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Calculating Kc or Keq Chapter 13 Pg

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In order to calculate the value of K, we need to know the concentration in mol/L of all reactants and products at equilibrium. Sometimes we do not immediately know all of the concentrations at equilibrium, but we are given enough information to determine all of the concentrations at equilibrium and then K.

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For Example mol/L Br 2 and mol/L F 2 were initially mixed together and allowed to come to equilibrium according to the following reaction: Br 2 (g) + F 2 (g) 2BrF(g) At equilibrium the concentration of Br 2 was found to be mol/L. Calculate the value of K.

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Fill this in… Br 2 (g)F 2 (g)2BrF(g) Initial Conc.0.100M 0 Change in conc. Final Conc M

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Should Look like: Br 2 (g)F 2 (g)2BrF(g) Initial Conc.0.100M 0 Change in conc M +[2 x M] =0.1973M Final Conc M M

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Good ol’ Stoichiometry We used stoichiometry (the relationship with the balanced chemical equation to help us solve for the final concentrations) Since Br 2 and F 2 were a 1:1 mole ratio and had the same initial concentration, they would have the same final. But BrF was a 1:2 mole ratio with Br 2, so we had to double the change in concentration and therefore the final answer.

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We’re not finished! Now we can solve for K because we found the final concentrations (at equilibrium) K=[BrF] 2 / [Br 2 ][F 2 ] =[0.1973M] 2 / [ M][ M] =2.15 x 10 4

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SAMPLE PROBLEM 1 The following concentrations were found at equilibrium for the reaction: N 2 (g) + 3H 2 (g) 2NH 3 (g) [N 2 ] = 0.25 M; [H 2 ] = 0.45 M; [NH 3 ] = 0.60 M Calculate K. Answer: K=16

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Sample Problem 2: A closed container initially had a CO(g) concentration of M and a H 2 O(g) concentration of M. It was allowed to reach equilibrium. At equilibrium, analysis showed a CO 2 (g) concentration of M. The equationfor the reaction is: CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) Calculate K Answer: K=5.00

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SAMPLE PROBLEM 3 A closed chemical system initially contained 6.0 M SO 2 ; 2.5 M NO 2 ; and 1.0 M SO 3. Equilibrium was eventually reached for the reaction SO 2 (g) + NO 2 (g) SO 3 (g) + NO(g) At equilibrium, the container was found to have 3.0 M SO 3 present. Calculate K. Answer: K=3.00

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Homework: Work on questions 11&12 pg. 508 (practicing with Kc) Tough questions: Pg 515 #25-29

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