Homework 6. CHEM 102 Chapter 17.
1.Which of the following processes involve an increase in Srxn?
1. Br2 (l)
-->
Br2 (g)
2. 2NO(g) + O2 (g) -->
2NO2 (g)
3. MgSO4 (s)
-->
MgO(s) + SO3 (g)
4. 2HgO(s)
-->
2Hg(l) + O2 (g)
a.
1 and 2
b. 2 and 3
c. 3 and 4
d. 1, 3, and 4
2. In which
a.
b.
c.
d.
e.
e. 1, 2, 3, and 4
one of the following processes will Srxn be negative?
2NH3 (g)
----->
N2(g) + 3H2(g)
BaO(s) + CO2(g) ----->
BaCO3(s)
H2O(g)
----->
H2(g) + 1/2O2(g)
I2(s)
----->
I2(g)
Hg(s)
----->
Hg(l)
3. The heat of vaporization of methanol, CH3OH, is 36.8 kJ/mol. Its boiling point is
64.7oC. The change in entropy of the surrounding (Ssurr) for the vaporization of
methanol in J/mol K is
a. -17.8.
b.
3.41.
c. 17.8.
d. 109. e. 569.
4.Calculate the enthalpy change, Ho, for the combustion of C3H6(g):
C3H6(g) + 9/2O2 (g) ------> 3CO2(g) + 3H2O(l); Ho = ?
Hfo values in kJ/mol are as follows: C3H6 (g) = 21; CO2(g) = -394; H2O(l)= -286
kJ/mol.
a.
-2061 kJ
b. -2019 kJ c.
-701 kJ d. 2019 kJ
e. 2061 kJ
5.Calculate S° for the following reaction:
Fe2O3 (s) + 3H2 (g) -----> 2Fe (s) + 3H2O (g)
S°( Fe2O3(s)) =90 J/mol K ; S°(H2(g)) =130.6 J/mol K;S°(Fe(s)) = 27 J/mol K;
S°(H2O(g)) = 188.7 J/mol K.
a.
4.9 J/mol K b.
228.3 J/mol K c. 620.1 J/mol K d. 173.3 J/mol K
e.
138.3 J/mol K
6.Consider the following chemical reaction which proceeds at 298 K (25°C).
Standard free energies G°f in kJ/mol are given in parentheses.
2H2S(g) + SO2(g) ----> 3S(s) + 2H2O(g)
G° = ?
 G°f
(-34.0) (300.0)
(0.0)
(-229.0)
What is G° for this reaction?
a.
310 kJ/mol
b. -690 kJ/mol
c. 100 kJ/mol
d. 300 kJ/mol
e.
405 kJ/mol
7.What is the G°, if H° = 72.75 kJ/mol and S° = -145.3 J/K°mol, at 25°C for the
following reaction?
H2S(aq) --> 2H+(aq) + S2-(aq)
a. -72.7 kJ/mol
b. -43.2 kJ/mol
c. -29.5 kJ/mol d. 116.1 kJ/mol
8. Which of the following compounds has the highest entropy in J/mol K at 298 K?
a. CH3OH(l)
b. CO(g)
c. SiO2(s)
d. H2O(l)
e. CaCO3(s)
9. The Ka experssion for the dissociation of acetic acid in water is based on the
following equilibrium:
HC2H3O2(l) <===> H+(aq) + C2H3O2-(aq)
What is G° if Ka=1.8 x 10-5? [G°=-RTlnK]
a. -9.74 kJ/mol b. -27.1 kJ/mol
c. 27.1 kJ/mol
d. 9.74 kJ/mol
10. For a reactions for which H<0, that is, H is negative, using G = H - TS, which
of the following is true?
a. are spontaneous under all conditions.
b. are spontaneous when S>0.
c. are endothermic.
d. take place very slowly.
Answer Key
1.
>
d
2.
>
b
3.
>
d
4.
>
a
5.
>
e
6.
>
b
7.
>
d
1, 3, and 4
BaO(s) + CO2(g)----->
109.
-2061 kJ
138.3
-690 kJ/mol
116.1 kJ/mol
BaCO3(s)