Chap15-part4

# Chap15-part4 - Alternative method Calculating equilibrium...

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Alternative method: Calculating equilibrium concentrations when the starting concentrations and the equilibrium constant are known: 1) Make a table with the known information. 2) Use the stoichiometry of the reaction to write the changes in concentrations as a function of an unknown variable (e.g., “x”). 3) Calculate the equilibrium concentrations as a function of x. 4) Write an algebraic expression for K as a function of x. 5) Find the value of x. 6) Calculate the equilibrium concentrations.

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Sample Exercise 15.12 Information: The initial concentration of H 2 is 1.000 M. The initial concentration of I 2 is 2.000 M. The initial concentration of HI is 0. The equilibrium constant is K c = 50.5
1) Make a table with the known information. H 2 (M) I 2 (M) HI (M) initial 1.000 2.000 0 change final

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2) Use the stoichiometry of the reaction to write the changes in concentrations as a function of an unknown variable (e.g., “x”). H 2 (M) I 2 (M) HI (M) initial 1.000 2.000 0 change -x -x +2x final
3) Calculate the equilibrium concentrations as a function of x. H 2 (M) I 2 (M) HI (M) initial 1.000 2.000 0 change -x -x +2x final 1.000 - x 2.000 - x 2x

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4) Write an algebraic expression for K c as a function of x. H 2 (M) I 2 (M) HI (M) initial 1.000 2.000 0 change -x -x +2x final 1.000 - x 2.000 - x 2x
5) Find the value of x. K c = 4 x 2 (1 x )(2 x ) 50.5(1 x )(2 x ) = 4 x 2 50.5(2 3 x + x 2 ) = 4 x 2 46.5 x 2 151.5 x + 101 = 0 x = b ± 2 4 ac 2 a x = ( 151.5) ± 151.5) 2 4(46.5)(101) 2(46.5) x = 151.5 ± 64.5 93.0 = 2.32, 0.935

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6) Calculate the equilibrium concentrations. Choose x = 0.935 so that the concentrations are positive. H 2 (M) I 2 (M) HI (M) initial 1.000 2.000 0 change -x -x +2x final 1.000 – 0.935 = 0.065 2.000 – 0.935 = 1.065 2(0.935) = 1.870
Example: Unimolecular Isomerization K equilibrium constant A equil concentration of A at equilibrium B equil concentration of B at equilibrium A initial initial concentration of A B initial initial concentration of B C total concentration (constant) A B

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Unimolecular Isomerization Solve for B equil : Solve for A equil :
Unimolecular Isomerization: Sample Calculation K = 3 A initial = 0.60 M B inital = 0.05 M Note that

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Efect oF Temperature and Pressure on the Haber Process The percent of NH 3 at equilibrium in mixtures starting with a 3:1 molar ratio of H 2 and N 2 varies with temperature and pressure. Larger amounts of NH 3 are found at lower temperatures and higher pressures.
Le Châtelier’s 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.”

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If a system is at equilibrium and is disturbed by a change in 1) one or more reactant or product concentrations 2) pressure 3) temperature the system will shift to re-achieve equilibrium.
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## This note was uploaded on 09/13/2011 for the course CHEM 102 taught by Professor Todd during the Spring '08 term at UNC.

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Chap15-part4 - Alternative method Calculating equilibrium...

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