Thermodynamic Quantities for Selected Substances at 298.15 K (25 °C) Substance H
ID: 522357 • Letter: T
Question
Thermodynamic Quantities for Selected Substances at 298.15 K (25 °C)
Substance H°f (kJ/mol) G°f (kJ/mol) S (J/K-mol)
Carbon
C (s, diamond) 1.88 2.84 2.43
C (s, graphite) 0 0 5.69
C2H2 (g) 226.7 209.2 200.8
C2H4 (g) 52.30 68.11 219.4
C2H6 (g) -84.68 -32.89 229.5
CO (g) -110.5 -137.2 197.9
CO2 (g) -393.5 -394.4 213.6
Hydrogen
H2( g) 0 0 130.58
Oxygen
O2 (g) 0 0 205.0
H2O (l) -285.83 -237.13 69.91
Sulfur
S (s, rhombic) 0 0 31.88
SO2(g) -269.9 -300.4 248.5
SO3(g) -395.2 -370.4 256.2
The combustion of acetylene in the presence of excess oxygen yields carbon dioxide and water:
2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l)
The value of S° for this reaction is ________ J/K mol.
A) +689.3
B) +122.3
C) +432.4
D) -122.3
E) -432.4
Explanation / Answer
The value is given by
S0 = n.S0 (products) – n.S0(reactants) = [(4moles)*S0(CO2,g) + (2 moles)*S0(H2O,g)] – [(2 moles)*S0(C2H2,g) + (5 moles)*S0(O2,g)] = [(4 moles)*(213.6 J/mol.K) + (2 moles)*(69.91 J/mol.K)] – [(2 moles)*(200.8 J/mol.K) + (5 moles)*(205.0 J/mol.K)] = -432.38 J/K - 432.4 J/K (ans).
Ans: (E) -432.4 J/K
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