This energy comes from a rearrangement of chemical

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Unformatted text preview: nd the products. However, in combustion systems, the evaluation of the enthalpies is not straightforward because the species entering the system are different than those coming out due to chemical reactions. Qrxn,p is often referred to as the enthalpy of reaction or heat of reaction, with the subscript p indicating that the value was calculated at constant pressure. The enthalpy of reaction is related to the enthalpy of combustion by Qrxn,p ¼ ÀQC. 2.3.1.1 Enthalpy of Formation In combustion processes, reactants are consumed to form products and energy is released. This energy comes from a rearrangement of chemical bonds in the ^ reactants to form the products. The standard enthalpy of formation, Dho , quantifies i the chemical bond energy of a chemical species at standard conditions. The enthalpy of formation of a substance is the energy needed for the formation of that substance from its constituent elements at STP conditions (25 C and 1 atm). ^ The molar base enthalpy of formation, Dho , has units of MJ/kmol, and the mass base i ^ enthalpy of formation, Dho , has units of MJ/kg. Elements in their most stable forms, i such as C(graphite), H2, O2, and N2, have enthalpies of formation of zero. Enthalpies of formation of commonly encountered chemical species are tabulated in Table 2.2. A departure from standard conditions is accompanied by an enthalpy change. For thermodynamic systems without chemical reactions, the change of enthalpy of an ideal gas is described by the sensible enthalpy, 2.3 Heating Values 25 Table 2.2 Enthalpy of formation of common combustion species ^ Species Dho (MJ/kmol) Species H2O (g) CO2 CO CH4 C3H8 C7H16 (g) (n-heptane) C8H18 (g) (isooctane) CH3OH (g) (methanol) CH3OH (l) (methanol) C2H6O (g) (ethanol) C2H6O (l) (ethanol) À241.83 À393.52 À110.53 À74.87 À104.71 À224.23 À259.25 À201.54 À238.43 À235.12 À277.02 ^ Dho (MJ/kmol) +217.99 +472.79 +90.29 +33.10 +249.19 +39.46 +715.00 +226.73 +52.28 À84.68 À126.15 H N NO NO2 O OH C (g) C2H2 (acetylene) C2H4 (ethylene)...
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