Section 4 : Measurement of Enthalpy and Internal Energy using Flow Calorimeter
Clearly, if we chose the enthalpy H1 to be a datum state and arbitrarily assign it a zero value, then:
H2 = Q
(3.42)
The last equation then allows one to uniquely determine the value of H2 at any condition achieved at state 2 by applying a known quantity of heat through the electric heating system. The internal energy at the same state can next be determined using the relation:
U = H – PV
(3.43)
Further one can measure the density (in terms of mass or mole) at the same state, and one may rewrite the last equation as
U = H – P/ρ
(3.44)
Thus using the above relations, U and H may be obtained experimentally at any P, T (and/or V) for any substance. The steam tables discussed at the concluding portion of section 2.1 constitutes such a tabulation of values of standard thermodynamic properties (saturation vapour pressure, internal energy, enthalpy (and entropy) of water obtained empirically over a wide range temperature and pressure.
