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EquilibriumTopic 7
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7.2.1 THE EQUILIBRIUM CONSTANT (Kc or K)7.2.1 THE EQUILIBRIUM CONSTANT (Kc or K)
For any type of chemical equilibrium of the type
a A + b B c C + d D
the following is a CONSTANT (at a given T)
K =[C]c [D]d
[A]a [B]b
conc. of products
conc. of reactantsequilibrium constant
If Kc is known, then we can predict concs. of products or reactants.
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Determining KcDetermining Kc
2 NOCl(g) 2 NO(g) + Cl2(g)
[NOCl] [NO] [Cl2]
Initial 2.00 0 0
Change -0.66 +0.66+0.33
Equilibrium 1.34 0.660.33
K = [NO]2[Cl2 ]
[NOCl]2
K = [NO]2[Cl2 ]
[NOCl]2 =
(0.66)2(0.33)
(1.34)2 = 0.080
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Chemical Equilibrium from the labFe3+ + SCN- FeSCN2+
Chemical Equilibrium from the labFe3+ + SCN- FeSCN2+
After a period of time, the concentrations of reactants and products are constant.
The forward and reverse reactions continue after equilibrium is attained.
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Writing and Manipulating K ExpressionsWriting and Manipulating K Expressions
Solids and liquids NEVER appear in equilibrium expressions.
S(s) + O2(g)
SO2(g)
K = [SO2 ]
[O2 ]
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Writing and Manipulating K ExpressionsWriting and Manipulating K Expressions
Solids and liquids NEVER appear in equilibrium expressions.
NH3(aq) + H2O(liq)
NH4+(aq) + OH-(aq)
K = [NH4
+ ][OH- ]
[NH3 ]
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The Meaning of KThe Meaning of K
1.Can tell if a reaction is product-favored or reactant-favored.
For N2(g) + 3 H2(g) 2 NH3(g)
Kc = [NH3 ]2
[N2 ][H2 ]3 = 3.5 x 108
Conc. of products is much greater than that of reactants at equilibrium. The reaction is strongly
product-favored.
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The Meaning of KThe Meaning of K
For AgCl(s) Ag+(aq) + Cl-(aq)
Kc = [Ag+] [Cl-] = 1.8 x 10-5
Conc. of products is much less than that of reactants at equilibrium.
The reaction with small K is strongly
reactant-favored.
Ag+(aq) + Cl-(aq)
AgCl(s)is product-favored.
Ag+(aq) + Cl-(aq)
AgCl(s)is product-favored.
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Product- or Reactant Favored
Product-favoredK > 1
Reactant-favoredK < 1
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EQUILIBRIUM AND EXTERNAL EFFECTS
Temperature, catalysts, and changes in concentration affect equilibria.
The outcome is governed by
LE CHATELIER’S PRINCIPLE “...if a system at equilibrium is
disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”
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Le Chatelier’s Principle
Change in temperature change in Kc therefore change in P or concentrations at
equilibrium Use a catalyst: reaction comes more
quickly to equilibrium. Kc not changed. Add or take away reactant or product:
Kc does not change Reaction adjusts to new equilibrium “position”
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7.2.5 Apply the concepts of kinetics and equilibrium to industrial processes. A good example of this is the production of
ammonia.
Fertilizers and many plastics are derived from ammonia.
High pressure is used to increase yield.
The reaction is exothermic so a lower temperature is used to increase yield.
A catalyst is used to speed up the reaction, but it doesn’t increase the yield.
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N2(g) + 3 H2(g) 2 NH3(g) + heat
K = 3.5 x 108 at 298 K
Haber-Bosch Process for NH3