Chap 17 - 17 Chemical Equilibrium 1 Basic Concepts...

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1 17 Chemical Equilibrium
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2 Basic Concepts Reversible reactions do not go to completion. Occur when products are not stable.
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3 Basic Concepts aA (g) + bB (g) cC (g) + dD (g)
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4 Basic Concepts Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate. A chemical equilibrium is a reversible reaction that the forward reaction rate is equal to the reverse reaction rate. Chemical equilibria are dynamic equilibria; molecules are continually reacting.
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5 Basic Concepts This plots shows the rates for the forward and reverse reactions are equal at equilibrium. aA (g) + bB (g) cC (g) + dD (g)
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6 Basic Concepts One of the fundamental ideas of chemical equilibrium is that equilibrium can be established from either the forward or reverse direction.
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7 Basic Concepts
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8 Basic Concepts
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9 The Equilibrium Constant For a simple one-step mechanism reversible reaction such as: The rates of the forward and reverse reactions can be represented as: [ ][ ] [ ][ ] rate. reverse the represents which D C k Rate rate. forward the represents which B A k Rate r r f f = = aA (g) + bB (g) cC (g) + dD (g)
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10 The Equilibrium Constant When system is at equilibrium: Rate f = Rate r [ ][ ] [ ][ ] [ ][ ] [ ][ ] B A D C k k to rearranges which D C k B A k : give to ip relationsh rate for the Substitute r f r f = =
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11 The Equilibrium Constant Because the ratio of two constants is a constant we can define a new constant as follows : [ ][ ] [ ][ ] k k K and K C D A B f r c c = =
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12 The Equilibrium Constant Similarly, for the general reaction: we can define a constant [ ] [ ] [ ] [ ] reactions. all for valid is expression This B A D C K products reactants b a d c c = aA (g) + bB (g) cC (g) + dD (g)
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13 The Equilibrium Constant K c (equilibrium constant) is defined for a reversible reaction at a given temperature as the product of the equilibrium concentrations (in M) of the products, each raised to a power equal to its stoichiometric coefficient in the balanced equation, divided by the product of the equilibrium concentrations (in M) of the reactants, each raised to a power equal to its stoichiometric coefficient in the balanced equation.
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14 The Equilibrium Constant Example 17-1: Write equilibrium constant expressions for the following reactions at 500 o C. All reactants and products are gases at 500 o C. PCl 5(g) PCl 3(g) + Cl 2(g) [ ] [ ] [ ] 5 2 3 c PCl Cl PCl K =
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15 The Equilibrium Constant H 2(g) + I 2(g) 2 HI (g) [ ] [ ][ ] 2 2 c I H HI K 2 =
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16 The Equilibrium Constant [ ] [ ] [ ] [ ] 5 4 6 4 2 3 2 c O NH O H NO = K 4 NH 3(g) + 5 O 2(g) 4 NO (g) + 6 H 2 O (g)
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The Equilibrium Constant Equilibrium constants are dimensionless because they actually involve a thermodynamic quantity called activity. Activities are directly related to molarity.
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This note was uploaded on 09/18/2010 for the course CHEM 1212 taught by Professor Suggs during the Fall '08 term at UGA.

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Chap 17 - 17 Chemical Equilibrium 1 Basic Concepts...

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