Chem 1B
Dr. White
Worksheet 15 – Crossover Temperature, ΔG°, and ΔS°
1. The enthalpy and entropy change of a reaction are -4.7 kJ/mole and -69.5 J/mole K
respectively over a wide range of temperatures. What is the crossover temperature? Is the
reaction spontaneous at temperatures above or below this temperature?
2. The enthalpy and entropy change of a reaction are 34.7 kJ/mole and 6.95 J/mole K
respectively over a wide range of temperatures. What is the crossover temperature? Is the
reaction spontaneous at temperatures above or below this temperature?
3. Using the listed [ΔGof values] calculate ΔG° for the reaction:
Mn3O4(s) [-1283.2 kJ/mol] + 4 CO(g) [-137.2 kj/mol] → 3 Mn(s) + 4 CO2(g) [-394.4 kJ/mol]
(ans: 254 kJ)
4. Using the listed information calculate ΔG° (kJ) for the reaction at 25°C and 100.°C:
C6H12(g) + 9 O2(g) → 6 CO2(g) + 6 H2O(l)
ΔH f (kJ/mole)
-156.4
-393.5
-285.9
So (J/K mole)
204.4
205.1
213.7
69.9
(NOTE: the ΔH° and ΔS° values are constant over 25°C and 100°C)
o
(ans: ΔH° = -3920 kJ and ΔS° = -348 J/K; ΔG° = -3816 kJ at 273 K and ΔG° = -3790 kJ at
373 K)
5. Calculate ΔHof at 298K for ClF3 from the ΔG° of reaction and the S° values.
S° (J/K.mole)
3 F2(g) + Cl2(g) → 2 ClF3(g)
ΔG° = -246.0 kJ
202.8 223.1
281.6
o
(ans: ΔH f ,ClF3 = −163.0kJ / mol )
6. From the table of values to below, determine the boiling point of bromine (in °C). (ans:
58.7°C)
∆H°f
∆Gf°
S°
€
[kJ/mol]
[kJ/mol]
[J/mol.K]
Br2(l)
—
—
152.23
Br2(g)
30.91
3.13
245.38
Br(g)
111.9
82.40
174.90
Br–(g)
–218.9
–102.9
80.71
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