CHEM 1312.001
EXAM 3
Wednesday April 23, 1997

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1. A non-equilibrium system will reach equilibrium quickly if

a) Kc for the reaction is a very small number

b) Kc for the reaction is approximately equal to 1

c) Kc for the reaction is a very large number

d) You can't tell, since the value of the equilibrium constant does not give any information about the time required to reach equilibrium

2. A stoichiometric mixture of reactants (no products initially present) will be converted almost entirely to products at equilibrium if

a) Kc for the reaction is a very small number

b) Kc for the reaction is approximately equal to 1

c) Kc for the reaction is a very large number

d) You can't tell, since the value of the equilibrium constant does not give any information about the position of equilibrium for a chemical reaction

3. True or False: There is no necessary relationship between the exponents in the equilibrium constant expression and the coefficients in the balanced equation if the reaction is non-elementary.
a) Trueb) False
4. When do chemical reactions have negative values for the equilibrium constant Kc?

a) they always do

b) they never do

c) when the equilibrium favors the right hand side of the reaction

d) when the equilibrium favors the left hand side of the reaction

5. Phosphorous pentachloride (PCl5) decomposes when heated. If PCl5 decomposes in a closed container, it soon reaches an equilibrium with its decomposition products, phosphorous trichloride (PCl3) and elemental chlorine (Cl2). The chemical equation for the equilibrium is

PCl5(g)       <---------->       PCl3(g)       +       Cl2(g)

If a chemist starts with any arbitrary concentration of PCl5 (and no PCl3 or Cl2) in a closed reaction vessel (the preceeding was the initial composition), what statement can be made about the equilibrium composition? Make use of information given in this problem only. Do not rely on any equilibrium constant or concentrations you may remember from a previous quiz involving this reaction. You don't know if those numbers are valid here.

a) At equilibrium, all three concentrations are equal     [PCl5]eq = [PCl3]eq = [Cl2]eq

b) At equilibrium, the product concentrations are equal     [PCl3]eq = [Cl2]eq

c) At equilibrium, the concentration of PCl3 is equal to the initial concentration of PCl5
[PCl3]eq = [PCl5]initial

d) Nothing at all can be said about the equilibrium composition, since the exact initial concentration of PCl5, the size of the reaction vessel, and the value of Kc were not specified

6. At a certain temperature, the reaction

3NO2(g)       <---------->       N2O5(g)       +       NO(g)

has Kc = 1.0 x 10-11. At the same temperature, what would be the numerical value of Kc for the reaction written in the form

2N2O5(g)       +       2NO(g)       <---------->       6NO2(g)
a) 1.0 x 10-22 b) 5.0 x 10-12 c) 2.0 x 10-11
d) 5.0 x 1010 e) 1.0 x 1022
7. At a certain temperature, the reaction

N2(g)       +       3H2(g)       <---------->       2NH3(g)

has Kc = 6.0 x 10-2. At the same temperature, what would be the numerical value of Kc for the reaction written in the form

3N2(g)       +       9H2(g)       <---------->       6NH3(g)
a) 2.2 x 10-4 b) 3.6 x 10-3 c) 2.0 x 10-2
d) 1.8 x 10-1 e) 3.9 x 10-1
8. The reaction

CO(g)       +       H2O(g)       <---------->       HCHO2(g)

has Kp = 6.40 x 10-7 at 400 oC. What is the value of Kc for the reaction at this same temperature? Note that R = 0.08205783 (L atm) / (K mol). Temperatures must always be in Kelvin units when gases are involved in a problem.
a) 1.16 x 10-8 b) 9.12 x 10-8 c) 6.17 x 10-6
d) 2.10 x 10-5 e) 3.54 x 10-5
9. The reaction

2NO(g)       +       2CO(g) <---------->       N2(g)       +       2CO2(g)

has Kc = 2.2 x 1059 at 300 oC. What is Kp for this reaction at the same temperature?
a) 4.7 x 1057 b) 8.9 x 1057 c) 5.4 x 1060
d) 1.0 x 1061 e) 3.8 x 1061
10. At room temperature, the reaction

N2(g)       +       2O2(g)       <---------->       2NO2(g)

has Kc = 3 x 10-17. If a closed reaction vessel contained 1 mole of N2 and 2 moles of O2 and no NO2 (in other words, a stoichiometric mixture of reactants) what would the composition of the system be like at equilibrium?

a) Essentially all of your N2 and O2 would be gone. You would find almost entirely NO2. Any N2 and O2 left in the system would be in trace amounts.

b) Essentially all of your N2 and O2 would still be present. Only trace amounts of NO2 would be found in the system

c) There would be an equal number of moles of N2, O2, and NO2 present in the system.

d) You can't tell, since an equilibrium constant gives no information on the position of equilibrium for a chemical system.

11. At room temperature, the reaction

2SO2(g)       +       O2(g)       <---------->       2SO3

has Kc = 8 x 1035. If a closed vessel contained 2 moles of SO2 and 1 mole of O2 and no SO3 (in other words, a stoichiometric mixture of reactants), what would the composition of the system be like at equilibrium?

a) Essentially all of your SO2 and O2 would be gone. You would find almost entirely SO3. Any SO2 and O2 left in the system would be in trace amounts.

b) Essentially all of your SO2 and O2 would still be present. Only trace amounts of SO3 would be found in the system.

c) There would be an equal number of moles of SO2, O2 and SO3 present in the system.

d) You can't tell, since an equilibrium constant gives no information on the position of equilibrium for a chemical system.

12. A chemist put 0.1500 moles of CO and 0.3000 moles of H2 in a 1.500 L container at 500 oC. No other substances were initially present. These substances reacted to produce methanol (CH3OH), and after a period of time an equilibrium was established. The equation for the equilibrium system is

CO(g)      +       2H2(g)       <---------->       CH3OH(g)

At equilibrium, it was found that 0.1187 moles of CO were present in the container. What is the numerical value of the equilibrium constant Kc for this reaction at 500 oC? Hints: Equilibrium constants (Kc) are defined in terms of molar concentrations. You need to use an ICE table to solve this problem. Keep the stoichiometry of the reaction in mind as you fill in numbers in your ICE table.
a) 1.22 b) 3.17 c) 5.30 d) 10.5 e) 26.8
13. The reaction

CH4(g)       +       2H2S(g)       <---------->       CS2(g)       +       4H2(g)

has Kc = 3.59 at 900 oC. If a chemist prepared a mixture at 900 oC in which the following concentrations exist:

[CH4] = 1.15 M       [H2S] = 1.20 M       [CS2] = 1.51 M       and       [H2] = 1.08 M

the system will

a) be at equilibrium and no net reaction will occur

b) react to the right

c) react to the left

Note: Assume Qc = Kc if they are the same within 3 significant figures.

14. Considering again, the equilibrium reaction in problem 13, if a chemist prepares a mixture at 900oC with the following concentrations:

[CH4] = 1.07 M       [H2S] = 1.20 M       [CS2] = 0.90 M       [H2] = 1.78 M

the system will

a) be at equilibrium and no net reaction will occur

b) react to the right

c) react to the left

Note: Assume Qc = Kc if they are the same within 3 significant figures

15. Considering again, the equilibrium reaction in problem 13, if a chemist prepares a mixture at 900 oC with the following concentrations:

[CH4] = 1.10 M       [H2S] = 1.49 M       [CS2] = 1.10 M       [H2] = 1.68 M

the system will

a) be at equilibrium and no net reaction will occur

b) react to the right

c) react to the left

Note: Assume Qc = Kc if they are the same within 3 significant figures

ANSWERS:

1)   d            2)   c            3)   b            4)   b            5)   b      

6)   e            7)   a            8)   e            9)   a            10)   b

11)   a            12)   d            13)   b            14)   c            15)   a           

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