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2046chapter15 - Chapter 15 Chemical Equilibrium Learning...

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1 Learning goals and key skills: head2right Understand what is meant by chemical equilibrium and how it relates to reaction rates head2right Write the equilibrium-constant expression for any reaction head2right Relate K c and K p head2right Relate the magnitude of an equilibrium constant to the relative amounts of reactants and products present in an equilibrium mixture. head2right Manipulate the equilibrium constant to reflect changes in the chemical equation head2right Write the equilibrium-constant expression for a heterogeneous reaction head2right Calculate an equilibrium constant from concentration measurements head2right Predict the direction of a reaction given the equilibrium constant and concentrations of reactants and products head2right Calculate equilibrium concentrations given the equilibrium constant and all but one equilibrium concentration head2right Calculate equilibrium concentrations given the equilibrium constant and the starting concentrations head2right 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
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