17. See notes and textbook, pp. 653-657.
18. Calculate the ΔS̊ for the following rxn.: 2 S(s, rhombic) + 3
Use Table B.1 in your textbook.
S(s)
31.8 J∕K∙mol × 2 mol = 63.6 J∕K
O2(g)
SO3(g)
O2(g) ➝ 2 SO3(g)
205.0 J∕K∙mol × 3 mol = 615.0 J∕K
256.6 J∕K∙mol × 2 mol = 513.2 J∕K
ΔS̊ = ΣSfinal − ΣSinitial
ΔS̊ = 513.2 J∕K − (63.6 J∕K + 615.0 J∕K)
ΔS̊ = −165.4 J∕K
(We would expect S to decrease because we are going from 2 solid and 3
gas phase molecules to 2 larger gas phase molecules.)
19: For benzene, given ΔHvap = 30.7 kJ∕mol and ΔSvap = 87.0 J∕(K∙mol), calculate the boiling point for
benzene.
Boiling occurs at the temperature (T) where the vapor pressure = the atmospheric pressure above the surface of
the liquid. For 1 atm pressure, ΔGvap = 0 when the T = the boiling point. Said another way, as you increase the
T at 1 atm, boiling becomes spontaneous at the boiling temperature. Therefore,
0 = ΔHvap −T ΔSvap
ΔHvap = T ΔSvap
30.7 kJ∕mol × 1000 J∕1 kJ = 3.07 × 104 J∕mol
ΔHvap ∕ ΔSvap = T
3.07 × 104 J∕mol ∕87.0 J∕(K∙mol) = 352.874 ➝ 353 J∕mol