1. Using Equilibrium Constants
General, Organic, and Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 9
Reaction Rates and Chemical Equilibrium
9.4
Using Equilibrium Constants
Lectures
© 2013 Pearson Education, Inc.

2. Reaching Chemical Equilibrium
The values of equilibrium constants can be large or
small.
The size of the Kc depends on whether equilibrium is
reached with
more products than reactants (Kc is large), or
more reactants than products (Kc is small).

3. Kc is Large. When a reaction has a large equilibrium constant, it
means
the forward reaction produced a large amount of products when equilibrium was reached.
the equilibrium mixture contains mostly products.

4. Kc is Large.

5. Kc is Small.
When a reaction has a small equilibrium constant, it means
the reverse reaction converted a large amount of products back to reactants.
the equilibrium mixture contains mostly reactants.

6. Kc is Small.

7. 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.

8. Large and Small Kc Values

9. Learning Check
For each Kc, indicate whether the reaction at equilibrium
contains mostly reactants or products.

10. Solution For each Kc, indicate whether the reaction at equilibrium
contains mostly reactants or products.
Kc is large and contains mostly products.
Kc is small and contains mostly reactants.

11. Guide to Using the Kc Value

12. Calculating Equilibrium Concentration
At equilibrium, the reaction,
has a Kc of and contains
What is the equilibrium concentration of PCl5?
Problem Facts

13. Calculating Equilibrium Concentration
At equilibrium, the reaction,
has a Kc of 4.2 x 10–2 and contains
What is the equilibrium concentration of PCl5?
Step 1 Write the Kc expression for the equilibrium
equation.
Step 2 Solve the Kc expression for the unknown
concentration.

14. Calculating Equilibrium Concentration
At equilibrium, the reaction,
has a Kc of and contains
What is the equilibrium concentration of PCl5?
Step 3 Substitute the known values into the
rearranged Kc expression and calculate.

15. Learning Check The Kc is 2.0 for the reaction,
If the equilibrium concentrations are and [ what is the equilibrium concentration of Br2?
A M
B M
C M

16. Solution The Kc is 2.0 for the reaction,
If the equilibrium concentrations are and [ what is the equilibrium concentration of Br2?
Problem Facts

17. Solution The Kc is 2.0 for the reaction,
If the equilibrium concentrations are [ and [ what is the equilibrium concentration of Br2?
Step 1 Write the Kc expression for the equilibrium
equation.

18. Solution The Kc is 2.0 for the reaction,
If the equilibrium concentrations are and [ what is the equilibrium concentration of Br2?
Step 2 Solve the Kc expression for the unknown
concentration.
Step 3 Substitute the known values into the
rearranged Kc expression and calculate.
Answer is B.