Lecture 6--Applications of the Equilibrium Constant

Lecture 6--Applications of the Equilibrium Constant -...

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Applications of the Equilibrium Constant Ch 302, Fall 2009 B. A. Rowland September 23, 2009
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Outline In the last lecture we described the idea of chemical equilibrium. We learned that equilibrium is a dynamic process. We learned how to write equilibrium expressions using the law of mass action. Given any chemical equation we can write the equilibrium expression. We discussed the concept of the equilibrium constant . Today, we will learn how to apply the equilibrium constant in order to predict the composition of systems at equilibrium.
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Equilibrium Expressions Involving Gases Thus far we have expressed our equilibrium expressions in terms of concentrations . What happens when the system is gaseous? For the ammonia reaction: N 2(g) + 3H 2(g) 2NH 3(g) . We can express K as: For systems which are gaseous, we can write an alternative form of the equilibrium constant (K p ) in terms of the partial pressures of the gases: [ ] [ ] [ ] 2 3 3 2 2 NH K N H = ( 29 ( 29 3 2 2 2 3 NH p N H P K P P =
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Converting K and K p Remember the ideal gas law: pV = nRT. We can express the concentration (C). The ideal gas law becomes: p/RT = C. Substituting this into the equilibrium expression for ammonia, we get: We see that a general rule for the conversion of K to K p is: In this expression, n is the difference between the number of moles of gaseous products minus the number of moles of gaseous reactants. Note that K and K p will have the same numerical value if n = 0. [ ] [ ] [ ] ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 3 3 2 2 2 2 2 2 2 2 2 2 3 3 3 3 2 2 3 NH NH p N H N H P P NH RT K RT K RT N H P P P P RT RT = = = × =               ( 29 n p K K RT = V
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Activities Thermodynamics often refers to a reference state. For gases, the “true” reference state is defined as 1 bar.
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This note was uploaded on 01/30/2010 for the course GOV 312L taught by Professor Madrid during the Fall '07 term at University of Texas at Austin.

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Lecture 6--Applications of the Equilibrium Constant -...

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