what is enthalpy

Add heat and, because the pressure is constant, the gas expands, so the heat splits into a rise of internal energy and expansion work. The heat does not all go into temperature. So to raise the temperature by the same amount, constant pressure, where the piston is pushed out and part of the heat leaves as expansion work, costs more heat than a tank whose volume is fixed.

Enthalpy is the internal energy plus PV. Change the volume and watch the bar grow, and H = U + PV piles up as one property of the state, and for an ideal gas the enthalpy is a function of temperature alone. Unlike work or heat, enthalpy is fixed by the state now, so whatever path was taken the enthalpy difference of two states is always the same.

Switch the two cases and the split is plain. At constant volume the work is 0, so the heat in is the change of internal energy, and at constant pressure the heat in is the enthalpy change entire, so the heat at constant pressure is the enthalpy change. Reaction heats in a chemistry book are mostly written as enthalpy because the experiment happens at constant pressure open to the air, and the heat exchanged is the enthalpy change.

Now an open system. To push a fluid in across the boundary you have to do work against the pressure, and pushing it in with the slider shows that this flow work is exactly PV. When water is pushed along a pipe or a pump draws a fluid in, every lump of fluid that enters carries the share that shoves aside the pressure in front.

Why enthalpy at all. Compare with the button and a closed system is fine with internal energy, but a flow that crosses always carries PV flow work, and the enthalpy that has already added that share to the internal energy is the natural language of flow devices such as a pump, a turbine, or a boiler. The next lesson moves to the ideal gas. Judging a refrigerator or a jet engine is in the end reading the enthalpy difference of the fluid that crosses, and the simplest start for that fluid is the ideal gas of the next lesson.