The Total Heat Content
Enthalpy ($H$) is a thermodynamic property of a system. It represents the total heat content of a system and is equal to the internal energy of the system plus the product of pressure and volume.
While internal energy represents the total microscopic kinetic and potential energy of the molecules, enthalpy accounts for the energy required to create the physical space for the system to exist by displacing its surroundings.
Practical Applications
- Chemical Reactions: In chemistry, measuring the change in enthalpy ($\Delta H$) determines if a reaction is exothermic (releases heat, $\Delta H < 0$) or endothermic (absorbs heat, $\Delta H > 0$). When you burn wood, the massive negative change in enthalpy is what you feel as the fire's heat.
- Steam Turbines: Engineers use enthalpy charts to design power plants. By tracking the enthalpy of superheated steam as it enters a turbine and the much lower enthalpy as it exits, they can calculate the exact amount of energy converted into rotational mechanical work.
- HVAC Systems: Air conditioning systems rely on the phase change of refrigerants. The enthalpy of vaporization tells engineers exactly how much heat energy the refrigerant absorbs from the room when it boils from a liquid into a gas inside the evaporator coils.
The Formula
H = U + PV
Example Calculation
A gas system has an internal energy of $10,000 , \text{Joules}$. It is contained at standard atmospheric pressure ($101,325 , \text{Pa}$) and occupies a volume of $0.5 , \text{m}^3$.
- Calculate PV Work: $101,325 \cdot 0.5 = 50,662.5 , \text{Joules}$.
- Add to Internal Energy: $10,000 + 50,662.5 = 60,662.5 , \text{Joules}$.
The total enthalpy of the system is $60,662.5 , \text{Joules}$. This tells us the total thermodynamic potential of the gas, including the energy required to simply 'push' the atmosphere out of the way to create its $0.5 , \text{m}^3$ volume.