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(CENG131)[2009](s)quiz6~kqyan^_10028.pdf
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CENG 131
CHEMICAL ENGINEERING THERMODYNAMICS
Quiz6.png
HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY, SPRING 2009

QUIZ #6
1. A piston-cylinder assembly contains 1 kg of steam at 1 bar, 520 K. It is first compressed isothermally until the first drop of water condenses. At this state, it is then compressed reversibly and adiabatically until the pressure reaches 200 bar. What is the change in the total Gibbs free energy of this process? Mark the process path on the T-S diagram. Approximate answers are acceptable.


The Gibbs free energy is a state function. Therefore, to calculate its change, we only need information about the initial and final states. From the diagram (next page), the initial state is: ..1=520 K; ..1=1 bar;..1=3600 kJ kg-1;..1=11.55 kJ kg-1K-1
We can calculate the specific Gibbs free energy of the initial state: ..1=..1...1..1=.2400 kJ kg-1
And the final state is: ..2=750 K; ..2=200 bar;..2=3800 kJ kg-1;..2=9.6 kJ kg-1K-1
The specific Gibbs free energy of the final state is: ..2=..2...2..2=.3400 kJ kg-1
For 1 kg of steam, the total change in Gibbs free energy is: ...=.. ..2...1 = 1 kg .3400 kJ kg-1+2400 kJ kg-1 =.1000 kJ