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Chapter 15 Chemical Equilibrium John D. Bookstaver St. Charles Community College Cottleville, MO Lecture Presentation 2012 Pearson Education, Inc. Slide 2 Equilibrium The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. 2012 Pearson Education, Inc. Slide 3 Equilibrium 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. 2012 Pearson Education, Inc. Slide 4 Equilibrium A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant. 2012 Pearson Education, Inc. Slide 5 Equilibrium Depicting Equilibrium Since, in a system at equilibrium, both the forward and reverse reactions are being carried out, we write its equation with a double arrow: N2O4(g)N2O4(g)2NO 2 (g) 2012 Pearson Education, Inc. Slide 6 Equilibrium The Equilibrium Constant 2012 Pearson Education, Inc. Slide 7 Equilibrium The Equilibrium Constant Forward reaction: N 2 O 4 (g) 2NO 2 (g) Rate law: Rate = k f [N 2 O 4 ] 2012 Pearson Education, Inc. Slide 8 Equilibrium The Equilibrium Constant Reverse reaction: 2 NO 2 (g) N 2 O 4 (g) Rate law: Rate = k r [NO 2 ] 2 2012 Pearson Education, Inc. Slide 9 Equilibrium The Equilibrium Constant Therefore, at equilibrium Rate f = Rate r k f [N 2 O 4 ] = k r [NO 2 ] 2 Rewriting this, it becomes kfkrkfkr [NO 2 ] 2 [N 2 O 4 ] = 2012 Pearson Education, Inc. Slide 10 Equilibrium The Equilibrium Constant The ratio of the rate constants is a constant at that temperature, and the expression becomes K eq = kfkrkfkr [NO 2 ] 2 [N 2 O 4 ] = 2012 Pearson Education, Inc. Slide 11 Equilibrium 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 + bBcC + dD 2012 Pearson Education, Inc. Slide 12 Equilibrium The Equilibrium Constant 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 ) 2012 Pearson Education, Inc. Slide 13 Equilibrium Relationship Between K c and K p From the ideal-gas law we know that Rearranging it, we get PV = nRT P = RT nVnV 2012 Pearson Education, Inc. Slide 14 Equilibrium Relationship Between K c and K p Plugging this into the expression for K p,, for each substance, the relationship between K c and K p becomes where K p = K c (RT) n n = (moles of gaseous product) (moles of gaseous reactant) 2012 Pearson Education, Inc. Slide 15 Equilibrium Equilibrium Can Be Reached from Either Direction As you can see, the ratio of [NO 2 ] 2 to [N 2 O 4 ] remains constant at this temperature no matter what the initial concentrations of NO 2 and N 2 O 4 are. 2012 Pearson Education, Inc. Slide 16 Equilibrium Equilibrium Can Be Reached from Either Direction These are the data from the last two trials from the table on the previous slide. 2012 Pearson Education, Inc. Slide 17 Equilibrium Equilibrium Can Be Reached from Either Direction It doesnt matter whether we start with N 2 and H 2 or whether we start with NH 3, we will have the same proportions of all three substances at equilibrium. 2012 Pearson Education, Inc. Slide 18 Equilibrium What Does the Value of K Mean? If K>>1, the reaction is product-favored; product predominates at equilibrium. If K Equilibrium If Q > K, there is too much product, and the equilibrium shifts to the left. 2012 Pearson Education, Inc. Slide 36 Equilibrium If Q < K, there is too much reactant, and the equilibrium shifts to the right. 2012 Pearson Education, Inc. Slide 37 Equilibrium Le Chteliers Principle 2012 Pearson Education, Inc. Slide 38 Equilibrium Le Chteliers Principle If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance. 2012 Pearson Education, Inc. Slide 39 Equilibrium The Haber Process The transformation of nitrogen and hydrogen into ammonia (NH 3 ) is of tremendous significance in agriculture, where ammonia-based fertilizers are of utmost importance. 2012 Pearson Education, Inc. Slide 40 Equilibrium The Haber Process If H 2 is added to the system, N 2 will be consumed and the two reagents will form more NH 3. 2012 Pearson Education, Inc. Slide 41 Equilibrium The Haber Process This apparatus helps push the equilibrium to the right by removing the ammonia (NH 3 ) from the system as a liquid. 2012 Pearson Education, Inc. Slide 42 Equilibrium Changes in Temperature Co(H 2 O) 6 2+ (aq) + 4Cl(aq)CoCl 4 (aq) + 6H 2 O(l) 2012 Pearson Education, Inc. Slide 43 Equilibrium Catalysts 2012 Pearson Education, Inc. Slide 44 Equilibrium Catalysts Catalysts increase the rate of both the forward and reverse reactions. 2012 Pearson Education, Inc. Slide 45 Equilibrium Catalysts When one uses a catalyst, equilibrium is achieved faster, but the equilibrium composition remains unaltered. 2012 Pearson Education, Inc.