1. 10.4 Using Equilibrium Constants
In the reaction of SO2(g) and O2(g), the equilibrium mixture contains mostly product SO3(g), which results in a large Kc.
Learning Goal Use an equilibrium constant to predict the extent of the reaction and to calculate equilibrium concentrations.

2. Using Equilibrium Constants
The values of Kc can be large or small, depending on whether equilibrium is reached with
more products than reactants.
more reactants than products.
However, the size of the equilibrium constant does not affect how fast equilibrium is reached.

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. At equilibrium, the reaction mixture contains mostly product and few reactants.
2SO2(g) + O2(g) SO3(g)

4. Equilibrium with a Small Kc
Reactions with a small Kc have an equilibrium mixture with a low concentration of products and high concentration of reactants. The equilibrium constant for the reaction of N2 and O2 has a small Kc. At equilibrium, the reaction mixture contains mostly reactants and few products. N2(g) + O2(g) 2NO(g)

5. 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.

6. Equilibrium with a Kc close to 1
A few reactions have equilibrium constants close to 1, which means they have about 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.

7. Equilibrium Constants: Kc

8. Guide to Using the Equilibrium Constant
Core Chemistry Skill Calculating Equilibrium Concentrations

9. Calculating Concentrations at Equilibrium
For the reaction of carbon dioxide and hydrogen, the equilibrium concentrations are 0.25 M CO2, 0.80 M H2, and 0.50 M H2O. What is the equilibrium concentration of CO(g)? CO2(g) + H2(g) CO(g) + H2O(g) Kc = 0.11 STEP 1 State the given and needed quantities.
ANALYZE Given Need
THE M CO2, 0.80 M H2, [CO]
PROBLEM 0.50 M H2O
Equation
CO2(g) + H2(g) CO(g) + H2O(g) Kc= 0.11

10. Calculating Concentrations at Equilibrium
For the reaction of carbon dioxide and hydrogen, the equilibrium concentrations are 0.25 M CO2, 0.80 M H2, and 0.50 M H2O. What is the equilibrium concentration of CO(g)? CO2(g) + H2(g) CO(g) + H2O(g) Kc = 0.11 STEP 2 Write the Kc expression for the equilibrium and solve for the needed concentration.

11. Calculating Concentrations at Equilibrium
For the reaction of carbon dioxide and hydrogen, the equilibrium concentrations are 0.25 M CO2, 0.80 M H2, and 0.50 M H2O. What is the equilibrium concentration of CO(g)? CO2(g) + H2(g) CO(g) + H2O(g) Kc = 0.11 STEP 3 Substitute the equilibrium (molar) concentrations and calculate the needed concentration.

12. Study Check
For each Kc, indicate whether the reaction mixture at equilibrium contains mostly reactants or products.
H2(g) + F2(g) HF(g) Kc = 1 × 1095
3O2(g) O3(g) Kc = 1.8 × 10−7

13. Solution
For each Kc, indicate whether the reaction mixture at equilibrium contains mostly reactants or products.
H2(g) + F2(g) HF(g) Kc = 1 × 1095
Kc is large; products are favored at equilibrium.
B. 3O2(g) O3(g) Kc = 1.8 × 10−7
Kc is small; reactants are favored at equilibrium.

14. Study Check
At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) Kc = 4.2 × 10−2 has the concentrations [PCl3] = [Cl2] = 0.10 M. What is the concentration of PCl5 at equilibrium?

15. Solution
At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) Kc = 4.2 × 10−2 has the concentrations [PCl3] = [Cl2] = 0.10 M. What is the concentration of PCl5 at equilibrium? STEP 1 State the given and needed quantities.
ANALYZE Given Need
THE [PCl3] = [Cl2] = 0.10 M [PCl5]
PROBLEM
Equation
PCl5(g) PCl3(g) + Cl2(g) Kc = 4.2 × 10−2

16. Solution
At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) Kc = 4.2 × 10−2 has the concentrations [PCl3] = [Cl2] = 0.10 M. What is the concentration of PCl5 at equilibrium? STEP 2 Write the Kc expression for the equilibrium and solve for the needed concentration.

17. Solution
At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) Kc = 4.2 × 10−2 has the concentrations [PCl3] = [Cl2] = 0.10 M. What is the concentration of PCl5 at equilibrium? STEP 3 Substitute the equilibrium (molar) concentrations and calculate the needed concentration.