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Unformatted text preview: Previous Midterm Questions and Solutions These questions were taken from previous midterm examinations from CHEM 120, which may be of assistance to you in studying. Please remember however that the course, and instructors, change from year to year, so many of these questions will not be as relevant to this year’s CHEM 120 as some. This gives the impression of being more difficult than they were when given, since different homework was assigned, and more time may have been spent in class in some of these topics. Some terminology and variables may have changed somewhat too. Good luck studying. 1. What is the work in joules done on the system to compress He gas from 24.0 L to 12.5 L against a pressure of 1.5 atm at a constant temperature of 37.4 ºC ? a. 5.5 × 10 3 b. –17 c. 17 d. 1.7 × 10 3 e. –1.7 × 10 3 Work is equal to P ∆ V, and the sign of the work in this case will be +, since work is being done ON the system BY the surroundings (compression). From the last page, 1 Joule = 1kPaL and 1 atm = 101 kPa, so the pressure here of 1.5 atm = 151.5 kPa. ∆ V = 11.5L (24L – 12.5L), so P ∆ V = (151.5 kPa)(11.5L) = 1742 kPaL = 1.7 x 10 3 Joules. The Temperature was not relevant. 2. 246 g of hot coffee at 86.0 ºC are placed in a 137 g mug at 20.0 ºC. The specific heat of coffee is 4.00 J/g ºC, while that of the mug is 0.752 J/g ºC. Assuming that no heat is lost to the surroundings, what is the final temperature of the system: mug + coffee? a. 79.7 ºC b. 93.7 ºC c. 98.4 ºC d. 76.0 ºC e. 53.0 ºC This was a difficult question, and I will refer everyone to the detailed solution to homework problem 7.29 in problem set 2 for the full calculation. In short however, conceptually, you need to figure out the total heat transferred as the variable, which will decrease the temperature of the hot coffee proportional to its specific heat and mass, and increase the temperature of the mug proportional it its specific heat and mass. 3. If the volume of a gas is doubled but the temperature remains constant, then: a. the pressure stays the same. b. the molecules move faster. c. the kinetic energy increases. d. the molecules move more slowly. e. none of these answers is correct. If we double the volume of a gas at constant T, the ONLY thing that is allowed to happen by PV = nRT is for the pressure to decrease to ½. If T is constant, then by definition the molecules are the same speed and kinetic energy, so none of a, b, c, or d can be correct. 4 . If we consider the Universe as one large thermodynamic system, we can say that over time : a. both the total Energy and the total Entropy are increasing. b. the total Energy is increasing and the total Entropy is decreasing....
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 Fall '07
 BrianMcGill
 Thermodynamics, Enthalpy, Energy, Entropy, b. c. d.

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