1422-Exam-1-2009-Answers

1422-Exam-1-2009-Answers - CHEM 1422 Exam # 1 (Feb 19,...

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CHEM 1422 Exam # 1 (Feb 19, 2009) ANSWER KEY Thermo, Kinetics & 1 st part of Equilibrium Put a big X this box if you want your graded exam put out in the public racks outside Prof. Stanley’s office after grading . If you don’t check it, Prof. Stanley will keep your exam and you will have to stop by to pick it up from him personally. 1. (5 pts) Qualitatively, for which of the following reactions will S rxn increase the most ? Circle the correct answer and provide clear justification/reasoning for your choice in the space to the left of the answers. a) Cu 2+ ( aq ) + Zn( s ) Cu( s ) + Zn 2+ ( aq ) b) AgCl( s ) + Br ( aq ) AgBr( s ) + Cl ( aq ) c) 3C 2 H 2 ( g ) C 6 H 6 ( l ) d) 2HgO( s ) 2Hg( l ) + O 2 ( g ) e) 2SO 2 ( g ) + O 2 ( g ) 2SO 3 ( g ) 2. (5 pts) Which of the following substances has the highest absolute Entropy? Circle the correct answer and provide clear justification/reasoning for your choice in the space to the right of the answers. a) H 2 Te( g ) b) H 2 S( g ) c) NH 3 ( g ) d) H 2 O( l ) e) LiBH 4 ( s ) 3. (5 pts) What is rxn for the following reaction? f (PCl 3 ) = 320 kJ/mol; P 4 = elemental P. Circle the correct answer and provide clear justification/reasoning for your choice in the space to the right of the answers. P 4 ( s ) + 6Cl 2 ( g ) 4PCl 3 ( l ) a) 1280 kJ/mol b) 320 kJ/mol c) 0 kJ/mol d) 320 kJ/mol e) 1280 kJ/mol 5. (5 pts) Calculate S rxn for: 3O 2 ( g ) 2O 3 ( g ) Sº(O 2 ) = 205 J/mol . K, Sº(O 3 ) = 240 J/mol . K Circle the correct answer and provide clear justification/reasoning for your choice in the space to the right of the answers. a) 620 J/mol . K b) 310 J/mol . K c) 135 J/mol . K d) 310 J/mol . K e) 620 J/mol . K 2 “molecules” of HgO( s ) forming 3 “molecules” of Hg( l ) and O 2 ( g ) will have the largest increase in entropy due to: A) 2 particles going to 3 particles, and B) 2 solids forming 2 liquids and 1 gas molecule. All the other reactions are staying about the same (same phases and # of particles on each side of the rxn) or decreasing. When comparing compounds of roughly similar size and molecular weight (MW), gasses will always have the highest entropy. Between the 3 gas phase molecules, the one with the highest MW, H 2 Te (MW = 130), will have the highest entropy. rxn = [ f (products)] – [ f (reactants)] Recognize that f for P 4 ( s ) and Cl 2 ( g ) are both = 0 (elements in their standard states) and REMEMBER to multiply by the coefficients!! rxn = [(4)(–320 kJ/mol)] – [0] = –1280 kJ/mol rxn = (S° (products) – S° (reactants) REMEMBER to multiply by the coefficients!! rxn = [(2)(240 J/mol)] – [(3)(205 J/mol)] = 480 J/mol – 615 J/mol = –135 J/mol
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CHEM 1422 - Exam # 1 (Thermo, Kinetics, Equilibrium) 2 6. (5 pts) Which potential energy diagram best represents a spontaneous reaction that will have the largest temperature dependence with regards to its rate of reaction? Indicate the letter for the diagram and provide clear justification/reasoning for your choice.
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1422-Exam-1-2009-Answers - CHEM 1422 Exam # 1 (Feb 19,...

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