# Example Problem with Complete Solution

3B-1 : ΔU and ΔH for Isothermal Expansion of Superheated Water Vapor 2 pts
A piston-and-cylinder device contains superheated steam at 350oC and 10 atm. The piston is held in place by a pin. When the pin is removed, the steam expands isothermally until the piston hits a second pin.
At this point, the pressure is 5 atm. Determine the change in the molar enthalpy and molar internal energy of the steam in units of KJ/mol.

Read : Because the water vapor is superheated, it has 2 degrees of freedom.  In this case both the T and P must be specified to completely determine the state.  Because the state is completely determined, we can use the given T and P values to look up properties like U and H in the Superheated Tables in the Steam Tables.
Diagram:
Given: T1 350 oC P1 8 atm
T2 350 oC P2 2 atm
Find: DU ??? kJ/mol
DH ??? kJ/mol
Assumptions: None.
Equations / Data / Solve:
Use the NIST WebBook to determine the properties of superheated water vapor at the initial and final pressures.  As always, use the ASHRAE convention.  A portion of the thermodynamic table used in this problem is provided below.
Temp.
(oC)
Pressure (atm) Internal Energy (kJ/mol) Enthalpy (kJ/mol) Phase
350 1 #VALUE! #VALUE! vapor
350 2 #VALUE! #VALUE! vapor
350 3 #VALUE! #VALUE! vapor
350 4 #VALUE! #VALUE! vapor
350 5 #VALUE! #VALUE! vapor
350 6 #VALUE! #VALUE! vapor
350 7 #VALUE! #VALUE! vapor
350 8 #VALUE! #VALUE! vapor
The internal energy and enthalpy at the given pressures are:
P = 8 atm P = 1 atm
U1 #VALUE! KJ/mol U2 #VALUE! KJ/mol
H1 #VALUE! KJ/mol H2 #VALUE! KJ/mol
Remember that the change in any property is defined as the final state minus the initial state.
DU = U2 - U1 = #VALUE! KJ/mol DH = H2 - H1 = #VALUE! KJ/mol
Verify: No assumptions to verify this time.
Answers : DU #VALUE! kJ/mol DH #VALUE! kJ/mol