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Chapter 9 Chemical Equilibrium
9.4 Using Equilibrium Constants
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Reaching Chemical Equilibrium
A container initially filled with SO2(g) and O2(g) or only SO3(g) contains mostly SO3(g) and small amounts of O2(g) and SO3(g) at equilibrium reaches equilibrium in both situations
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Equilibrium Can Favor Product
If equilibrium is reached after most of the forward reaction has occurred, the system favors the products
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Equilibrium with a Large Kc
At equilibrium, a reaction with a large Kc produces a large amount of product; very little of the reactants remain Kc = [NCl3]2 = x 1011 [N2][Cl2]3 a large Kc favors the products N2(g) + 3Cl2(g) NCl3(g) When this reaction reaches equilibrium, it will essentially consist of the product NCl3.
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Equilibrium Can Favor Reactant
If equilibrium is reached when very little of the forward reaction has occurred, the reaction favors the reactants
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Equilibrium with a Small Kc
At equilibrium, a reaction that produces only a small amount of product has a small Kc Kc = [NO] = 2.3 x 10-9 [N2][O2] a small Kc favors the reactants N2(g) + O2(g) NO(g) When this reaction reaches equilibrium, it will essentially consist of the reactants N2 and O2.
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Summary of Kc Values A reaction that favors products has a large Kc
with about equal concentrations of products and reactants has a Kc close to 1 that favors reactants has a small Kc
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Summary of Kc Values (continued)
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Large and Small Kc Values
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Learning Check For each Kc, indicate whether the reaction at equilibrium contains mostly of (R) reactants or (P) products. __A. H2(g) + F2(g) HF(g) Kc = 1 x 1095 __B. 3O2(g) O3(g) Kc = 1.8 x 10-7
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Solution For each Kc, indicate whether the reaction at equilibrium
contains mostly of (R) reactants or (P) products. A. P (Kc>>1) H2(g) + F2(g) 2HF(g) Kc = 1 x 1095 B. R (Kc<<1) 3O2(g) O3(g) Kc = 1.8 x 10-7
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Guide to Using the Kc Value
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Using Kc to Solve for an Equilibrium Concentration
At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) has a Kc of 4.2 x 10–2 and contains [PCl3] = [Cl2] = 0.10 M. What is the equilibrium concentration of PCl5?
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Using Kc to Solve for an Equilibrium Concentration (continued)
STEP 1 Write the Kc expression: Kc = [PCl3][Cl2] [PCl5] STEP 2 Solve for the unknown concentration: [PCl5] = [PCl3][Cl2] Kc STEP 3 Substitute the known values and solve: [PCl5] = [0.10][0.10] = M 4.2 x 10–2 STEP 4 Check answer by placing concentrations in Kc: Kc = [0.10][0.10] = x 10–2 [0.24]
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Learning Check The Kc is 2.0 for the reaction 2NOBr(g) 2NO(g) + Br2(g)
If the equilibrium concentrations are [NOBr] = 0.50 M and [NO] = 0.80 M, what is the equilibrium concentration of Br2? 1) 0.39 M 2) 0.78 M 3) 1.3 M
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Solution STEP 1 Write the Kc expression: Kc = [NO]2 [Br2] [NOBr]2
STEP 2 Solve for the unknown concentration: [Br2] = [NOBr]2 Kc [NO]2 STEP 3 Substitute known values and solve: [Br2] = [0.50] = M (2) [0.80]2 STEP 4 Check answer by placing concentrations in Kc: Kc = [0.80]2[0.78] = 2.0 [0.50]2
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