Elements of Energy Balance Calculations

In this section, we outline a procedure for solving energy balance problems that will be applied to both nonreaclive processes (this chapter) and reactive processes (Chapter 9). Section 

Recall that we can never know the absolute values of U and H fui a species at a given state. 0(kj/mol) is the sum of the energies of motion of all 6.02 X10 molecules in one gram-mole of the species plus the intramolecular kinetic and potential energies of all the atoms and subatomic particles, which are quantities we cannot determine. Since H = H + PV and we cannot know the value of V, we also cannot know the value of H at a specified state. 

Fortunately, we never need to know the absolute values of H or // at specified states we only need to know AH and AH for specified changes of state, and we can determine these quantities experimentally. We may therefore arbitrarily choose a reference state tor a species and determine AH = 0 - Href for the transition from the reference state to a series of other states. If we set Href equal to zero, then H(= AH) for a specified state is the specific internal energy at that state relative to the reference state. The specific enthalpies at each state can then be calculated from the definition, H = U + PV, provided that the specific volume (V) of the species at the given temperature and pressure is known. 

The values of H and H in the steam tables were generated using this procedure. The reference state was chosen to be liquid water at the triple point [H20(l, 0.01 C, 0.00611 bar)], at which point H was defined to be zero. According to Table B.7, for water vapor at 400°C and 

H = 2958 kJ/kg. This does not mean that the absolute value of 0 for water in the specified state is 2958 kJ/kg remember, we cannot know the absolute value of H. It means that H of water vapor at 400 C and 10.0 bar is 2958 kJ/kg relative to water at the reference state, or 

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