2 StandardKnow how to use Le Châtelier’s principle to predict the effect of changes in concentration, pressure, and temperature.
3 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
4 II.A. 3 Types of Stress Three kinds of stress: changes in the concentrations of reactants or productschanges in pressurechanges in temperatureWhen a stress is first applied to a system, equilibrium is disturbed and the rates of the forward and backward reactions are no longer equal.
5 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 substanceIf a substance is taken out of a reaction at equilibrium, the system will shift to produce more of the substance that was removed
7 II.A.1. Changes in Concentration Pencils downIn a reaction of two colored complex ions:pale blue blue-purpleWhen 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.
8 Pencils downPredicting the Effect of a Change in Concentration on the Equil PositionWhich way would the following equilibria shift in order to re-establish equilibrium:1. CO(g) + Cl2(g) COCl2(g)a. Remove COCl2Answer: Shift to RIGHTb. Add chlorine gasAnswer: Shift to RIGHT2. 2PbS(s) + 3O2(g) PbO(s) + 2SO2(g)a. Remove O2Answer: Shift to LEFTb. Add PbSAnswer: NO Changec. Add SO2Answer: Shift to LEFT3. 2H 2S (g) + O2(g) S(s) + 2H2O (g)a. Add sulfurAnswer: No Changeb. Remove water vaporAnswer: Shift to RIGHTc. Remove H2SAnswer: Shift to LEFT
9 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)ChangeShifts the Equilibrium↑ pressureSide with fewest moles of gas↓ volumeSide with fewest moles of gas↓ pressureSide with most moles of gas↑ volumeSide with most moles of gas
10 II.A.2. Pressure Changes cont’d Pencils downN2(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?
11 An equilibrium reaction that has the same # of moles of gas on both sides will not be affected by changes in pressure.
12 II.A.2. Predicting the Effect of a Change in Pressure on the Equilibrium Position Pencils downPROBLEM: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) ICl(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.
13 II.A.3. Temperature Changes H2(g) + I2(g) HI(g) + heatChangeExothermic RxEndothermic Rx↑ temperatureKeq ↓ (less products)Keq ↑↓ temperatureKeq ↑ (more products)Keq ↓***Temp—only factor that affects the value of Keq
14 II.A.3. Predicting the Effect of a Change in Temperature on the Equilibrium Position Pencils downPROBLEM: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 = 297kJPLAN: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) heatAn 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.
15 Le Châtelier’s Principle Change EquilibriumConstantChangeShift EquilibriumConcentrationyesnoPressureyesnoVolumeyesnoTemperatureyesyes
16 QUIZ 3-5Which way will the equilibrium shift with the following changes?2PbS(s)+3O2(g)↔2PbO(s)+2SO2(g)a. Remove O2b. Add PbSc. Add SO2
17 S (s) + 3F2(g) ↔ SF6(g) ΔH = -35 kJ QUIZ 3-6S (s) + 3F2(g) ↔ SF6(g) ΔH = -35 kJWhich way would the equilibrium shift if:Increase pressure.Increase temperatureIncrease the amount of SIncrease the amount of F2Increase the amount of SF6
18 Cl2 (g) + I2 (g) ↔ 2ICl (g) ΔH = +35 kJ QUIZ 3-7Cl2 (g) + I2 (g) ↔ 2ICl (g) ΔH = +35 kJWhich way would the equilibrium shift if:Increase pressure.Increase temperatureIncrease the amount of Cl2Increase the amount of I2Increase the amount of ICl
19 2H2S(g)+ O2(g) ↔ 2S(s)+ 2H2O(g) ΔH = 80kJ QUIZ 3-82H2S(g)+ O2(g) ↔ 2S(s)+ 2H2O(g) ΔH = 80kJHow 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.
20 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
21 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
22 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?