Using Standard Molar Enthalpies of Formation
Benzene, C_6H_6, is a commercially important hydrocarbon that is present in gasoline, where it enhances the octane rating. Calculate its enthalpy of combustion per mole; that is, find the value of ΔH° for the reaction
C_6H_6(\ell ) + \frac{15}{2} O_2(g) → 6 CO_2(g) + 3 H_2O(\ell )
For benzene, ΔH^\circ_f\left\{ C_6H_6(\ell)\right\}= 49.0 kJ/mol. Use Table 6.2 for any other values you may need.
Strategy and Explanation To calculate ΔH° you need standard molar enthalpies of formation for all compounds (and elements, if they are not in their standard states) involved in the reaction. (Since O_2(g) is in its standard state, it is not included.) From Table 6.2,
C(graphite) + O_2(g) → CO_2(g) ΔH^\circ_f = – 393.509 kJ/mol
H_2(g) + \frac{1}{2} O_2(g) → H_2O(\ell) ΔH^\circ_f = – 285.830 kJ/mol
Using Equation 6.11,
ΔH° = \sum\left\{(moles of product) × ΔH^\circ_f (product)\right\} – \sum\left\{(moles of reactant) × ΔH^\circ_f (reactant)\right\} [6.11]
ΔH° = [6 mol× ΔH^\circ_f \left\{ CO_2(g)\right\}+ 3 mol × ΔH^\circ_f \left\{H_2O(\ell)\right\} ] – [1 mol C_6H_6(\ell) × ΔH^\circ_f \left\{C_6H_6(\ell)\right\}
= [6 mol × (- 393.509 kJ/mol) + 3 mol × (- 285.830 kJ/mol)]
– [1 mol × (49.0 kJ/mol)] = – 3267.5 kJ
Reasonable Answer Check As expected, the enthalpy change for combustion of a fuel is negative and large.
Table 6.2 Selected Standard Molar Enthalpies of Formation at 25 °C* | |||||
Formula | Name | Standard Molar Enthalpy of Formation (kJ/mol) | Formula | Name | Standard Molar Enthalpy of Formation (kJ/mol) |
Al_2O_3(s) | Aluminum oxide | -1675.7 | HI(g) | Hydrogen iodide | 26.48 |
BaCO_3(s) | Barium carbonate | -1216.3 | KF(s) | Potassium fluoride | -567.27 |
CaCO_3(s) | Calcium carbonate | -1206.92 | KCl(s) | Potassium chloride | -436.747 |
CaO(s) | Calcium oxide | -635.09 | KBr(s) | Potassium bromide | -393.8 |
C( s, diamond) | Diamond | 1.895 | MgO(s) | Magnesium oxide | -601.7 |
CCl_4(\ell) | Carbon tetrachloride | -135.44 | MgSO_4(s) | Magnesium sulfate | -1284.9 |
CH_4(g) | Methane | -74.81 | Mg(OH)_2(s) | Magnesium hydroxide | -924.54 |
C_2H_5OH(\ell) | Ethyl alcohol | -277.69 | NaF(s) | Sodium fluoride | -573.647 |
CO(g) | Carbon monoxide | -110.525 | NaCl(s) | Sodium chloride | -411.153 |
CO_2(g) | Carbon dioxide | -393.509 | NaBr(s) | Sodium bromide | -361.062 |
C_2H_2(g) | Acetylene (ethyne) | 226.73 | NaI(s) | Sodium iodide | -287.78 |
C_2H_4(g) | Ethylene (ethene) | 52.26 | NH_3(g) | Ammonia | -46.11 |
C_2H_6(g) | Ethane | -84.68 | NO(g) | Nitrogen monoxide | 90.25 |
C_3H_8(g) | Propane | -103.8 | NO_2(g) | Nitrogen dioxide | 33.18 |
C_4H_{10}(g) | Butane | -126.148 | O_3(g) | Ozone | 142.7 |
C_6H_{12}O_6(s) | \alpha-D-Glucose | -1274.4 | PCl_3(\ell) | Phosphorus trichloride | -319.7 |
CuSO_4(s) | Copper(II) sulfate | -771.36 | PCl_5(s) | Phosphorus pentachloride | -443.5 |
H_2O(g) | Water vapor | -241.818 | SiO_2(s) | Silicon dioxide (quartz) | -910.94 |
H_2O(\ell) | Liquid water | -285.830 | SnCl_2(s) | Tin(II) chloride | -325.1 |
HF(g) | Hydrogen fluoride | -271.1 | SnCl_4(\ell) | Tin(IV) chloride | -511.3 |
HCl(g) | Hydrogen chloride | -92.307 | SO_2(g) | Sulfur dioxide | -296.83 |
HBr(g) | Hydrogen bromide | -36.40 | SO_3(g) | Sulfur trioxide | -395.72 |
*From Wagman, D. D., Evans, W. H., Parker, V. B., Schuman, R. H., Halow, I., Bailey, S. M., Churney, K. L., and Nuttall, R. The NBS Tables of Chemical Thermodynamic Properties. Journal of Physical and Chemical Reference Data, Vol. 11, Suppl. 2, 1982. (NBS, the National Bureau of Standards, is now NIST, the National Institute for Standards and Technology.) |