TA practice exam 3 key

# 17 calculationentropychangeataphasetransition

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Unformatted text preview: kJ 16000 kJ correct Using the equation E q w 16000 kJ q 12000 kJ q 4000 kJ It asked for the magnitude, so make it a positive number. 16. Which of the following reactions would have a positive entropy change? I. H 2O(g) H 2O l II. CO2 ( s ) CO2 ( g ) III. C 3H 8 ( g ) 5O 2 3CO 2 ( g ) 4H 2O (l ) 1. I only 2. II only 3. III only 4. I and II only 5. II and III only correct 6. I and III only 7. I, II, and III 8. None of the above Process I describes the condensation of water vapor, which has a negative entropy change due to the reduction of the volume that the gas molecules can occupy. Process II describes sublimation of carbon dioxide, which increases the disorder in the system. Process III describes the combustion of propane, which greatly increases the disorder of the system due to the higher number of moles of gas in the products. 17. Calculation entropy change at a phase transition Which of the following transitions would have the largest value for Ssystem ? 1. H 2O(g) H 2O l 2. H 2O(l ) H 2O g correct 3. H 2O(s ) H 2O l 4. H 2O(l ) H 2O s Processes 1 and 4 have a negative value for Ssystem . Process 2 has a larger entropy change because it has a much larger change in volume. 18. When deciding whether or not a process is spontaneous, one only needs to look at the sign of _____________. 1. Ssystem 2. H 3. Suniverse 4. Ssurroundings correct This is a version of the second law of thermodynamics. If a process is spontaneous, then Suniverse is positive. 19. Which of the following is a consequence of the third law of thermodynamics? 1. The energy of the universe is constant 2. The entropy of the universe increases for any spontaneous process. 3. The entropy of any system is 0 at 0 K. 4. The entropy of a perfect crystal with only one possible orientation is 0 at 0 K. correct 5. Both 3 and 4 are correct. Statement 1 corresponds to the 1st law. Statement 2 corresponds to the 2nd law. Statement 3 is not necessarily true (consider a molecule with 2 orientations). Statement is correct because it has all of the proper qualifiers. 20. If a molecule has two possible orientations how does the polarity of the molecule affect the entropy at 0...
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## This note was uploaded on 10/10/2011 for the course CH 50995 taught by Professor Laude during the Fall '10 term at University of Texas.

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