exam --1 - pokharel (yp624) Homework 2 Sutcliffe (52410) 1...

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Unformatted text preview: pokharel (yp624) Homework 2 Sutcliffe (52410) 1 This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. This HW is due at 11pm the day before Exam 1. 001 10.0 points Calculate the standard Gibbs free energy at 298 K for the reaction 4 NH 3 (g) + 7 O 2 (g) 4 NO 2 (g) + 6 H 2 O( ) Species H f S kJ/mol J/mol K NH 3 (g)- 46 . 11 192 . 3 O 2 (g) . 205 . NO 2 (g) +33 . 2 240 . H 2 O( )- 285 . 8 69 . 91 1. +1398 kJ / mol rxn 2.- 1152 kJ / mol rxn correct 3. +244 , 363 kJ / mol rxn 4.- 180 , 455 kJ / mol rxn 5. +1152 kJ / mol rxn 6.- 1643 kJ / mol rxn 7.- 825 kJ / mol rxn 8. +1643 kJ / mol rxn 9.- 1398 kJ / mol rxn 10. +825 kJ / mol rxn Explanation: H rxn = summationdisplay n H f prod- summationdisplay n H f rct = bracketleftBig 6(- 285 . 8 kJ / mol) +4(33 . 2 kJ / mol) bracketrightBig- 4(- 46 . 11 kJ / mol) =- 1397 . 56 kJ / mol S rxn = summationdisplay n S f prod- summationdisplay n S f rct = bracketleftBig 6(69 . 91 J / mol K) +4(240 . 0 J / mol K) bracketrightBig- bracketleftBig 7(205 . 0 J / mol K) +4(192 . 3 J / mol K) bracketrightBig =- 824 . 74 J / mol K =- . 82474 kJ / mol K G = H- T S =- 1397 . 56 kJ / mol- (298 K) (- . 82474 kJ / mol K) =- 1151 . 79 kJ / mol 002 10.0 points For a given reaction at 300 K, G =- 412 kJ/mol rxn and at 400 K, G =- 439 kJ/mol rxn. The enthalpy for the reaction is the same at both temperatures ( H =- 331 kJ/mol rxn). What is the S (entropy) for this reaction? 1.- 27000 J/mol K 2. 27000 J/mol K 3. 270 J/mol K correct 4. 80000 J/mol K 5. 0.0 J/mol K Explanation: G 1 =- 412 kJ / mol rxn T 1 = 300 K G 2 =- 439 kJ / mol rxn T 2 = 400 K G = H- T S S = G- H- T and H = G + T S pokharel (yp624) Homework 2 Sutcliffe (52410) 2 H 1 = H 2 =- 331 kJ / mol rxn S 1 = (- 412 + 331) kJ / mol rxn- 300 K = 0 . 27 kJ / K mol rxn = 270 J / mol K S 2 = (- 439 + 331) kJ / mol rxn- 400 K = 0 . 27 kJ / K mol rxn = 270 J / mol K Alternate Explanation : G 1 + T 1 S = G 2 + T 2 S G 1- G 2 = T 2 S- T 1 S = S ( T 2- T 1 ) S = G 1- G 2 T 2- T 1 = (- 412 + 439) kJ / mol rxn (400- 300) K = 0 . 27 kJ / K mol rxn = 270 J / mol K 003 10.0 points For the reaction 2 SO 3 (g) 2 SO 2 (g) + O 2 (g) H r = +198 kJ mol 1 at 298 K. Which state- ment is true for this reaction? 1. G r will be negative at high tempera- tures. correct 2. The reaction will not be spontaneous at high temperatures. 3. The reaction is driven by the enthalpy. 4. G r will be positive at high tempera- tures....
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This note was uploaded on 05/17/2009 for the course CH 52410 taught by Professor Sutcliffe during the Spring '09 term at University of Texas at Austin.

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exam --1 - pokharel (yp624) Homework 2 Sutcliffe (52410) 1...

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