The amount of heat transfer is estimated by q0 v

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Unformatted text preview: Bomb calorimeter 2.3 Heating Values 29 Because the final water temperature is close to room temperature, the water in the combustion products is usually in liquid phase. Therefore the measurement leads to the HHV from a constant-volume combustion process as described by Eq. 2.32: ( HHV ¼ ÀQ0 ;v rxn À X Ni;P À X i ! ) Â Ã ^ Ni;R Ru T0 = Nfuel Mfuel ; i where Nfuel is the number of moles of fuel burned and Mfuel is the molecular weight of the fuel. The negative sign in front of Q0 ;v ensures that HHV is positive. In a rxn bomb calorimeter, if the final temperature of the combustion products is higher than the reactants by only a few degrees (<10 C), the error is negligible. The amount of heat transfer is estimated by À Q0 ;v ¼ ðmsteel Á cp;steel þ mwater Á cp;water ÞDT ; rxn (2.35) where DT is the temperature change of the water and the steel container. The bomb calorimeter can also measure the enthalpy of formation of a chemical species. For instance, to determine enthalpy of formation of H2O, we start out with 1 mol of H2 and 0.5 mol of O2. These element species have zero enthalpy of formation; therefore X ^0 Ni;R Dhi;R ¼ 0: i The only product is the species of interest, namely H2O. We therefore can write the ^ enthalpy of formation of H2O, Dhi0P , as ; ^ Dhi0P ; ¼ Q0 ;v rxn þ  P Ni;P À i P  ^ Ni;R Ru T0 i ¼ Ni;P ^ Q0 ;v þ DN Ru T0 rxn Ni;P (2.36) where DN ¼ X Ni;P À i 2.3.3 X Ni;R : i Representative HHV Values Listed in Table 2.3 are higher heating values of some common and less common fuels. Example 2.4 A table of thermodynamic data gives the enthalpy of formation ^0 for liquid water as DhH2 OðlÞ ¼ À285.8 kJ/mol. A bomb calorimeter burning 1 mol of H2 with O2 measures 282.0 kJ of heat transfer out of the reacted mixture. Estimate the error of the enthalpy measurement. 30 2 Thermodynamics of Combustion Solution: We start out with the combustion stoichiometry H2 ðgÞ þ 0:5O2 ðgÞ ¼ H2 OðliqÞ; DN ¼ À1:5 ðChange in moles of gas in the mixtureÞ Applying the ideal gas approximation to...
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This document was uploaded on 01/20/2014.

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