Practice Quiz 6 (3)

Practice Quiz 6 (3) - This print—out should have 8...

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Unformatted text preview: \/ This print—out should have 8 questions. Multiple—choice questions may continue on the next column or page — find all choices before answering. LDE Stat Thermo Theory 001 001 10.0 points When calculating positionai entropy using the Boltzmann formula (3 = it In W), which of the following statements is / are true? v/ I) An experimentally determined value of W will be less than the ideai value. II) For the same number of molecules at ab— solute zero, the experimentai value of W for. HP will be greater than the expert» mentai value for CO. III) At absolute zero, the positional entropy of a group of molecules with one orienta» tion will equal zero. 1. II only 2. I only 3. I, II, III , III correct 5. n, In 6. I, II 7. III oniy Explanation: Statement I is true because ideal values assume that IMF aren’t present and therefore don’t influence the likelihood of a moiecule adopting a particular orientation. Statement II is false because HF has a stronger IMF which thus more dramatically perturbing the value of W. Statement III is true because I raised to any power is i and the natural log of 11s 0. LDE Stat Thermo Energy 001 002 10.0 points Assuming vibrational modes are excited, how much internal energy would be associ— . ‘i ., 5 fl L '3‘ \J " a ' ' ‘31:. . e» Version PREVIEW — Quiz 6 m hawkins —— (53755) 1 ated with just the vibrational motion of 10 molecules of 002? @0kT correct 2. BOkT 3. 15kT 4. 40kT 5. 45kT Explanation: The energy of any given mode of move- . merit for any given atom or molecule is equal to 1/2kT. Since carbon dioxide has 9 to— tal modes, 3 of which are translational and 2 of which are rotational (because it is lin» ear), it has 4 vibrational modes. Since there are 10 moiecules and 4 modes, each of with are worth I/ZkT, we have a total of 20kT in internal energy. LDE Stat Their-mo Entropy 001 003 10.0 points If you had 1 mole of HF molecules, each of which had two possible orientations, what would their total theoretical positionai en~ tropy be at absolute zero? W I I 1.9.57x10 24J-K 1 So; 1%,, g 2.461.}vK‘1 . era 7, 3. 7.65 x 10—22 J - K"1 4. 5.76 J - K“3L correct Explanation: s = almost) we em = 5.76 J . Krl LDE Internal Energy 001 004 10.0 points Which of the following statements concerning internal energy is / are true? I) If the expansion work is small, AH and AU are close in value. ii) The internal energy of a system is equal Version PREVIEW — Quiz 6 — hawkins - (53755) to g at constant voinrne. III) Assuming no heat is exchanged, when pressure—voiurne work is done on the sys— tem, AU is positive. 1. I, III 2. II, III 3. Ionly 4. I, II 5. II only 6. III only 91:, III correct xplanation: Statement I follows from the identity AH = AU + pAV, because pAV is the expansion work. Statement II follows from AU = q a w, "because to z 0 for processes that occur at constant volume. Statement III also follows from AU = q + in, because w is the pressure- volurne work. (Note: for reversible processes, expansion work and pressurevolume work are identicai.) LDE Internal. Energy Cale 001 005 10.0 points If a man is jogging and releases 800 kJ of heat and does 200 kJ of work, what is the man’s change in internai energy equal to? 1. “600 kJ k3 correct 3. 1000 Ltd 4. 600 kJ Explanation: AU = q+w 2 —800k5 + —200kJ = ~1000 kJ LDE Ranking Entropies 001 10 006 10.0 points Bank the following systems in terms of in: creasing entmpy: H (D8 \ gr” a) 1 H101 of pure ice b) 1 mol of water with I moi of salt dissoIve in it. c) 1 mol of pure waterhi IOUJ l.b<c<a @ < c < b correct 3.b<a<c 4.a<b<c 5.c<a<b 6.c<h<a Explanation: Entropy increases as systems go through endothermic phase transitions and when there is more matter or more dispersed matter present. LDE Entropy Change Gate 001 007 10.0 points If a given system releases 900 J of heat, what will A5 of the surroundings be if this takes place at 27 CC? M in) .th J - KW} correct 1. 33.3J’K4 Explanation: Heat released by the system will increase the entropy of the surroundings by an amount equal to q/T, in this case 900 J _3 LEE Thermo 2nd Law Oak: 001 008 10.0 points Version PREVIEW m Quiz 6 — hawkins M (53755) Consider the condensation of water at standard pressure at 10100. Even with out knowing the exact values in ASWP and Affmp, what can you say about the value of Afluniveme for this process? 1. ASum-Ueme would be a large positive num- ber. 2. asumme would be a iarge negative num» ber. @Smmrse would be a small negative number. correct 4. ASmmrse would be a small-positive nuns». bet. ' Explanation: The process would be non—Spontaneous (water doesn’t condense at 1 atm at 101 °C), so ASmmme would be negative. We are near the phase change temperature of water, so it would also be small. ...
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Practice Quiz 6 (3) - This print—out should have 8...

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