Posted 13 April 2012 - 08:34 AM

1. Suppose an horizontal adiabatic fluid flow (without shaft work). At any point the energy per unit mass of the fluid can be considered as the sum of internal energy (U), kinetic energy, and pressure energy (p*V), that is U + v^{2}/2 + p*V. It is convenient to combine U+p*V into a single quantity H (enthalpy).

Note: "Principles of Unit Operations" by A Foust, L Wenzel, C Clump (Wiley, 1960), chapter 20 - The energy balance and its applications, offer a clear presentation.

2. As Shivshankar has pointed out, heat absorbed or given by a system under constant pressure actually represents its enthalpy change. Constant pressure (isobaric) process is quite common in Chemical Engineering. Hence enthalpy is sometimes called heat content, but this is valid under isobaric process. This is its physical meaning, ΔH=Cp*ΔT (Cp=constant pressure heat capacity), or better ΔH=integral of Cp*dT.

3. Consequently convenience may be the reason of using the enthalpy function H, instead of U+p*V. It is used more frequently than U, entropy, free energy.

4. A good reference for those interested in Thermodynamics "philosophy" is "Heat and Thermodynamics" by M Zemansky, McGraw-Hill, several editions from 1937 on. Practical applications or exercises also improve understanding, since only theory may not be adequate. But I have weak points, not having studied theory systematically.

5. Videos by P McCord enjoyed, intensive and extensive (e.g. volume) properties have been clear now.