Use bond-energy data to calculate the enthalpy of formation for each of the following compounds at 25°C.
a. n-Octane, C8H18(g)
b. Naphthalene, C10H8(g)
c. Formaldehyde, H2CO(g)
d. Formic acid, HCOOH(g)
Compare your answers with enthalpies of formation given in tables A.5-A.7 in the appendix. (The enthalpy of vaporization of formic acid is 46.15 kJ at 25°C.) Give the most likely reasons for the largest discrepancies between your calculated values and the ones in the tables.
[
egin{array}{|c|c|c|c|}
hline
ext{Compound} & Delta H^{circ}_{f} ( ext{kJ mol}^{-1}) & s^{circ} ( ext{J K}^{-1} ext{mol}^{-1}) & Delta G ( ext{kJ mol}^{-1}) \
hline
ext{Acetylene, CH} & 226.73 & 200.93 & 209.24 \
ext{Benzene, CH6} & 82.9 & 269.31 & 129.86 \
ext{Benzene, CH} & 49.0 & 173.26 & 124.60 \
ext{Butane, CH4} & 125.6 & 310.23 & 16.27 \
ext{Cyclohexane, CH6} & 124.6 & 298.30 & 30.70 \
ext{Ethane, CH3} & 84.0 & 229.71 & 32.08 \
ext{Ethylene, CH2} & 52.47 & 219.32 & 68.49 \
ext{Heptane, CH7} & 187.8 & 427.9 & 8.56 \
ext{Hexane, CH6} & 167.1 & 388.93 & 0.17 \
ext{Isobutane, CH4} & 134.2 & 294.75 & 20.26 \
ext{Methane, CH4} & 74.87 & 186.25 & 50.70 \
ext{Naphthalene, CH10} & 151.602 & 335.75 & 224.59 \
ext{n-Octane, C8H18} & 208.7 & 467.17 & 16.66 \
ext{Pentane, CH5} & 146.8 & 347.93 & 8.01 \
ext{Propane, CH3} & 104.7 & 270.02 & 24.09 \
ext{Propylene, CH3} & 20.41 & 267.05 & 62.91 \
hline
end{array}
]
[
egin{array}{|c|c|c|c|}
hline
ext{Compound} & Delta H^{circ}_{f} ( ext{kJ mol}^{-1}) & s^{circ} ( ext{J K}^{-1} ext{mol}^{-1}) & Delta G ( ext{kJ mol}^{-1}) \
hline
ext{Acetaldehyde, CH3CHO (g)} & 166.36 & 264.22 & 133.27 \
ext{Acetate} & - & - & 372.29 \
ext{Acetic acid, CH3COOH (l)} & 484.1 & 159.83 & 389.32 \
ext{Acetone, CH3COCH3 (l)} & 248.1 & 200.4 & 155.34 \
ext{Adenine, C5H5N5 (s)} & 95.98 & 151.00 & 299.56 \
ext{L-Alanine, C3H7NH3COO- (s)} & 562.7 & 129.20 & 370.17 \
ext{L-Alanylglycine, C7H12N2O3 (s)} & 826.42 & 195.05 & 532.52 \
ext{L-Aspartate} & - & - & 698.61 \
ext{Aspartic acid, C4H7NO4 (s)} & 973.37 & 170.12 & 730.15 \
ext{Butyric acid, C4H7COOH (s)} & 533.9 & 226.4 & 377.62 \
ext{Citrate} & - & - & 1168.24 \
ext{Creatine, C4H9N3O2 (s)} & 537.18 & 189.5 & 264.83 \
ext{L-Cysteine, C3H7SCH2CH(NH2)COO- (s)} & 533.9 & 169.9 & 343.90 \
ext{L-Cystine, C6H12N2O4S2 (s)} & 1051.9 & 280.58 & 693.21 \
ext{Ethanol, C2H5OH (l)} & 276.98 & 160.67 & 174.09 \
ext{Formaldehyde, CH2O (g)} & 115.90 & 218.78 & 109.89 \
ext{Formamide, HCONH2 (g)} & 186.2 & 248.45 & 101.1 \
ext{Formic acid, HCOOH (l)} & 424.76 & 128.95 & 361.43 \
hline
end{array}
]