CHM152Chapter 13

Chemistry (MasteringChemistry Series)

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9/3/2009 1 Chapter 13 - Chemical Equilibrium The Equilibrium State Not all chemical reactions go to completion. When you hear “equilibrium”, what do you think of? Example: weather patterns: ocean water evaporates at the same rate that it rains. They are in equilibrium. The Equilibrium State Equilibrium: The state reached when the concentrations of reactants and products remain constant over time. Most reactions are reversible. ± A + B C + D ± A and B react to make C and D ± But C and D can also react to make A and B
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9/3/2009 2 The Equilibrium State Figure 13.1 The Equilibrium State We commonly use concentrations to describe equilibrium. ± [ ] = M = moles/L ± At equilibrium, the concentrations of substances remain constant. ± The rate of the forward reaction is the same as the rate of the reverse reaction. Usually one side of the equation is favored (reactants or products). The Equilibrium Constant, K c The Equilibrium Constant, K c , tells us which side of the reaction is favored. 2 H 2 (g) + O 2 (g) 2 H 2 O (g) K =[H O 2 /[H 2 [O K c = [H 2 O] / [H 2 ] [O 2 ] ± K c is the equilibrium constant, a value ± The fraction is the equilibrium constant expression K c is constant at a particular T, K c is unitless.
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9/3/2009 3 The Equilibrium Constant, K c Write the equilibrium constant expressions (K c ) for the following equations. (Note: Expressions don’t include solids or liquids!) ± CO(g) + 3H 2 (g) CH 4 (g) + H 2 O(g) ² 2NH 3 (g) N 2 (g) + 3H 2 (g) ³ 2SO 2 (g) + O 2 (g) 2SO 3 (g) ± K c =[CH 4 ][H 2 O] / [CO][H 2 ] 3 ² K c = [N 2 ][H 2 ] 3 / [NH 3 ] 2 ³ K c = [SO 3 ] 2 / [SO 2 ] 2 [O 2 ] The Equilibrium Constant, K c 2 H 2 (g) + O 2 (g) 2 H 2 O (g) If K c = 2.4 x 10 47 (e.g., a large number) what does this tell you about the equilibrium of the reaction? ´ A. There are more reactants than products ´ B. There are more products than reactants ´ C. More information is needed. How can you figure this out? The Equilibrium Constant, K c 2 HBr (g) H 2 (g) + Br 2 (g) K c = 2 x 10 -19 (e.g., a small number), what does this tell you about the equilibrium of the reaction? ´ A. There are more reactants than products ´ B. There are more products than reactants ´ C. More information is needed.
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9/3/2009 4 Calculating K c 2NH 3 (g) N 2 (g) + 3H 2 (g) At 500 K, the following concentrations were measured: [N 2 ] = 3.0 x 10 -2 M, [H 2 ] = 3.7 x 10 -2 M, 2 [NH 3 ] = 1.6 x 10 -2 M. What is K c ?
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CHM152Chapter 13 - 9/3/2009 Chapter 13 - Chemical...

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