Chemistry 2
Practice Exam 1

2. Which statement(s) is(are) TRUE about a system before it comes to equilibrium if it initially starts with 0.10 M N2O4(g)? (See graph above.)

N2O4(g) ↔  2NO2(g)

  I. Rate = (delta)(N2O4)
             (delta)t
  II. Rate = k(N2O4)
  III. The concentration of N2O4(g) is decreasing.

a) II
b) I & II
c) I & III
d) II & III
e) I & II & III

3. What are the equilibrium concentrations of NH3 and O2 if initially 0.0150 M NH3 and 0.0150 M O2 are injected into a flask, and at equilibrium the concentration of N2 is 1.96 x 10–3 M ?

4 NH3(g) + 3 O2(g) ↔ 2 N2(g) + 6 H2O(g)

a) [NH3] = 0.0007 M      [O2] = 0.0032 M
b) [NH3] = 0.0072 M      [O2] = 0.0091 M
c) [NH3] = 0.0111 M      [O2] = 0.0121 M
d) [NH3] = 0.0130 M      [O2] = 0.0130 M
e) [NH3] = 0.0140 M      [O2] = 0.0137 M

4.  When a system reaches equilibrium, which statement(s) is(are) TRUE ?

  I.  The forward reaction comes to a stop.
  II. Both the forward and reverse reactions occur.
  III. There is no net change in the concentrations of reactants or products.

a)  I
b)  I & II
c)  I & III
d)  II & III
e)  I & II & III.

5.  What is Kc for the reaction below at 500°C if the equilibrium concentrations are 0.20 M H2, 0.019 M N2, and 0.0025 M NH3 ?

N2(g) + 3 H2(g)  ↔  2 NH3(g)

a) 0.0061
b) 0.041
c) 0.66
d) 3.95
e) 24.3

6. Which is the correct equilibrium constant expression for the reaction below?

SF6(g) + 2 SO3(g) ↔ 3 SO2F2(g)

a)   [SO2F2]
    [SF6] [SO3]
b) [SF6] [SO3]
     [SO2F2]
c  [SF6] [SO3]2
     [SO2F2] 3
d)  [SO2F2]3__
  [SF6] [SO3]2
e   [3 SO2F2]3__
  [SF6] [2 SO3]2

7. What is the equilibrium concentration of SO2 when 0.50 M SO3 and 0.10 M O2 are mixed at 300°C and Kc = 1.6 x 10–10 at this temperature?

2 SO3(g) ↔ 2 SO2(g) + O2(g)

a) 1.0 x 10–5 M
b) 1.4 x 10–5 M
c) 2.0 x 10–5 M
d) 2.8 x 10–5 M
e) 4.0 x 10–5 M

8. Given that Kc = 0.110 for Reaction A. What is the Kc for Reaction B ?

A. 2 ICl(g) ↔ I2(g) + Cl2(g)
B. I2(g) + Cl2(g) ↔ 2 ICl(g)

a) 0.0121
b) 0.110
c) 0.220
d) 9.09
e) 82.6

9. Which variable(s) change(s) the value of an equilibrium constant?

a) pressure
b) temperature
c) product concentration
d) reactant concentration
e) all of the above

10. At equilibrium which equation is TRUE ?

a) Qc < Kc
b) Kc = k[Reactant]
c) Kc = rateforward_
        ratereverse
d) rateforward > ratereverse
e) Kc = kforward x kreverse

11. What is the equilibrium concentration of COCl2(g) if initial concentration of CO is 0.70 M and Cl2 is 0.70 M? Kc = 4.95 x 10–3

CO(g) + Cl2(g) ↔ COCl2(g)

a)  0.0024 M
b)  0.0035 M
c)  0.0069 M
d)  0.0097 M
e)  0.0101 M

12. If the pressure of the equilibrium below is doubled, what will occur?

N2(g) + 3 H2(g) ↔ 2 NH3(g)

a) The reaction will shift to the left.
b) The reaction will shift to the right.
c) The change has no effect on the system.
d) The rate of the forward reaction will increase.
e) The rate of the reverse reaction will increase.

13. Which of the following changes will shift the endothermic reaction to the right?

SO2Cl2(g) ↔ SO2(g) + Cl2(g)

a) Increasing the temperature.
b) Increasing the concentration of Cl2
c) Decreasing the concentration of SO2Cl2
d) Increasing the pressure of the reaction chamber.
e) None of these will shift the reaction to the right.

14. What will happen to a mixture if 1.00 M CH4 and 2.00 M H2S, 1.00 M CS2, and 2.00 M H2 were mixed together at 960°C ?
At this temperature Kc = 0.036.

CH4(g) + 2 H2S(g) ↔ CS2(g) + 4 H2(g)

a) The concentrations of H2 and CS2 will increase at equilibrium.
b) The concentrations of CH4 and H2S will decrease at equilibrium.
c) The reaction system will shift to the left until it reaches equilibrium.
d) The reaction system will shift to the right until it reaches equilibrium.
e) The system is at equilibrium, so no changes in concentrations will occur.

15.  Which conditions will maximize the production of lime, CaO, in the endothermic reaction by thermal decomposition of limestone, CaCO3 ?

CaCO3(s)  ↔ CaO(s) + CO2(g)

a) low Temperature & low Pressure
b) low Temperature & high Pressure
c) high Temperature & low Pressure
d) high Temperature & high Pressure
e) More information is needed to make this prediction.

16. What is the equilibrium concentration of NO when the initial concentration of N2 was 0.100M and O2 was 0.090 M? Kc= 3.3 x 10–10 at this temperature.

N2(g) + O2(g) ↔ 2 NO(g)

a) 1.48 x 10–12 M
b) 8.62 x 10–7 M
c) 1.72 x 10–6 M
d) 2.44 x 10–6 M
e) 3.45 x 10–6 M

17. It is said that Le Châtelier's principle also applies to other fields such as economics and ecology. For example, the vegetation in the Louisiana wetlands is in balance (or equilibrium) with the musk rats (plant eating animals). When nutria, a plant-eating animal, is introduced to a balanced area, the amount of plants in the area decrease. To counter this effect, what needs to happen?

a) The number of nutria in the wetlands must stay the same.
b) The total number of musk rats & nutria in the wetland must increase.
c) The total number of musk rats & nutria in the wetland must decrease.
d) Fifty scientists are needed to study this before an answer can be given.

18. BONUS: After 22.7 g of PH3BCl3(s) was introduced into a 3.0 L container and allowed to reach equilibrium, 2.8 x 10–3 M PH3 present at equilibrium. What is Kc for this reaction ?

PH3BCl3(s) ↔ PH3(g) + BCl3(g)

a) 1.2 x 10–1
b) 5.6 x 10–3
c) 2.8 x 10–3
d) 1.7 x 10–4
e) 7.8 x 10–6