Practice Problems: Equilibrium
CHEM 1B
1. Write the equilibrium expression for each of the following reactions:
a) SiCl4 (g) + 2 H2 (g) ⇌ Si (s) + 4 HCl (g)
b) Al2(SO4)3·6H2O (s) ⇌ 2 Al3+ (aq) + 3 SO42– (aq) + 6 H2O (l)
c) H3PO4 (aq) + H2O (l) ⇌ H3O+ (aq) + H2PO4– (aq)
2. Given the following equilibrium constants, predict whether reactants or products will dominate the
reaction mixture at equilibrium (circle one for each line).
a) Ksp = 8.4 x 10–12 Reactants or Products
b) Kf = 1.7 x 107
Reactants or Products
c) Ka = 1.8 x 10–8
Reactants or Products
3. Given the following relationships between the reaction quotient and the equilibrium constant, predict
which direction the reaction must proceed in order to get to equilibrium (circle one for each line).
a) Q > K
Forward or Already at Equilibrium or Reverse
b) Q = K
Forward or Already at Equilibrium or Reverse
c) Q < K
Forward or Already at Equilibrium or Reverse
4. The initial concentrations in the reaction below are [N2O4] = 0.798 M and [NO2] = 0.000 M. What
will the equilibrium concentration of NO2 be if K = 4.63 x 10–3 at the current temperature?
N2O4 (g) ⇌ 2 NO2 (g)
5. What are the partial pressures of each gas at equilibrium (at 700 K) if initially there are 0.227 atm H2,
0.152 atm I2, and 1.989 atm HI? Kp = 57 at 700 K
Remember, Kp uses the same equilibrium expression as any other K, but with partial pressures
instead of molarity.
H2 (g) + I2 (g) ⇌ 2 HI (g)
6. The initial concentrations in the reaction below are [SO2] = 0.465 M, [O2] = 0.195 M, and [SO3] =
0.000 M. What will the equilibrium concentration of SO3 be if K = 2.7 x 102 at the current
temperature?
2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g)
7. Complete the stress/shift/equilibrium diagrams below for the indicated stresses (applied to a system
at equilibrium). Use vertical arrows to describe changes in concentration and a horizontal arrow to
indicate shifts to the right or left (as predicted by Le Châtelier’s principle). Use medium sized
arrows in the Stress and Shift rows, and the appropriate sized arrows (small, medium, or large) in the
Equilibrium row.
a) Stress: Product ammonia is removed
Stress:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) + heat
Shift:
Equilibrium:
b) Stress: The volume of the container is decreased
Stress:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) + heat
Shift:
Equilibrium:
c) Stress: The temperature in increased
Stress:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) + heat
Shift:
Equilibrium:
8. Write the word “heat” into the equation below, and list four stresses that would shift the reaction to
the right (be specific).
1)
H = −58.6 kJ
2 NO2 (g)
N2O4 (g)
2)
3)
4)
9. Which stress in Questions 7 and 8 changes the equilibrium constant?