Physical meaning of enthalpy

In physics one of the most fundamental concepts is the conservation of energy and in thermodynamics we systematize, in an ideal manner how to account for the energy and changes in energy in systems. So basically a means of categorical naming, bookkeeping.

The units of enthalpy are energy units such as Joules. And for a homogeneous system, the enthalpy is the sum of the system internal energy and the pressure energy.

As energy, enthalpy is potential in a system in the form of chemical bonds; the making and breaking of these bonds. The direction in which enthalpy changes tells us which way heat is flowing: if $Delta H < 0$ heat flows out of the system (exothermic), and if $Delta H > 0$ heat flows into the system (endothermic).

Thermodynamics was developed largely with gases in mind. In this case work can be done on the gas, the $pDelta V$ term. But there is also the internal energy U to consider, so when one wants to compare experiments done on the same substance under different conditions it is useful to define a new quantity, which is the enthalpy, as the internal energy plus work done. Thus one has a measure with which to properly compare and one set of measurements with another.

A chemical engineer's viewpoint, which will definitely differ from a physicist's viewpoint:

For enthalpy calculations, a standard state is defined, such that the enthalpy of an ideal gas at a stated temperature (e.g., 0K, 273K, etc.) and pressure (e.g., 1 atm) is defined to be 0 BTU/lb (yes, I worked in U.S. units). Ideal gases that exist at a higher temperature than this obviously have more internal energy than the standard state, and they are considered to have a higher heat content than the standard state.

A liquid at the standard temperature had to lose an amount of heat equal to the heat of vaporization for that compound, meaning that its enthalpy can easily be negative. Likewise, gases that have a low enough temperature also may have a negative enthalpy (if the standard temperature is not 0K), recognizing that most gases are not ideal, mixtures of gases may involve non-ideal mixing rules, etc.

The concept of "heat content" is useful as long as the standard state is kept in mind, and heat transfer calculations can still easily be done by calculating enthalpy changes for a given substance, or in the case of published data, by looking up enthalpies that correspond to two different temperatures, and calculating the heat transfer into or out of that substance as the difference between the enthalpies at those two different temperatures.

Thus, there are definitions of enthalpy that can be given the physical meaning of "heat content". In addition, for typical real world problems, fluid flow rates for process design problems are normally not known beforehand, so it is convenient to define a specific enthalpy (e.g., BTU/lb) for a given substance, and multiply by a flow rate (when that data is known) to determine total enthalpy.

Enthalpy, H, is H = U + PV where U is internal energy, P is pressure, and V is volume. Specific enthalpy, h (enthalpy per unit mass), is h = u + pv where u is internal energy per unit mass, P is pressure, and v is specific volume (inverse of density). Physically, enthalpy represents energy associated with mass flowing into and out of an "open" thermodynamic system. An "open" system is one with mass transfer in and out, in contrast to a "closed" system where there is no mass transfer. From a Lagrangian viewpoint (following a fixed mass) pv is work; from an Eulerian viewpoint (considering a fixed region) pv is energy. Most developments of fluid dynamics use the Eulerian viewpoint. Specific heat at constant pressure is based on enthalpy; specific heat at constant temperature is based on internal energy. For an ideal gas, enthalpy- like internal energy- is a function of temperature only. Engineering texts on thermodynamics provide more details. A texbook I like is an old one, Elements of Thermodynamics and Heat Transfer, by Obert and Young, because of the clarity of the definitions and explanation of concepts.

Enthalpy is heat at constant pressure: $ dH = dU + pdV + Vdp $ $ dU = delta Q - pdV $ $ delta Q = dU + pdV $ $ dH = delta Q + Vdp $
$ dp = 0 $ for constant pressure

So on a psychrometric chart of air, enthalpy is heat (energy) content per unit weight.