Answer Key
Chapter 15
Practice Problems
1. 1674 J or 400 cal
2. Specific heat  0.898
J/(g•C). The specific heat is
very close to the value for
aluminum.
3. 44.3C
4. 112C
5. 0.40 J/(g•C)
6. 47.9C
7. a. 10.7 kJ
b. 83.8 kJ
The heat needed for boiling
is much larger than the heat
needed for melting.
8. 833 kJ is evolved.
9. 239 g
10. Hrxn  112 kJ
11. Hrxn  129 kJ
12. a. 40.9 kJ
b. 153.87 kJ
c. 1322.91 kJ
d. 3.0 kJ
13. a. negative
b. positive
c. positive
d. positive
e. negative
14. a. 175 kJ, nonspontaneous
b. 8 kJ, spontaneous
c. 27 kJ, nonspontaneous
15. 391 K, or 118C
Chapter 15 Review
16. Use the equation for the amount of heat absorbed.
q  c  m  T
Both samples have the same mass and the same temperature change,
so the sample with the higher specific heat absorbs more energy as
it is heated. From Table 15.2 in your textbook, the specific heat of
ethanol is 2.44 J/(g•C), whereas that of aluminum is 0.897 J/(g•C),
so the ethanol absorbs more energy.
17. The universe is defined as the system plus the surroundings. The
system is the reaction or process being studied. The surroundings
include everything in the universe other than the system.
Answer Key (continued)
18. Melting of a solid and boiling of a liquid are endothermic processes,
so H is positive. Condensation of a vapor and solidification of a
liquid are exothermic, so H is negative.
19. The standard heat of formation (H f ) of ethane gas is the enthalpy
change that accompanies the formation of one mole of ethane in its
standard state from its constituent elements, carbon and hydrogen,
in their standard states. The standard state of a substance is its
normal state at one atmosphere pressure and 298 K (25C). The
equation for the formation of ethane is as follows.
2C(s)  3H2(g)  C2H6(g)
20. Gsystem  Hsystem  TSsystem
a. Gsystem  (positive term)  T(negative term)
Gsystem  (positive term)  (positive term)
Gsystem  0 not spontaneous
b. Gsystem  (negative term)  T(negative term)
Gsystem  (negative term)  (positive term)
The sign of G is uncertain, so spontaneity is uncertain.
c. Gsystem  (negative term)  T(positive term)
Gsystem  (negative term)  (negative term)
Gsystem  0 spontaneous