2046chapter15

2046chapter15 - Chapter 15 Chemical Equilibrium Learning...

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1 Learning goals and key skills: h Understand what is meant by chemical equilibrium and how it relates to reaction rates h Write the equilibrium-constant expression for any reaction h Relate K c and K p h Relate the magnitude of an equilibrium constant to the relative amounts of reactants and products present in an equilibrium mixture. h Manipulate the equilibrium constant to reflect changes in the chemical equation h Write the equilibrium-constant expression for a heterogeneous reaction h Calculate an equilibrium constant from concentration measurements h Predict the direction of a reaction given the equilibrium constant and concentrations of reactants and products h Calculate equilibrium concentrations given the equilibrium constant and all but one equilibrium concentration h Calculate equilibrium concentrations given the equilibrium constant and the starting concentrations h Understand how changing the concentrations, volume, or temperature of a system at equilibrium affects the equilibrium position. Chapter 15 Chemical Equilibrium The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. The Concept of Equilibrium • As a system approaches equilibrium, both the forward and reverse reactions are occurring. • At equilibrium, the forward and reverse reactions are proceeding at the same rate . • Once equilibrium is achieved, the amount of each reactant and product remains constant.
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2 Chemical equilibrium occurs when opposing reactions are proceeding at equal rates. Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow. Forward reaction: N 2 O 4 (g) → 2 NO 2 (g) Rate Law: Rate = k f [N 2 O 4 ] N 2 O 4 ( g ) 2 NO 2 ( g ) Reverse reaction: 2 NO 2 (g) → N 2 O 4 (g) Rate Law: Rate = k r [NO 2 ] 2 Equilibrium Constant • Therefore, at equilibrium Rate f = Rate r k f [N 2 O 4 ] = k r [NO 2 ] 2 • Rewriting this, it becomes k f k r [NO 2 ] 2 [N 2 O 4 ] = K eq = k f k r [NO 2 ] 2 [N 2 O 4 ] = The Equilibrium Constant • Consider the generalized reaction The equilibrium expression for this reaction would be K c = [C] c [D] d [A] a [B] b aA + bB cC + dD Since pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written K p = ( P C ) c ( P D ) d ( P A ) a ( P B ) b
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This note was uploaded on 12/13/2011 for the course CHM 2046h taught by Professor Lufaso during the Fall '11 term at UNF.

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2046chapter15 - Chapter 15 Chemical Equilibrium Learning...

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