Chapter 14 - 1 Chapter 14 Chemical Equilibrium Dr. David P....

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Unformatted text preview: 1 Chapter 14 Chemical Equilibrium Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II 2 Learning Objectives I- Know what a dynamic equilibrium is.- Know how to determine mass action expressions for a reaction.- Understand the relationship between Q and K .- Be able to convert reaction coefficients to equilibrium expression superscripts for equilibrium reactions.- Be capable in manipulating equations for chemical equilibria.- Know under what conditions Kc can be converted into Kp .- Understand the meaning of the magnitude of K . 3 General Concepts A reaction system attains Dynamic Equilibrium when there is no further change in concentrations. When such a system is at equilibrium, the forward and reverse reaction are proceeding at the same rate Equilibrium is signified with double arrows or the equal sign 4 The equilibrium mixture is independent of whether we start on the reactant side or the product side The equilibrium between N 2 O 4 and NO 2 . 5 The same equilibrium composition is reached from either the forward or reverse direction, provided the overall system composition is the same. Pure NO 2 is brown and pure N 2 O 4 is colorless. The amber color of the equilibrium mixture indicates that both species are present at equilibrium. Reaction Reversibility 6 There is a simple relationship among the concentrations of the reactants and products for any chemical system at equilibrium It is called the mass action expression , and is derived from thermodynamics (discussed in Chapter 20) Consider the equilibrium: ) ( HI 2 ) ( I ) ( H 2 2 g g g + Equilibrium Laws 7 The numerical value of the mass action expression is called the reaction quotient, Q At equilibrium (and 440 o C) for this reaction the reaction quotient has the value 49.5 (a unitless number) This relationship is called the equilibrium law for the system Q = = ] ][ [ 2 2 I H [HI] 2 Mass Action Expression 8 The value 49.5 is called the equilibrium constant, K c , and characterizes the system For chemical equilibrium to exist, the reaction quotient Q must be equal to the equilibrium constant K c C) 440 (at 5 . 49 ] I ][ H [ [HI] o 2 2 2 = = c K gG fF eE dD + + The Equilibrium Constant , K c 9 Consider the general system at equilibrium: The exponents in the mass action expression are the same as the stoichiometric coefficients At equilibrium c ] [ ] [ ] [ ] [ K E D G F e d g f = gG fF eE dD + + 10 Your Turn! Which of the following is the correct mass action expression (MAE) for the reaction: Cu 2+ (aq) + 4NH 3(aq) [Cu(NH 3 ) 4 2+ ] (aq) these of none D . Q ] ) [C u(N H ] ][N H [C u C ....
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This note was uploaded on 04/24/2010 for the course CHE 1120 taught by Professor Brown during the Spring '10 term at St. John's.

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Chapter 14 - 1 Chapter 14 Chemical Equilibrium Dr. David P....

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