Physical Chemistry Third Edition

146 3 The Second and Third Laws of Thermodynamics: Entropy

Exercise 3.22

Use Trouton’s rule to estimate∆vapHfor ethane from its normal boiling temperature,− 88. 6 ◦C.

Compare with the correct value in Table A.7 of the Appendix.

PROBLEMS

Section 3.5: The Third Law of Thermodynamics and
Absolute Entropies

3.33 Assign the following nonzero constant values for the
standard-state molar entropies at 0 K:

C(s) 10.00JK−^1 mol−^1

O 2 (s) 20.00JK−^1 mol−^1

H 2 (s) 30.00JK−^1 mol−^1

Accepting as experimental fact that the entropy changes of

all reactions between pure solids at 0 K are equal to zero,

assign values for the standard-state molar entropies of

CO 2 (s) and H 2 O(s) at 0 K.

3.34 Calculate∆S◦at 298.15 K for each of the following
reactions:
a.2H 2 (g)+O 2 (g)→2H 2 O(l)
b.2H 2 (g)+O 2 (g)→2H 2 O(g)
c.CaCO 3 (s)→CaO(s)+CO 2 (g)
d.PCl 5 (g)→PCl 3 (g)+Cl 2 (g)

3.35 Calculate∆S◦at 298.15 K for each of the following
reactions:
a.C 3 H 8 (g)+5O 2 (g)→3CO 2 (g)+4H 2 O(g)
b.C 3 H 8 (g)+5O 2 (g)→3CO 2 (g)+4H 2 O(l)
c.C 2 H 2 (g)+2H 2 (g)→C 2 H 6 (g)
d. 2 n-C 8 H 18 (g)+25O 2 (g)→16CO 2 (g)+18H 2 O(g)

3.36 Using absolute entropy values from Table A.8 and
heat-capacity values from Table A.6 or Table A.8, calculate
Sm◦values for the substances at the indicated temperatures.
If no polynomial representations are available, assume
constant heat capacities.
a.H 2 (g) at 100.0◦C
b.O 2 (g) at 100.0◦C

c.H 2 O(l) at 100.0◦C

d.H 2 O(g) at 100.0◦C

3.37 Using absolute entropy values from Table A.8 and

heat-capacity values from Table A.6 or Table A.8,

calculateSm◦values for the substances at the indicated

temperatures. If no polynomial representations are

available, assume constant heat capacities.

a.CH 4 (g) at 200.0◦C

b.H 2 O(g) at 200.0◦C

3.38 a.Using absolute entropy values calculated in

Problem 3.36, calculate∆vapSm◦, the standard molar

enthalpy of vaporization for H 2 O at 100.0◦C.

b.Calculate∆vapS◦mfor H 2 Oat25◦C.

c.Calculate∆vapS◦mat 100.0◦C from data in Table A.7.

d.Comment on your values in light of Trouton’s rule.

3.39 Tabulated entropy changes of formation could be used to

calculate entropy changes of chemical reactions instead of

absolute entropies.

a.Using absolute entropies, calculate the standard-state

entropy change of formation at 298.15 K for CO(g),

O 2 (g), and CO 2 (g).

b. Calculate the standard-state entropy change at

298.15 K for the reaction:

2CO(g) + O 2 (g)→2CO 2 (g)

using your values of entropy changes of formation

from part a. Compare your result with that of

Example 3.16.

3.40 UsingS◦mvalues calculated in Problem 3.36, calculate∆S◦

at 100◦C for the reaction:

0=2H 2 O(g) – O 2 (g)–2H 2 (g)