1422-Chapt-17-Equilibria

1422-Chapt-17-Equilibria - Chapters 17 & 20-1 to 20-3...

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Chapters 17 & 20-1 to 20-3 Chemical Equilibria
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Equilibrium 2 Chemical Equilibrium: It is the condition of a chemical reaction in which the rate of formation of products (from reactants ) equals the rate of formation of the reactants (from products ). rate 1 A + B C + D rate -1 Equilibrium occurs when rate 1 = rate -1 . Although chemists usually want reactions to go completely to products (and ideally only to a single product), many do not. Theoretically all reactions are in equilibrium. A reaction will not generally reach equilibrium if: 1) The rxn is very exothermic (exoergic) 2) One (or more) of the products (or reactants) is removed from the rxn 3) One of the products (or reactants) is insoluble
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Equilibrium 3 Consider the very important industrial reaction ( the Haber process ) of nitrogen and hydrogen to produce ammonia , which is used as a fertilizer : N 2 ( g ) + 3H 2 ( g ) 2NH 3 ( g ) This is a very difficult reaction (large activation barrier) that requires high temperatures, pressures and a catalyst. At the high temperatures required to make the reaction proceed at a reasonable rate, the thermodynamics favors the N 2 + H 2 reactants producing the following behavior: Time P H 2 N 2 NH 3 Note that the rxn does not go to completion, rather the forward and backward rxns reach a state of chemical equilibrium .
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Equilibrium 4 Equilibrium is a dynamic process. This means that when a reaction has reached a state of equilibrium, the forward and backward reactions making up the overall reaction have not stopped !! The equilibrium definition states that equilibrium is reached when the forward and backward reaction rates become equal ! For example consider a saturated solution of NaCl (no additional salt will dissolve): NaCl( s ) Na + ( aq ) + Cl ( aq ) If we add 5 g more NaCl( s ) to this solution, the amount of solid NaCl in the container will increase by 5 g (that is, no additional solid NaCl will dissolve into solution). This does not mean, however that some of the new NaCl that we just added won't dissolve at all. Some of it will dissolve, while some Na + ( aq ) + Cl ( aq ) in solution elsewhere will precipitate out! We could follow this by adding radioactive 24 Na 38 Cl to the container: radioactive salt TIME
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Equilibrium 5 Demonstration:
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Equilibrium 6 Law of Mass Action One can set up a general mathematical expression to describe the following chemical equilibria: A + B wx C + D y z K eq = [C] [D] [A] [B] y z products reactants K eq is called the equilibrium constant The equilibrium expression for the Haber process reaction would be written as: N (g) + 3H (g) 2NH (g) K eq = [NH ] [N ] [H ] products reactants 22 3 3 2 2 2 3
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Equilibrium 7 Consider, for example, the equilibrium between N 2 O 4 ( g ) and NO 2 ( g ): N 2 O 4 ( g ) 2NO 2 ( g ) K eq = [] NO N O 2 24 2 Listed below is experimental data giving initial concentrations for N 2 O 4 ( g ) and NO 2 ( g ) . After some time the reaction reaches equilibrium and the concentrations listed.
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This note was uploaded on 11/12/2011 for the course CHEM 1422 taught by Professor Staff during the Fall '08 term at LSU.

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1422-Chapt-17-Equilibria - Chapters 17 & 20-1 to 20-3...

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