# What is enthalpy of a system, Macroscopic thermodynamics.

Tags:
1. Nov 26, 2015

### Apoorv3012

What is enthalpy of a system based on macroscopic POV of thermodynamics and not chemistry? And how do we use it to calculate the total heat transfer in isentropic processes?

2. Nov 26, 2015

### stockzahn

Enthalpy H is the sum of the internal energy U and the flow work p⋅V.

H = U + p⋅V

In an isentropic process no heat is transferred, the enthalpy difference of the fluid corrisponds to the work performed by/on the fluid.

3. Nov 26, 2015

### Staff: Mentor

This is incorrect. In a reversible Carnot cycle, which is an isentropic process, the change in enthalpy is zero, and the work is equal to the net heat transferred. Even for an isentropic reversible single-step change for a closed system, it is the internal energy which is equal to minus the work done on the surroundings, not the enthalpy.

(The definition given for enthalpy given here is correct, however.)

Apoorv312: Are you asking about an open system operating at steady state when you ask "how do we use it to calculate the total heat transfer in isentropic processes?"

Chet

4. Nov 26, 2015

### Apoorv3012

Yes, sorry I forgot to add that.

5. Nov 26, 2015

### Staff: Mentor

For a flow system like the one described, you first follow each little parcel of gas going through the system, and treat it as a closed system that is subjected to an adiabatic reversible expansion to the final pressure exiting your device. On this basis, you determine the temperature change for each of the parcels. This will determine the exit temperature from your flow system. The change in enthalpy per unit mass flowing through your system is then equal to Cp times the temperature change.

Chet

6. Nov 26, 2015

### Apoorv3012

Thank you sir, that really helped.