# Calculating Kc or Keq Chapter 13 Pg 499-508.

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Calculating Kc or Keq Chapter 13 Pg

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.

Br2(g) + F2(g) <-->2BrF(g)
For Example 0.100 mol/L Br2 and mol/L F2 were initially mixed together and allowed to come to equilibrium according to the following reaction: Br2(g) + F2(g) <-->2BrF(g) At equilibrium the concentration of Br2 was found to be mol/L. Calculate the value of K.

Fill this in… Br2(g) F2(g) 2BrF(g) Initial Conc. 0.100M
Change in conc. Final Conc. M

Should Look like: Br2(g) F2(g) 2BrF(g) Initial Conc. 0.100M
Change in conc. M +[2 x M] =0.1973M Final Conc. M 0.1973M

Good ol’ Stoichiometry
We used stoichiometry (the relationship with the balanced chemical equation to help us solve for the final concentrations) Since Br2 and F2 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 Br2, so we had to double the change in concentration and therefore the final answer.

We’re not finished! Now we can solve for K because we found the final concentrations (at equilibrium) K =[BrF]2 / [Br2][F2] =[0.1973M]2/ [ M][ M] =2.15 x 104

SAMPLE PROBLEM 1 The following concentrations were found at equilibrium for the reaction: N2(g) + 3H2(g)<--> 2NH3(g) [N2] = 0.25 M; [H2] = 0.45 M; [NH3] = 0.60 M Calculate K. Answer: K=16

Sample Problem 2: A closed container initially had a CO(g) concentration of M and a H2O(g) concentration of M. It was allowed to reach equilibrium. At equilibrium, analysis showed a CO2(g) concentration of M. The equationfor the reaction is: CO(g) + H2O(g) <-->CO2(g) + H2(g) Calculate K Answer: K=5.00

SAMPLE PROBLEM 3 A closed chemical system initially contained 6.0 M SO2; 2.5 M NO2; and 1.0 M SO3. Equilibrium was eventually reached for the reaction SO2(g) + NO2(g)<--> SO3(g) + NO(g) At equilibrium, the container was found to have 3.0 M SO3 present. Calculate K. Answer: K=3.00

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