Lec-14-Chap15-4-bare

Lec-14-Chap15-4-bare - Lec-14-Chap15-4-bare 10/5/2011...

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Lec-14-Chap15-4-bare 10/5/2011 Chemical Equilibrium Calculations 1 Lec-14: Chemical Equilibrium Chapter 15 1 C+ D+ A+ B+ ab cd For a reaction         D AB C c K the equilibrium constant expression is Then, for chemical reactions in the gas phase we can choose to work with         CD o r cp K PP P K P  Equilibrium Constants: General Case K c allows us to answer questions concerning: At equilibrium , do reactants of products predominate? Given initial concentrations of reactants and products , in which way does the reaction proceeds? What are the concentrations of reactant and products at equilibrium?         D C c K 2
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Lec-14-Chap15-4-bare 10/5/2011 Chemical Equilibrium Calculations 2 Prediction of the direction of a reaction Given the initial composition, comparison of the values of Q c and K c allows us to decide which way the reaction will proceed. 3 For the reaction C+ D+ A+ B+ ab cd         D AB C c K The concentrations of reactants and products are those at equilibrium.         0 0 0 0 B C A D b c a Q The concentrations of reactants and products are those before the reaction takes places (that is, how the system is prepared). Prediction of the direction of a reaction 4                 00 concentrations are at equilibrium concentrations are initial or instan equilibrium constant expression reaction quotient taneo 0 u 0 s ; CD C A B A D B c c b d c a K Q the reaction is alread if th y at e the react quilibriu ion proceeds towar the reaction pro e ceeds towards the pro ds rea m c du t ct n s ants c c Q K     = > <
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Lec-14-Chap15-4-bare 10/5/2011 Chemical Equilibrium Calculations 3 Prediction of the direction of a reaction We begin by calculating the initial concentrations :                 3 2 3 4 3 3 4 2 2 0 3 3 2 0 7 25.0 10 m 8.0 10 mol ol NO g reactan N O g product 78.0 10 mol mol N O 7 t 25.0 10 mo of in 10 10.0 of i l m .8 10 ol NO 2.5 10 n 10 10.0 V V  5         2 2 24 2 2 4 2 2NO g N 1.7 N 10 at O Ng 5 O C O 2 c K Suppose that at 25°C we add to an empty container of volume 10.0 L 25.0×10 –3 mol of NO 2 (g) and 78.0×10 –3 mol of N 2 O 4 (g). What happens? Prediction of the direction of a reaction To determine whether the reaction will occur and in what direction we calculate the reaction quotient Q c       3 0 22 3 3 2 0 NO 7.8 2.5 10 10 1.248 10 c Q and compare it with the value of K c = 1.7×10 2 . Clearly c c Q K The reaction system is not at equilibrium and the reaction will proceed spontaneously in the direction such that the concentrations of the reactants increase and the concentrations of the products decrease so that as time progresses Q c changes towards the value of K c . 6
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This note was uploaded on 03/06/2012 for the course CHE 132 taught by Professor Hanson during the Fall '08 term at SUNY Stony Brook.

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Lec-14-Chap15-4-bare - Lec-14-Chap15-4-bare 10/5/2011...

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