CHE 104 Chapter 15 - CHE 104 Chapter 15 Dr Mark Shuman Chemical Equilibrium Every reaction can proceed in a forward or a reverse direction aA bB dD eE

CHE 104 Chapter 15 - CHE 104 Chapter 15 Dr Mark Shuman...

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CHE 104 Chapter 15 Dr. Mark Shuman
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Chemical Equilibrium Every reaction can proceed in a forward or a reverse direction aA + bB dD + eE Arbitrarily called the “forward” direction Has a rate law which is established experimentally Has a rate constant, k dD + eE aA + bB Arbitrarily called the “reverse” direction Has a rate law which is established experimentally Has a rate constant, k - Shown with a double arrow with k and k - written on the top and bottom of the arrows Draw on blackboard
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Chemical Equilibrium (cont.) When the forward and reverse reactions are proceeding at the same rate: We have chemical equilibrium This is a dynamic state Molecules on both sides are still reacting Concentrations of reagents are no longer changing The reagent concentrations at equilibrium are shown as: [A] eq [B] eq [D] eq [E] eq
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Chemical Equilibrium (cont.) All elementary reactions can be written as reversible. We can always write down the rate law of an elementary reaction. Consider the reaction that forms N 2 O 4 from NO 2 : NO 2 + NO 2 N 2 O 4 “Production” with k p N 2 O 4 NO 2 + NO 2 “Decomposition” with k d
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Chemical Equilibrium (cont.) The rate laws for these elementary reactions are: Rate p = k p [NO 2 ] 2 “Production” Rate d = k d [N 2 O 4 ] “Decomposition” At equilibrium , the rate of the “production” reaction must equal the rate of the “decomposition” reaction, i.e. Rate p = Rate d Rate p = k p [NO 2 ] eq 2 Rate d = k d [N 2 O 4 ] eq Therefore: k p [NO 2 ] eq 2 = k d [N 2 O 4 ] eq k p /k d = [N 2 O 4 ] eq / [NO 2 ] eq 2 k p /k d is defined as the equilibrium constant, K eq In general, K eq = k f /k r or k 1 /k -1 or k 2 /k -2 etc.
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Chemical Equilibrium (cont.) Consider the 2 step mechanism for the reaction: 2 NO 2 2 NO + O 2 NO 2 + NO 2 NO + NO 3 with k 1 NO 3 NO + O 2 with k 2 Each of these elementary reactions can be written as reversible An equilibrium expression can be derived for each These equilibrium expressions can be combined K eq = [NO] eq 2 [O 2 ] eq /[NO 2 ] eq 2 = k 1 k 2 /k -1 k -2
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Chemical Equilibrium (cont.) In general, for any reaction (elementary or otherwise): aA + bB dD + eE with k f dD + eE aA + bB with k r • K eq = [D] eq d [E] eq e /[A] eq a [B] eq b = k f /k r
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Chemical Equilibrium (cont.)
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