Chapter%2015%20-%20Chemical%20Equilibrium

Chapter%2015%20-%20Chemical%20Equilibrium - Chapter 15...

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1 Chapter 15 Chemical Equilibrium
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2 Class Business
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3 15.1 The Concept of Equilibrium Chemical equilibrium applies to reactions that can occur in both directions. In a reaction such as: Even though the reactants are constantly forming products and vice-versa the amount of reactants and products does become steady at some point of time. When the net change in the amount of reactants and products is zero, we can say that the reaction has reached equilibrium . The equilibrium is a dynamic equilibrium the amount of products and reactants are constant ( note: they are constant and need not be equal ) , both reactions are still occurring . CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g)
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4 Example Consider the hypothetic reaction A B as shown below (A is red and B is blue ): Does this diagram indicate that the system reaches an equilibrium state? Explain.
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5 The Concept of Equilibrium - Contd. .. Consider the decomposition of N 2 O 4 : N 2 O 4 ( g ) 2 NO 2 ( g ) Colorless Brown Only N 2 O 4 at low temperature Mixture of N 2 O 4 and NO 2 Although the color stops changing, both reactions are still occurring. Chemical equilibrium occurs when a forward reaction and its reverse reaction proceed at the same rate .
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6 The Concept of Equilibrium - Contd. .. A system approaches equilibrium when both the forward and reverse reactions are occurring. At equilibrium , the forward and reverse reactions are proceeding at the same rate . N 2 O 4 ( g ) 2 NO 2 ( g )
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7 The Concept of Equilibrium - Contd. .. Once equilibrium is achieved, the amount of each reactant and product remains constant . N 2 O 4 ( g ) 2 NO 2 ( g )
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8 15.2 The Equilibrium Constant N 2 O 4 ( g ) 2 NO 2 ( g ) Consider the equilibrium: Forward reaction Reverse reaction N 2 O 4 ( g ) 2 NO 2 ( g ) N 2 O 4 ( g ) 2 NO 2 ( g ) Rate Law Rate = k f [N 2 O 4 ] Rate = k r [NO 2 ] 2 Rate Law Note that each step is an elementary step . So, the order of the reaction is the stoichiometric coefficient.
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9 The Equilibrium Constant - Contd. .. At equilibrium: k f [N 2 O 4 ] = k r [NO 2 ] 2 Rate f = Rate r k f k r = NO 2 [ ] 2 N 2 O 4 [ ] Equilibrium Constant, K eq What is the unit for K eq ?
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10 The Equilibrium Constant - Contd. .. In general, consider the following reaction: K c = C [ ] c D [ ] d A [ ] a B [ ] b a A + b B c C + d D The equilibrium constant expression in term of concentrations for this reaction is: Law of Mass Action a , b , c , d are the stoichiometric coefficient in the chemical reaction
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11 The Equilibrium Constant - Contd. .. The equilibrium constant expression depends only on the stoichiometry of the reaction, NOT on its mechanism The equilibrium constant expression and/or the value of K c do NOT say anything about the reaction mechanism or the reaction rate The value of K c is constant at any given temperature and does NOT depend on the initial concentrations of reactants and products: the equilibrium can be reached from both directions By looking at the chemical equation we CAN write the equilibrium constant expression for that reaction. However, the K c = C [ ] c D [ ] d A [ ] a B [ ] b a A + b B c C + d D
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This note was uploaded on 05/05/2008 for the course CHEM 178 taught by Professor Gupta during the Spring '08 term at N. Arizona.

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Chapter%2015%20-%20Chemical%20Equilibrium - Chapter 15...

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