A pistonandcylinder device contains superheated steam at
350^{o}C 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: 
T_{1} 
350 
^{o}C 



P_{1} 
8 
atm 

T_{2} 
350 
^{o}C 



P_{2} 
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.
(^{o}C) 
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 



U_{1} 
#VALUE! 
KJ/mol 



U_{2} 
#VALUE! 
KJ/mol 

H_{1} 
#VALUE! 
KJ/mol 



H_{2} 
#VALUE! 
KJ/mol 











Remember that the change
in any property is defined as the final state minus the initial state. 












DU
= U_{2}  U_{1} = 
#VALUE! 
KJ/mol 

DH
= H_{2}  H_{1} = 
#VALUE! 
KJ/mol 










Verify: 
No assumptions to verify
this time. 
















Answers : 
DU 
#VALUE! 
kJ/mol 



DH 
#VALUE! 
kJ/mol 









