CHM 116 Ch 15 II-Equilibrium

CHM 116 Ch 15 II-Equilibrium - 15.5 Calculating Equilibrium...

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1 1 1 15.5 Calculating Equilibrium Constants z If the equilibrium concentrations or partial pressures of all reactants and products are given, just plug into the K expression. z (We did this already) z If one or more is missing, use stoichiometry to figure out the missing values for the K expression (Example 15.9) 2 Calculating Equilibrium Constants z ICE Tables are useful for simplifying these calculations. 3 Calculating Equilibrium Constants z Sample Problem (Like 15.9) z H 2 (g) + I 2 (g) U 2HI(g) z 1.000x10 -3 M H 2 and 2.000x10 -3 M I 2 are allowed to react. z The equilibrium concentration of HI was determined to be 1.87 x 10 -3 M z Calculate the value of K. z To solve problem, first determine equilibrium concentrations if H 2 and I 2 , then plug into K expression (use an ICE table). 4 Determining Equilibrium Concentrations (ICE Table) H 2 (g) + I 2 (g) 2HI(g) Initial 1.000x10 -3 2.000x10 -3 Change Equilibrium 1.87x10 -3 5 Determining Equilibrium Concentrations (ICE Table) H 2 (g) + I 2 (g) U 2HI(g) Initial 1.000x10 -3 2.000x10 -3 0 Change -X -X +2X Equil. -3 - X -3 - X 1.87x10 -3 What is X? 6 Determining Equilibrium Concentrations to get K c z 2x = 1.87x10 -3 M z x = 0.935x10 -3 M z [H 2 ] eq = 1.000x10 -3 - x = 0.065x10 -3 M z [I 2 ] eq = 2.000x10 -3 - x = 1.065x10 -3 M z [HI] eq = 1.87x10 -3 M
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2 2 7 Determining Equilibrium Concentrations to get K c z K c = [HI] 2 / [H 2 ][I 2 ] z K c = (1.87x10 -3 ) 2 / (0.065x10 -3 )(1.065x10 -3 ) z K c = 51 8 15.5 Applications of Equilibrium Constants z If K value is unknown and/or if more than one equilibrium concentration is unknown, then initial concentrations must be used to calculate equilibrium concentrations. 9 Approach for Solving Equilibrium Problems z Write a balanced chemical equation. z Select one of the concentration changes and call it x. z Use stoichiometry to determine all the concentration changes in terms of x. z Make a table containing the substances, their initial concentrations, their changes in concentration, and their equilibrium concentrations (calculated from C o and x). (ICE table) 10 ICE Approach continued z Write the equilibrium constant expression. z Insert the equilibrium concentration expressions from the table into the equilibrium constant expression. z Solve the equation for x, applying any possible simplifications (perfect squares or dropping terms when small). z Check simplifications for validity (5% error is tolerable). If not valid, return to step 7 and do not use the simplification. 11 ICE Approach continued Check your answer: z Substitute the value of x into the expressions for the equilibrium concentrations and determine their values. z Use the equilibrium concentrations to calculate the reaction quotient, and compare it to the equilibrium constant to verify the accuracy of the answers.
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CHM 116 Ch 15 II-Equilibrium - 15.5 Calculating Equilibrium...

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