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CHAPTER 17 EQUILIBRIUM: THE EXTENT OF CHEMICAL REACTIONS 17.1 If the rate of the forward reaction exceeds the rate of reverse reaction, products are formed faster than they are consumed. The change in reaction conditions results in more products and less reactants. A change in reaction conditions can result from a change in concentration or a change in temperature. If concentration changes, product concentration increases while reactant concentration decreases, but the K c remains unchanged because the ratio of products and reactants remains the same. If the increase in the forward rate is due to a change in temperature, the rate of the reverse reaction also increases. The equilibrium ratio of product concentration to reactant concentration is no longer the same. Since the rate of the forward reaction increases more than the rate of the reverse reaction, K c increases (numerator, [products], is larger and denominator, [reactants], is smaller). K c = [ ] [] products reactants 17.2 The faster the rate and greater the yield, the more useful the reaction will be to the manufacturing process. 17.3 A system at equilibrium continues to be very dynamic at the molecular level. Reactant molecules continue to form products, but at the same rate that the products decompose to re-form the reactants. 17.4 If K is very large, the reaction goes nearly to completion. A large value of K means that the numerator is much larger than the denominator in the K expression. A large numerator, relative to the denominator, indicates that most of the reactants have reacted to become products. One cannot say with certainty whether the value of K for the phosphorus plus oxygen reaction is large or small (although it likely is large). However, it is certain that the reaction proceeds very fast. 17.5 No , the value of Q is determined by the mass action expression with arbitrary concentrations for products and reactants. Thus, its value is not constant. 17.6 The equilibrium constant expression is K = [O 2 ]. If the temperature remains constant, K remains constant. If the initial amount of Li 2 O 2 present was sufficient to reach equilibrium, the amount of O 2 obtained will be constant, regardless of how much Li 2 O 2 ( s ) is present. 17.7 a) On the graph, the concentration of HI increases at twice the rate that H 2 decreases because the stoichiometric ratio in the balanced equation is 1 H 2 :2 HI. Q for a reaction is the ratio of concentrations of products to concentrations of reactants. As the reaction progresses the concentration of reactants decrease and the concentration of products increase, which means that Q increases as a function of time. H 2 ( g ) + I 2 ( g ) ' 2 HI( g ) Q = [ ] 2 22 HI HI 17-1
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The value of Q increases as a function of time until it reaches the value of K . b) No , Q would still increase with time because the [I 2 ] would decrease in exactly the same way as [H 2 ] decreases.
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This note was uploaded on 02/14/2010 for the course MAE chm2046 taught by Professor Dick during the Spring '10 term at Universidade Federal do Rio de Janeiro.

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