E115_Problem_Set_12 - Considering the sign of your answer...

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e 115 Engineering Thermodynamics Fall 2009 University of California Problem set 12 Due: Friday, October 2 th , in lecture 1) (25 points) In this problem, we are going to investigate the entropy changes associated with the freezing of 1000 g of supercooled water at –7°C. You will need the following information: c P , liq = 75.3 J•mol 1 •K 1 c P , sol = 36.9 J•mol 1 •K 1 Δ H m 0 ° C ,1 atm ( ) = 6.02 kJ /mol M H 2 O = 18 g / mol First, you will calculate the change in entropy of the water when it freezes. a) Describe with a schematic and/or words a reversible path which takes liquid water at –7°C to ice at –7°C. (3 pts) b) Calculate the entropy change in the water along the path you described in a). (7 pts) c) The freezing of water at –7°C and 1 atm. is an irreversible process (water does not spontaneously melt at –7°C). The second law of thermodynamics says that the entropy of an isolated system may either remain constant or increase. If the entropy increases, an irreversible process occurred.
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Unformatted text preview: Considering the sign of your answer to part b), (it should be negative, indicating that the entropy of the water decreased) is the freezing of H 2 O at 7C an exception to the second law? Explain your answer in 2-3 sentences. (3 pts) d) Now concentrate on what happens to the surroundings. When the water freezes, heat is transferred from the water to the surroundings, and the entropy of the surroundings changes. Find the heat transferred to the surroundings when water freezes along the reversible path you described in part a). (6 pts) e) Find the entropy change of the surroundings along the reversible path you described in part a). (3 pts) f) What is the change in entropy of the universe as a result of the freezing? This change is either positive, negative, or zero. Discuss the significance of the sign of your answer in 2-3 sentences. (3 pts)...
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