Chpt 13 Lec - Chemistry 8/e Steven S. Zumdahl and Susan A....

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Chemistry 8/e Steven S. Zumdahl and Susan A. Zumdahl Chemical Equilibrium Chapter 13 1
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Equilibrium We’ve already used the phrase “equilibrium” when talking about reactions. In principle, every chemical reaction is reversible . .. capable of moving in the forward or backward direction. 2 H 2 + O 2 2 H 2 O Some reactions are easily reversible . .. Some not so easy . .. 2
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Equilibrium: the extent of a reaction In Stoichiometry we talk about theoretical yields, and the many reasons actual yields may be lower. Another critical reason actual yields may be lower is the reversibility of chemical reactions: some reactions may produce only 70% of the product you may calculate they can produce. Equilibrium looks at the extent of a chemical reaction. 3
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The Concept of Equilibrium Consider colorless frozen N 2 O 4 . At room temperature, it decomposes to brown NO 2 : N 2 O 4 ( g ) 2NO 2 ( g ). At some time, the color stops changing and we have a mixture of N 2 O 4 and NO 2 . Chemical equilibrium is the point at which the rate of the forward reaction is equal to the rate of the reverse reaction . At that point, the concentrations of all species are constant. Using the collision model: – At the beginning of the reaction, there is no NO 2 so the reverse reaction (2NO 2 ( g ) N 2 O 4 ( g )) does not occur. – as the amount of NO 2 builds up, there is a chance that two NO 2 molecules will collide to form N 2 O 4 . 4
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The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. 5
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The Concept of Equilibrium As the substance warms it begins to decompose: N 2 O 4 ( g ) 2NO 2 ( g ) When enough NO 2 is formed, it can react to form N 2 O 4 : 2NO 2 ( g ) N 2 O 4 ( g ). At equilibrium, as much N 2 O 4 reacts to form NO 2 as NO 2 reacts to re-form N 2 O 4 The double arrow implies the process is dynamic. N 2 O 4 ( g ) 2NO 2 ( g ) 6
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The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate. 7
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A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant. 8
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The Concept of Equilibrium 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. A B 9
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The Equilibrium Constant 10
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• No matter the starting composition of reactants and products, the same ratio of concentrations is achieved at equilibrium. • For a general reaction the equilibrium constant expression is where K c is the equilibrium constant.         b
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This note was uploaded on 10/27/2010 for the course CHEM 104 taught by Professor Quigley during the Summer '08 term at CUNY Hunter.

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Chpt 13 Lec - Chemistry 8/e Steven S. Zumdahl and Susan A....

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