if: Increase pressure. Increase temperature Increase the amount of Cl2 Increase the amount of I2 Increase the amount of ICl

2H2S(g)+ O2(g) ↔ 2S(s)+ 2H2O(g) ΔH = 80kJ QUIZ 3-8 2H2S(g)+ O2(g) ↔ 2S(s)+ 2H2O(g) ΔH = 80kJ How would you adjust concentration to get more of the products? How would you adjust pressure to get more of the products? How would you adjust temperature to get more products? Write an equilibrium expression.

The Haber Process ‘During peace time a scientist belongs to the world, but during war time a scientist belongs to his country.’ Allows nitrogen compounds to be produced from nitrogen in the air. - freed Germany from having to import gun powder and fertilizer

The Haber Process N2(g) + 3H2(g) ↔ 2NH3(g) ΔH=-90kJ Write an equilibrium expression for this reaction. Calculate Keq if at equilibrium: [N2] = 2.0 M ; [H2] = 2.0 M ; and [NH3] = 6.0 M

The Haber Process N2(g) + 3H2(g) ↔ 2NH3(g) ΔH=-90kJ List 5 ways to increase the yield of ammonia in this process. (Be precise.) The Haber process is carried out at high temperature even though the yield is lower. Why?