1. Chapter 9 Chemical Equilibrium
9.4
Using Equilibrium Constants

2. 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

3. Equilibrium Can Favor Product
If equilibrium is reached after most of the forward
reaction has occurred,
the system favors the products

4. 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.

5. Equilibrium Can Favor Reactant
If equilibrium is reached when very little of the
forward reaction has occurred,
the reaction favors the reactants

6. 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.

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. Summary of Kc Values (continued)

9. Large and Small Kc Values

10. 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

11. 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

12. Guide to Using the Kc Value

13. 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?

14. 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]

15. 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

16. 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