24 one way to determine the enthalpy of formation of

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Unformatted text preview: C2H6 (ethane) C4H10 (n-butane) Q = −393,522 kJ (heat out) Fig. 2.1 Constant-pressure flow reactor for determining enthalpy of formation 1 kmol C C + O2 → CO2 1 kmol O2 @ 25°C, 101.3 kPa ^ hsi ¼ CV 1 kmol CO2 @ 25°C, 101.3 kPa ZT ^ cp ðT ÞdT ; To where the subscript i refers to species i, T0 denotes the standard temperature (25 C), and ^ indicates that a quantity is per mole. Note that the sensible enthalpy of any ^ species is zero at standard conditions. The ‘absolute’ or ‘total’ enthalpy, hi , is thus 2 the sum of the sensible enthalpy and the enthalpy of formation: ^ ^ ^ hi ¼ Dho þ hsi i (2.24) One way to determine the enthalpy of formation of a species is to use a constantpressure flow reactor. For instance, the enthalpy of formation of CO2 is determined by reacting 1 kmol of C(graphite) with 1 kmol of O2 at 25 C at a constant pressure of 101.3 kPa. The product, 1 kmol of CO2, flows out of this reactor at 25 C as sketched in Fig. 2.1. An amount of heat produced in the reaction is transferred 2 When phase change is encountered, the total enthalpy needs to include the latent heat, ^ ^ ^ ^ hi ¼ Dho þ hsi þ hlatent . i 26 2 Thermodynamics of Combustion out of this system, therefore the enthalpy formation of CO2 is negative ^ Dho 2 ¼ À393.52 MJ/kmol. This means that CO2 at 25 C contains less energy CO than its constituent elements C(graphite) and O2, which have zero enthalpy of formation. The enthalpy of formation is not negative for all chemical species. ^ For instance, the enthalpy formation of NO is Dho ¼ +90.29 MJ/kmol, meaning NO that energy is needed to form NO from its elements, O2 and N2. For most unstable or “radical” species, such as O, H, N, and CH3, the enthalpy of formation is positive. 2.3.1.2 Evaluation of the Heat of Combustion from a Constant-Pressure Reactor Using the conservation of energy equation (2.23), we can now evaluate the enthalpies of the reactants and products. Inserting the expression for the total enthalpy,  X   X ^...
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