Chapter-14-Tro_1 (1)

Chapter-14-Tro_1 (1) - Chapter 14: Chemical equilibrium...

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Chapter 14: Chemical equilibrium Forward reaction : when a chemical reaction occurs between reactants to from products, we refer to this reaction as forward reaction. Example: Reactants Products Reverse reaction : when some of the products formed in a given forward reaction revert back to reactants, we refer to this reaction as reverse reaction. Example: Reactants Products Reaction at equilibrium : When both forward and reverse reactions occur and the rate at which reactants convert to products becomes equal to the rate at which products are converted back to reactants, then the reaction is said to be in equilibrium . The reaction at equilibrium is represented by k + k - k + k - ____ Example: Consider the reaction, aA(g) + bB(g) cC(g)+dD(g) k + k - Rate of forward reaction=k + [A] a [B] b ; (see the chapter on Chemical kinetics) Rate of reverse reaction=k - [C] c [D] d ; (see the chapter on Chemical kinetics) At equilibrium, rate of forward reaction=rate of reverse reaction. Thus,
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k + [A] a [B] b = k - [C] c [D] d ; K= k + k - = [C] c [D] d [A] a [B] b _______ For a general reaction, a A(g) + b B(g) c C(g) + d D(g), Law of mass action Note that equilibrium constant does not have units. For this reason each concentration in the equilibrium constant expression is divided by a reference concentration of 1M. (1). When pure liquids or solids are involved in a reaction, they do not enter the equilibrium constant expression (2). When solutions are involved in a reaction, they enter equilibrium constant expressions in terms of their concentrations (3). When gases are involved in a reaction they enter the equilibrium constant expressions in terms of their partial pressures; but partial pressures can also be written in terms of concentrations using ideal gas equation (see later). When the equilibrium constant is expressed in terms of concentrations of reactants and products, the equilibrium constant is designated as K c . b d a c M B M D M A M C K = 1 ] [ 1 ] [ 1 ] [ 1 ] [
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Use of Pressures Using PV=nRT, P= n _ V RT = CRT n o p c RT P K K or = ) tan ( ts reac gaseous of moles products gaseous of moles b a d c n - = - - + = When a reaction involves all gases, the equilibrium constant can be written in terms of the partial pressures of products and reactants, and designated as K p . However, K p can be converted to the one in terms of concentrations, K c . For a general reaction, a A(g) + b B(g) c C(g) + d D(g), Law of mass action (P D /P o ) d = [C D RT/P o ] d ; (P C /P o ) c = [C C RT/P o ] c ;(P A /P o ) a = [C A RT/P o ] a and (P B /P o ) b = [C B RT/P o ] b. Substituting these relations in to expression for K
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Chapter-14-Tro_1 (1) - Chapter 14: Chemical equilibrium...

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