1. Chapter 13 Reaction Rates and Chemical Equilibrium
13.4 Using Equilibrium Constants
Learning Goal Use an equilibrium constant to predict the extent of the reaction and to calculate equilibrium concentrations.
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2. Using Equilibrium Constants
The values of Kc can be large or small, depending on whether
equilibrium is reached with more products than reactants
equilibrium is reached with more reactants than products
However, the size of the equilibrium constant does not affect how fast equilibrium is reached.
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3. Equilibrium with a Large Kc
Reactions with a large Kc have large amounts of products produced from the forward reaction at equilibrium.
The equilibrium constant for the reaction of SO2 and O2 has a large Kc.
2SO2(g) + O2(g) SO3(g)
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4. Equilibrium with a Large Kc
At equilibrium, the reaction mixture contains mostly products and a small amount of reactants.
2SO2(g) + O2(g) SO3(g)
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5. Equilibrium with a Large Kc
Insert figure 13.8 with caption pg 451.
In the reaction of SO2(g) and O2(g), the equilibrium mixture contains mostly product SO3(g), which results in a large Kc.
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6. Equilibrium with a Small Kc
Reactions with a small Kc have an equilibrium mixture with a low concentration of products and a high concentration of reactants.
The equilibrium constant for the reaction of N2 and O2 has a small Kc.
N2(g) + O2(g) NO(g)
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7. Equilibrium with a Small Kc
At equilibrium, the reaction mixture contains a small amount of products and a large amount of reactants.
N2(g) + O2(g) NO(g)
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8. Equilibrium with a Small Kc
The equilibrium mixture contains a very small amount of the product NO and a large amount of the reactants N2 and O2, which results in a small Kc.
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9. Equilibrium Constants, Kc
Reactions with equilibrium constants close to 1 have equal concentrations of reactants and products at equilibrium.
At equilibrium, a reaction with a large Kc contains mostly products,
whereas a reaction with a small Kc contains mostly reactants.
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10. Equilibrium Constants, Kc
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11. Learning Check
For each Kc, indicate whether the reaction mixture at equilibrium contains mostly reactants
or products.
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12. For each Kc, indicate whether the reaction mixture
Solution
For each Kc, indicate whether the reaction mixture
at equilibrium contains mostly reactants
or products.
Kc is large; products are favored at equilibrium.
Kc is small; reactants are favored at equilibrium.
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13. Calculating Concentrations at Equilibrium
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14. At equilibrium, the reaction
Learning Check
At equilibrium, the reaction
has the concentrations: [PCl3] = [Cl2] = 0.10 M.
What is the concentration of PCl5 at equilibrium?
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15. At equilibrium, the reaction
Solution
At equilibrium, the reaction
What is the concentration of PCl5 at equilibrium?
Step 1 State the given and needed quantities.
Given
Need
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16. At equilibrium, the reaction
Solution
At equilibrium, the reaction
What is the concentration of PCl5 at equilibrium? Step 2 Write the Kc expression for the equilibrium and solve for the needed concentration.
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17. At equilibrium, the reaction
Solution
At equilibrium, the reaction
What is the concentration of PCl5 at equilibrium? Step 3 Substitute the equilibrium molar concentrations and calculate the needed concentration.
© 2014 Pearson Education, Inc.