3._Chapter16_equilibrium_post - Principles of Chemical...

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Ch. 16 Equilibrium 1 Principles of Chemical Equilibrium Chemical Equilibrium: That stat reached when the concentration of reactants and products remain constant over time. Up to this point we have focused only on the conversion of reactant into products. However, many reaction do not go to completion. Once products are formed they can combine to give back the reactants that you started with. Consider the rates of the forward and the reverse reactions in a equilibrium as elementary processes a A + b B + … k -1 k 1 c C + d D + … (Rate) for = k 1 [A] a [B] b (Rate) rev = k -1 [C] c [D] d Initially the rate of the forward reaction is large because their concentration is large and the concentration of C and D is zero or small. As A and B are consumed and C and D are produced the rate of the forward reaction decreases and the rate of the reverse reaction decreases. At some point the rate of the forward reaction will equal the rate of the reverse reaction and the concentrations of A, B, C and D remain constant. At this point a state of dynamic equilibrium has been reached.
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Ch. 16 Equilibrium 2 The Equilibrium Constant Expression a A + b B k -1 k 1 c C + d D (Rate) for = k 1 [A] a [B] b (Rate) rev = k -1 [C] c [D] d At equilibrium the rate of the forward reaction = rate of the reverse reaction k 1 [A] a eq [B] b eq = k -1 [C] c eq [D] d eq Where [ ] eq = the concentration mol/L at equilibrium. Rearrange: [ ] [ ] [ ] [ ] c b eq a eq d eq c eq K B A D C k k = = - 1 1 This expression is called the equilibrium constant expression and its numerical value is the equilibrium constant (K c ) [ ] [ ] [ ] [ ] reactants products = = b eq a eq d eq c eq c B A D C Q At equilibrium the Reaction quotient Q c is a function whose value changes as the concentration changes K c is a number equal to one particular but special value of Q c The subscript c reminds us that we are writing the terms with respect to concentration K c is a constant that is independent of the starting concentrations.
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Ch. 16 Equilibrium 3 Methanol synthesis is a reversible reaction given in the equation below: CO (g) + 2 H 2 (g) CH 3 OH (g) Note the change in arrows, which means the mixture has reached dynamic equilibrium Given the data below show that the reaction quotient has the same value at equilibrium Q c =
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Ch. 16 Equilibrium 4 Review The reaction quotient Q c has the same value at equilibrium regardless of the starting material K c = Q at equilibrium = Q c The composition of the system at equilibrium does depend on the starting concentrations. There are an infinite number of equilibrium states but each one gives the same value of Q c The spontaneous direction of the reaction can be predicted by comparing the instantaneous value of Q with its equilibrium value (K) When Q < K , the reaction must go in the forward direction to reach equilibrium i.e. Q must get bigger When Q > K , the reaction must go in the reverse direction to reach equilibrium i.e. Q must get smaller When Q = K , there is no net reaction i.e. the system has reach a state of dynamic equilibrium.
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