"Enthalpy" is a term that simply means total heat in a substance above a chosen reference condition of temperature and pressure. An approximately-only explanation might go like this:

Let us choose a reference of 0 dF and atmospheric pressure, and say that ice has zero enthalpy at this condition. Below freezing, water has a specific heat of about 0.5. If we warm a one-pound ice cube up to 32 dF, this takes 0.5 x 32 dF = 16 btu. This is sensible heat; that is, the temperature changes. When the ice cube reaches 32 dF, it melt into liquid water. This takes about 144 btu [144 btu/lb x 1 lb]. The temperature does not change; this is the latent heat of fusion. At this point the water contains 16 + 144 = 160 btu, which is its enthalpy.

Between freezing and boiling, it takes 1 btu per pound to raise the water temperature by 1 dF [with very slight variation]. Thus, to raise this pound of water from 32 dF to 212 dF requires 180 btu. At the verge of boiling, the water thus has 160 + 180 = 340 btu of enthalpy. This 180-dF step is again sensible heat, because of the increase of temperature. As further heat is added, the temperature does not (yet) increase; instead the water boils, changing state into vapor. This latent heat of evaporation is about 1000 btu per pound [depending on the pressure at which it occurs]. When all the water has vaporized, it now has about 340 + 1000 = 1340 btu of enthalpy. Finally, we can heat the vapor (steam) further. I don't do superheated steam, so I forget the specific heat in this range. But suppose it is x btu/lb. If we superheat the steam 100 dF further, it will contain 1340 + 100x btu, which is its enthalpy at this state.

If you think that's bad, wait until you get to entropy!

You're starting to bring back my memories of steam tables...........

http://en.wikipedia.org/wiki/Enthalpy

This is Enthalpy as refers to gases (air, not gasoline).

Enthalpy is a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure.

If you had a gas in a chamber and it was at 20°C and 2 atm (atmospheres) pressure and you heated it up, the pressure would increase as the heat increases. As the pressure increases the amount of work the gas could do increases as well. If your chamber was a cylinder with a piston on top the piston would rise (do work against the force of gravity) as the temperature increased.

Enthalpy has more to do with the physical volume change of the gas than the state change from a liquid to a gas or solid to liquid. It refers to the energy (heat) within a system (the gas in my example) plus the pressure of the gas.

Entropy is the thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work. Basically Enthalpy is a variable quantity depending on the temperature and pressure of a gas, entropy is the point where there is no longer enough energy in the system to do work.

Drew