CHEM 1312.001
EXAM 3
Wednesday April 23, 1997
Name________________________________________
SSN(optional)_________________________________
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.
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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