17-1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Equilibrium: The Extent of Chemical Reactions

17-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 17.1 Reaching equilibrium on the macroscopic and molecular levels. N 2 O 4 ( g ) 2NO 2 ( g )

17-3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Equilibrium - the condition in which the concentrations of all the reactants and products in a closed system cease to change with time At equilibrium: rate forward = rate reverse no further net change is observed because changes in one direction are balanced by changes in the other but it doesn’t mean that the reaction had stopped. The amount of reactants and products are constant but they are not necessarily equal.

17-4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. If rate forward = rate reverse then k forward [reactants] m = k reverse [products] n = = K the equilibrium constant k forward k reverse [products] n [reactants] m The values of m and n are those of the coefficients in the balanced chemical equation. The rates of the forward and reverse reactions are equal, NOT the concentrations of reactants and products. K is dependent only of the temperature. Note: The terms for pure solids or pure liquids do not appear in the equilibrium constant expression

17-5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Equilibrium constant expression for N 2 O 4 ( g ) 2NO 2 ( g ) K =

17-6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PRACTICE PROBLEM The subscript “c” in K c indicates the equilibrium constant is based on reactant and product concentrations The value of “K” is usually shown as a unitless number, BUT IT ACTUALLY DOES HAVE A UNIT EXPRESSION Write the Equilibrium Constant for the combustion of Propane gas C 3 H 8 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) 1. Balance the Equation C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(g)

17-7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 17.2 The range of equilibrium constants small K large K intermediate K

17-8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thus, a mixture of N2 and O2 will react to a very small extent to produce NO at equilibrium. Thus, a mixture of N2 and H2 will almost completely be converted to NH3 at equilibrium.

17-9 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SAMPLE PROBLEM 15.2 In a 500 ml stainless steel reaction vessel at 900 ⁰ C, carbon monoxide and water vapor react to produce carbon dioxide and hydrogen. Evidence indicates that this reaction establishes an equilibrium with only partial conversion of reactants to products. Initially 2.00 mol of each reactant is placed in the vessel. K c foe this reactionis 4.20 at 900 ⁰ C. What amount concentration of each substance will be present at equilibrium?

17-10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cont… VT=500 mlnCO (g) =nH 2 O (g) =2.00 molK c =4.20T=900 ⁰C [CO (g) ]=[H 2 O (g) ]=2.00 mol/0.500L = 4.00 M Solve for x  K c =4.20= [CO 2(g) ] [H 2(g) ] [CO (g) ] [H 2 O (g) ] Concentration[CO (g) ][H 2 O (g) ][CO 2(g) ][H 2(g) ] Initial Change (algebraic) -x +x Change (numeric) Equilibrium (algebraic) 4.00-x xx Equilibrium (numeric)

17-11 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HOMEWORK P. 688 # 1, 3, 4, 6, 8, 10

17-12 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. LeChatelier’s Principle When a chemical system at equilibrium is subjected to a stress, the system will return to equilibrium by shifting to reduce the stress. If the concentration increases, the system reacts to consume some of it. If the concentration decreases, the system reacts to produce some of it.

17-13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FACTORS AFFECTING EQUILIBRIUM Concentration Changes Pressure Changes Temperature Changes Addition of a Catalyst

17-14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. If the conc. of a substance is increased, the equilibrium will shift in a way that will decrease the conc. of the substance that was added. I. Concentration Changes H2H2 I2I2 HI Decrease HI - forward Decrease I 2 -backward Increase H 2 - forward Increase HI - backward Where will the reaction shift?

17-15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Note: adding/removing a solid/liquid to an equilibrium system will not cause any shift in the position of equilibrium. addition of an inert gas such as He, Ar, Kr, etc. at constant volume, pressure and temperature does not affect the equilibrium. Increase CO 2 : Increase CaO Decrease CO 2 Adding Kr - backward - No effect - forward - No effect

17-16 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Practice Exercise:Predicting the Effect of a Change in Concentration on the Equilibrium Position PROBLEM:To improve air quality and obtain a useful product, chemists often remove sulfur from coal and natural gas by treating the fuel contaminant hydrogen sulfide with O 2 ; 2H 2 S( g ) + O 2 ( g ) 2S( s ) + 2H 2 O( g ) In what direction will the rection shift if (a) O 2 is added? (b) H 2 O is added? (c) H 2 S is removed? (d) sulfur is added?

17-17 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + lower P (higher V) more moles of gas higher P (lower V) fewer moles of gas I. Pressure Changes

17-18 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An increase in pressure (decrease in volume) shifts the position of the equilibrium in such a way as to decrease the number of moles of gaseous component. When the volume is increased (pressure decreased), a net reaction occurs in the direction that produces more moles of gaseous component The effect of pressure (volume) on an equilibrium system. Decrease in pressure : Decrease in volume: backward forward

17-19 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sample Problem SOLUTION: Predicting the Effect of a Change in Volume (Pressure) on the Equilibrium Position PROBLEM:For the following reactions, predict the direction of the reaction if the pressure is increased: (a) CaCO 3 ( s ) CaO( s ) + CO 2 ( g ) (b) S( s ) + 3F 2 ( g ) SF 6 ( g ) (c) Cl 2 ( g ) + I 2 ( g ) 2 I Cl( g ) (a) CO 2 is the only gas present. The equilibrium will shift to the direction with less moles of gas. Answer: backward (b)There are more moles of gaseous reactants than products. Answer: forward (c) There are an equal number of moles of gases on both sides of the reaction. Answer: no effect

17-20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Effect of a Change in Temperature on an Equilibrium Consider heat as a product or a reactant. In an exothermic reaction, heat is a product,  H 0 rxn = negative In an endothermic reaction, heat is a reactant,  H 0 rxn = positive CO kJ Increase temperature:backward

17-21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sample Problem SOLUTION: Predicting the Effect of a Change in Temperature on the Equilibrium Position PROBLEM:In what direction will the reaction shift if there is a decrease in temperature (a) CaO( s ) + H 2 O( l ) Ca(OH) 2 ( aq )  H 0 = -82kJ (b) CaCO 3 ( s ) CaO( s ) + CO 2 ( g )  H 0 = 178kJ (c) SO 2 ( g ) S( s ) + O 2 ( g )  H 0 = 297kJ (a) CaO( s ) + H 2 O( l ) Ca(OH) 2 ( aq )  heat A decrease in temperature will shift the reaction to the right (b) CaCO 3 ( s ) + heat CaO( s ) + CO 2 ( g ) The reaction will shift to the left (c) SO 2 ( g ) + heat S( s ) + O 2 ( g ) The reaction will shift to the left

17-22 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Addition of a Catalyst The presence of a catalyst has no effect on the position of the chemical equilibrium, since a catalyst affects the rates of the forward and reverse reactions equally.

17-23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17-24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HOMEWORK Le Chatelier’s Principle P. 695 # 1-3 P. 699 # 1-6 Visual Dictionary Due