Here are some typical thermodynamics exam questions.
| 1. Which is not correct as it relates to the
three laws of thermodynamics? |
| a. The total energy gained by a system is
equal to the energy put into the system minus that used by the system to do work, as in
the expansion of a gaseous product. |
| b. The absolute entropy of a perfect crystal
at absolute zero is absolutely zero. |
| c. In any spontaneous
process, the change in entropy of the system is always positive. |
| d. To the best of our knowledge, the total
entropy of the universe is always increasing. |
| 2. Which of the following would
you expect to have the highest absolute entropy, S? |
| a. 1 mol NH3(g) |
d.1 mol NH3(aq) |
| b. 1 mol NH3(l) |
|
| c. 1 mol NH3(s) |
|
| 3. Which of the following
conditions would be best for favoring a spontaneous reaction? |
| a. +delta H, +delta S |
d. +delta H, -delta S |
| b. -delta H, -delta S |
|
| c. -delta H, +delta S |
|
| 4. Which of the following best expresses the
increased degree of disorder or randomness associated with the fusion (melting) and
sublimation processes? |
| a. delta Sfus =
9.0 J/mol-K; delta Ssub = 144.3 J/mol-K |
| b. delta Sfus = 144.3 J/mol-K;
delta Ssub = 9.0 J/mol-K |
| c. delta Sfus = 9.0 J/mol-K; delta
Ssub = 144.3 J/mol-K |
| d. delta Sfus = 144.3 J/mol-K;
delta Ssub = 9.0 J/mol-K |
| e. delta Sfus = delta Ssub
= 0 |
| 5. Which of the following reactions has a
negative delta S? |
| a. CH4(l) + F2(l)
-----> CH3F(g) + HF(g) |
| b. 2 CO(g) + O2(g)
-----> 2 CO2(g) |
| c. Al2Cl6(g)
-----> 2 AlCl3(g) |
| d. I2(s)-----> I2(g) |
| 6. Which reaction would you
expect to be spontaneous only at higher temperatures? |
| a. 2 F(g) -----> F2(g) |
c. Ni(CO)4(g)
-----> Ni(s) + 4 CO(g) |
| b. H2(g) + S(s)
-----> H2S(s) |
d. CH4(g) + 2 O2(g)
-----> CO2(g) + 2 H2O(l) |
|
| 7. Which of the following reactions has the
fastest rate at 25oC and 1 atm? |
| a. CH4(g) + 2 O2(g)
-----> CO2(g) + 2 H2O(g); delta G = 817 kJ |
| b. CH2Cl2(l) + O2(g)
-----> CO2(g) + 2 HCl(g); delta G = 517 kJ |
| c. SiO2(g) + Pb(s)
-----> PbO2(s) + Si(s) delta G = +639 kJ |
| d. Insufficient data
provided to determine the answer to this question. |
| 8. The standard free energy of
formation, delta Gf, of HCl(g) is -95 kJ/mol. What is the value of Kp
at 298K for the following reaction: H2(g)
+ Cl2(g) -----> 2 HCl(g)
|
| a. 5 x 1033 |
c. 2 x 1017 |
| b. 2 x 1033 |
d. 4 x 1016 |
|
|
9. The following thermodynamic data are to be used for this
question:
| |
delta Gf (kJ/mol) |
delta S (J/mol-K) |
C2H2(g) |
+209 |
+201 |
O2(g) |
0 |
+205 |
CO2(g) |
-394 |
+214 |
H2O(g) |
-229 |
+189 |
a. Calculate the standard free energy change for the
combustion of C2H2. The equation is:
2 C2H2(g) + 5 O2(g)
-----> 4 CO2(g) + 2 H2O(g)
answer: delta Grxn = -2452 kJ
b. Calculate the standard entropy change for this reaction:
answer: delta Srxn
= -193 J/K
c. Calculate the standard enthalpy change for this
reaction:
answer: delta Hrxn = -2509 kJ
d. Standard free energies of formation are given in the table above. Write the equation
for the reaction associated with the delta Gf for C2H2(g):
answer: 2
C(s, graphite) + H2(g) -----> C2H2(g)
e. If each component of this reaction is at a partial
pressure of 1.00 atm in a sealed container at 25C, in which direction would it move to
reach equilibrium? How can we tell how far it would move?
answer: Move to
the right and the extent is known through Kp
[HOME].
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