Equilibrium: The Extent of Chemical Reactions

Kinetics applies to the speed of a reaction, the concentration of product appearing (or of reactant disappearing) per unit time. Equilibrium applies to the extent of a reaction, the concentration of product that has appeared given unlimited time, or when no further change occurs. At equilibrium: rate forward = rate reverse A system at equilibrium is dynamic on the molecular level; no further net change is observed because changes in one direction are balanced by changes in the other.

N 2 O 4 ( g ) 2NO 2 ( g ) Reaching equilibrium on the macroscopic and molecular levels

small K large K intermediate K The range of equilibrium constants

Equilibrium: no net change reactants products Reaction Progress reactants products Reaction Progress Reaction direction and the relative sizes of Q and K

+ lower P (higher V) more moles of gas higher P (lower V) fewer moles of gas The effect of pressure (volume) on an equilibrium system. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

solids do not change their concentrations

Converting Between K c and K p PROBLEM:Calculate K c for the following, if CO 2 pressure is in atmospheres. CaCO 3 (s) CaO(s) + CO 2 (g) K p = 2.1x10 -4 at 1000K

LeChatlier’s PrinciplePredicting the Effect of a Change in Temperature on the Equilibrium Position PROBLEM:How does an increase in temperature affect the concentration of the underlined substance and K c for the following reactions? (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

LeChatlier’s PrinciplePredicting the Effect of a Change in Concentration on the Equilibrium Position PROBLEM:To improve air quality and obtain a useful product, sulfur is often removed 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 ) What happens to (a) [H 2 O] if O 2 is added? (b) [H 2 S] if O 2 is added? (c) [O 2 ] if H 2 S is removed? (d) [H 2 S] if sulfur is added?

LeChatlier’s PrinciplePredicting the Effect of a Change in Volume (Pressure) on the Equilibrium Position PROBLEM:How would you change the volume of each of the following reactions to increase the yield of products. (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 )

Calculating K c from Concentration Data PROBLEM:In a study of hydrogen halide decomposition, a researcher fills an evacuated 2.00-L flask with 0.200mol of H I gas and allows the reaction to proceed at C. 2H I ( g ) H 2 ( g ) + I 2 ( g ) At equilibrium, [H I ] = 0.078M. Calculate K c. Equilibrium Activity

Determining Equilibrium Concentrations from Initial Concentrations and K c PROBLEM:Fuel engineers use the extent of the change from CO and H 2 O to CO 2 and H 2 to regulate the proportions of synthetic fuel mixtures. If mol of CO and mol of H 2 O are placed in a 125-mL flask at 900K, what is the composition of the equilibrium mixture? At this temperature, K c is 1.56 for the equation CO( g ) + H 2 O( g ) CO 2 ( g ) + H 2 ( g ) In-class activity

Calculating Equilibrium Concentration with Simplifying Assumptions PROBLEM:Phosgene is a potent chemical warfare agent that is now outlawed by international agreement. It decomposes by the reaction COCl 2 ( g ) CO( g ) + Cl 2 ( g ) K c = 8.3x10 -4 (at C) Calculate [CO], [Cl 2 ], and [COCl 2 ] when the following amounts of phosgene decompose and reach equilibrium in a 10.0-L flask. (a) 5.00 mol COCl 2 (b) mol COCl 2 In-class activity

K relationships aA + bB cC + dD K c = [C] c [D] d [A] a [B] b cC + dD aA + bB K ’ = 1/Q c aA + bB cC + dD n K c ’ = (Q c ) n For a sequence of equilibria, K overall = K 1 x K 2 x K 3 x …