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. |
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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. |
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Diagram: |
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Given: |
T1 |
350 |
oC |
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P1 |
8 |
atm |
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T2 |
350 |
oC |
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P2 |
2 |
atm |
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Find: |
DU |
??? |
kJ/mol |
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DH |
??? |
kJ/mol |
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Assumptions: |
None. |
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Equations
/ Data / Solve: |
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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. |
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Temp.
(oC) |
Pressure (atm) |
Internal Energy
(kJ/mol) |
Enthalpy (kJ/mol) |
Phase |
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350 |
1 |
#VALUE! |
#VALUE! |
vapor |
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350 |
2 |
#VALUE! |
#VALUE! |
vapor |
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350 |
3 |
#VALUE! |
#VALUE! |
vapor |
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350 |
4 |
#VALUE! |
#VALUE! |
vapor |
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350 |
5 |
#VALUE! |
#VALUE! |
vapor |
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350 |
6 |
#VALUE! |
#VALUE! |
vapor |
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350 |
7 |
#VALUE! |
#VALUE! |
vapor |
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350 |
8 |
#VALUE! |
#VALUE! |
vapor |
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The internal energy and
enthalpy at the given pressures are: |
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P = 8 atm |
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P = 1 atm |
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U1 |
#VALUE! |
KJ/mol |
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U2 |
#VALUE! |
KJ/mol |
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H1 |
#VALUE! |
KJ/mol |
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H2 |
#VALUE! |
KJ/mol |
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Remember that the change
in any property is defined as the final state minus the initial state. |
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DU
= U2 - U1 = |
#VALUE! |
KJ/mol |
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DH
= H2 - H1 = |
#VALUE! |
KJ/mol |
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Verify: |
No assumptions to verify
this time. |
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Answers : |
DU |
#VALUE! |
kJ/mol |
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DH |
#VALUE! |
kJ/mol |
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