worksheet 13 key

# worksheet 13 key - Fall20

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Fall 20 CH301 Homework 13a Answer Key—Problems for Statistical Thermodynamics Lecture 1. Two quick ideas that make math with statistical thermodynamics a lot simpler: a) What is the amount of energy for Avagadro’s number multiplied by Boltzman’s constant? What is the famous name of this value? kN = R = 8.314J the ideal gas law constant b) Use what you know about log functions to simplify S=k lnW? Let # = number of orientations in a molecule and let n = the number of molecules. S=k ln W = k ln # n =n k ln # 2. Answer the question asked about internal motional energy for each of the systems listed: The number of degrees of freedom in a H atom in benzene? 3 The number of degrees of freedom in a benzene molecule? 36 The number of translational modes of motion in a molecule of benzene? 3 The number of rotational modes of motion in molecule of benzene? 3 The number of vibrational modes of motion in a molecule of benzene? 30 The number of degrees of freedom in 10 molecules of benzene? 360 The total internal motional energy in 10 molecules of benzene? 180kT The total non vibrational motional energy in 10 molecules of benzene? 30kT The total vibrational energy in 10 molecules of benzene? 150kT The total vibrational energy in a mole of benzene? 15RT The total translational energy in 10 moles of benzene? 15RT The number of degrees of freedom in 10 molecules of carbon monoxide? 60 The number of degrees of freedom in a O atom in carbon monoxide? 3 The number of translational modes of motion in a molecule of carbon monoxide? 3 The number of rotational modes of motion in molecule of carbon monoxide? 2 The number of vibrational modes of motion in a molecule of carbon monoxide? 1 The total internal motional energy in 10 molecules of carbon monoxide? 30kT The total non vibrational motional energy in 10 molecules of carbon monoxide? 25kT The total vibrational energy in 10 molecules of carbon monoxide? 5kT The total vibrational energy in a mole of carbon monoxide? 0.5RT The total translational energy in 10 moles of carbon monoxide? 15RT 3. Which of the following molecules would have no positional entropy at absolute zero? H 2 CH 4 NH 3 BH 3 SF 6 H 2 O CO 2 COCl CH 3 F I 3 O 3 Answer: H 2 CH 4 BH 3 SF 6 CO 2 All the others can form more than a single crystal orientation. 4. Which of the following molecules would have a positional entropy closest to zero? H 2 O H 2 S H 2 Se Answer: H 2 O because it has the greatest IMF (due to H bonding.) The greater the IMF the more non ideal the behavior and greater tendency to “order” thereby reducing the entropy. 5. Write an expression describing the maximum positional entropy of each of the following systems at absolute zero: a) 1 molecule of ammonia S = k ln 4 1 b) 10 molecules of carbon monoxide S = 10 k ln 2 c) a mole of chloromethane S = k ln 4N = kN ln 4 = R ln 4 d) 50 molecules of BH 2 Cl S = 50 k ln 3

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e) 5 moles of CO2 S = 5R ln 1 = 0
CH301 Worksheet 13b Answer Key—Internal Energy Lecture 1. Define

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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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worksheet 13 key - Fall20

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