exam3solns - Hou, Taylor Exam 3 Due: Dec 5 2007, midnight...

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Hou, Taylor – Exam 3 – Due: Dec 5 2007, midnight – Inst: Brodbelt 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. The due time is Central time. Brodbelt CH301 Exam 3 ±all 2007 Use oF programmable calculators or cell phones is considered cheating and thus is pro- hibited. You may not leave the room For any reason. You may not start the exam aFter 7;15 pm. GOOD LUCK! 001 (part 1 oF 1) 10 points Which statement about intermolecular Forces in liquids is ±ALSE? 1. The weaker the intermolecular Forces, the lower the surFace tension. 2. The weaker the intermolecular Forces, the more rapid the evaporation rate. 3. The greater the intermolecular Forces, the lower the vapor pressure. 4. The stronger the intermolecular Forces, the lower the boiling point. correct 5. The stronger the intermolecular Forces, the greater the molar heat oF vaporization. Explanation: The stronger the intermolecular Forces, the more energy is required to get the molecules to break away From one another, and the higher the boiling point and the higher the critical temperature. 002 (part 1 oF 1) 10 points The energy change Δ H associated with the reaction NBr 3 (g) + 3H 2 O(g) 3HOBr(g) + NH 3 (g) is +81 kJ/mol rxn. These bond energy values might be useFul: O-H 459 kJ/mol; N-H 386 kJ/mol; O-Br 201 kJ/mol. The strength oF the N Br bond is 1. 128 kJ/mol 2. 66 kJ/mol 3. 248 kJ/mol 4. 280 kJ/mol 5. 4 kJ/mol 6. 155 kJ/mol correct 7. 465 kJ/mol Explanation: Δ H = 81 kJ Bonds broken: 3 N Br bonds at x kJ each 6 O H bonds at 459 kJ each Bonds made: 3 N H bonds at 386 kJ each 3 O H bonds at 459 kJ each 3 O Br bonds at 201 kJ each. Δ H = (bonds broken) - (bonds made) 81 = [3 x + 6(459)] - [3(386) + 3(459) + 3(201)] 81 = 3 x + 2754 - 3138 3 x = 465 x = 155 kJ 003 (part 1 oF 1) 10 points Consider the reaction C 2 H 2 (g) + H 2 (g) C 2 H 4 (g) at constant pressure. Which response is true? 1. No work is done as the reaction occurs. 2. Work is done by the system as it occurs. 3. Work may be done on or by the system as the reaction occurs, depending upon the temperature.
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Hou, Taylor – Exam 3 – Due: Dec 5 2007, midnight – Inst: Brodbelt 2 4. Work is done on the system as it occurs. correct Explanation: The amount of work that is done is given by w = - P Δ V but, in this case, since - P Δ V = - n ) R T , w = - n ) R T Note, however, that two moles of gaseous re- actants (1 mol of C 2 H 2 and 1 mol of H 2 ) become one mol of gaseous product. Thus Δ n is - 1. The sign of w must then be positive , which indicates that work is done ON the system. 004 (part 1 of 1) 10 points The process of steam condensing to form liq- uid water is 1. an endothermic chemical reaction. 2.
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exam3solns - Hou, Taylor Exam 3 Due: Dec 5 2007, midnight...

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