Calori di combustione

Pagina 1

© 2000 CRC Press LLC HEAT OF COMBUSTION The heat of combustion of a substance at 25°C can be calculated from the enthalpy of formation (ΔfH°) data in the table “Standard Thermodynamic Properties of Chemical Substances” in this Section. We can write the general combustion reaction as For a compound containing only carbon, hydrogen, and oxygen, the reaction is simply and the standard heat of combustion ΔcH°, which is defined as the negative of the enthalpy change for the reaction (i.e., the heat released in the combustion process), is given by This equation applies if the reactants start in their standard states (25°C and one atmosphere pressure) and the products return to the same conditions. The same equation applies to a compound containing another element if that element ends in its standard reference state (e.g., nitrogen, if the product is N2); in general, however, the exact products containing the other elements must be known in order to calculate the heat of combustion. The following table gives the standard heat of combustion calculated in this manner for a few representative substances. Molecular formula Name ΔcH°/kJ mol–1 Inorganic substances C Carbon (graphite) 393.5 CO Carbon monoxide (g) 283.0 H2 Hydrogen (g) 285.8 H3N Ammonia (g) 382.8 H4N2 Hydrazine (g) 667.1 N2O Nitrous oxide (g) 82.1 Hydrocarbons CH4 Methane (g) 890.8 C2H2 Acetylene (g) 1301.1 C2H4 Ethylene (g) 1411.2 C2H6 Ethane (g) 1560.7 C3H6 Propylene (g) 2058.0 C3H6 Cyclopropane (g) 2091.3 C3H8 Propane (g) 2219.2 C4H6 1,3-Butadiene (g) 2541.5 C4H10 Butane (g) 2877.6 C5H12 Pentane (l) 3509.0 C6H6 Benzene (l) 3267.6 C6H12 Cyclohexane (l) 3919.6 C6H14 Hexane (l) 4163.2 C7H8 Toluene (l) 3910.3 C7H16 Heptane (l) 4817.0 C10H8 Naphthalene (s) 5156.3 Alcohols and ethers CH4O Methanol (l) 726.1 C2H6O…