ch14 MSJ jlm - Chapter 14 Chemical Equilibrium Henri L le...

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1 Chemical Equilibrium Chemical Equilibrium Chapter 14 Chapter 14 Henri L. le Chatlier 1850-1936. Adapted thermodynamics to equilibria; formulated the principle known by his name.
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2 The Concept of Equilibrium The Concept of Equilibrium Consider colorless N 2 O 4 . At room temperature, it decomposes to brown NO 2 : N 2 O 4 ( g ) 2NO 2 ( g ). (colorless) (brown) N 2 O 4 ( g ) 2NO 2 ( g ) The double arrow implies a dynamic equilibrium At some point, the forward and reverse rates become the same . This is equilibrium . Furthermore, it is a dynamic equilibrium. As NO 2 is produced, it reacts back to form N 2 O 4 : N 2 O 4 ( g ) 2NO 2 ( g )
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3 N 2 O 4 ( g ) 2NO 2 ( g )
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4 The Concept of Equilibrium The Concept of Equilibrium Consider Forward reaction: A B Rate = k f [A] Reverse reaction: B A Rate = k r [B] At equilibrium k f [A] = k r [B]. For an equilibrium we write A B As the reaction progresses [A] decreases to a constant, [B] increases from zero to a constant. When [A] and [B] are constant, equilibrium is achieved.
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5 The Concept of Equilibrium The Concept of Equilibrium Consider reaction: A(g) B(g) At beginning: P A = P Ao P B = 0 As reaction proceeds, P A decreases and P B increases Equilibrium occurs when there is no further change in concentration as time progresses . P A P B o A P
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6 The Concept of Equilibrium The Concept of Equilibrium Alternatively: As reaction proceeds , until the two rates become the same. This is equilibrium . Or, when k f [A] = k r [B], equilibrium is achieved. forward rate, k f [A] decreases reverse rate, k r [B] increases
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7 The Equilibrium Constant The Equilibrium Constant Consider N 2 ( g ) + 3H 2 ( g ) 2NH 3 ( g ) If we start with a mixture of N 2 and H 2 (in any proportions), the reaction will reach equilibrium with a constant set of pressures of N 2 , H 2 and NH 3 . If we start with just NH 3 and no N 2 or H 2 , the reaction will proceed in reverse and N 2 and H 2 will be produced until equilibrium is achieved, with a constant set of pressures of N 2 , H 2 and NH 3 .
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