exam4_solutions - Version 174 Exam 4 Sutcliffe(53770 This...

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Version 174 – Exam 4 – Sutcliffe – (53770) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Predict which of butane (C 4 H 10 ) or propanone (CH 3 COCH 3 ) has the greater vis- cosity. Assume that they are both at the same temperature and in their liquid form. 1. propanone correct 2. Equal viscosities 3. butane Explanation: Propanone ( a.k.a. acetone) should be more viscous than butane because it is more polar than butane. 002 10.0 points Which statement is FALSE? 1. Δ H is equal to Δ E for the process 2 H 2 (g) + O 2 (g) 2 H 2 O(g). correct 2. Δ H is sometimes exactly equal to Δ E . 3. Δ H is often nearly equal to Δ E . 4. The thermodynamic quantity most easily measured in a coffee cup calorimeter is Δ H . 5. No work is done in a reaction occurring in a bomb calorimeter. Explanation: For 2 H 2 (g) + O 2 (g) 2 H 2 O(g) , n i = 3 mol gas n f = 2 mol gas Δ n = 1 mol gas w = P Δ V = n ) RT negationslash = 0 , Since Δ E = q + w , q = Δ H and w negationslash = 0, Δ E negationslash = Δ H . 003 10.0 points Which of the molecules H 2 , HF, CH 4 , CH 3 OH can be involved in hydrogen bonding? 1. CH 3 OH, CH 4 2. HF, CH 3 OH correct 3. CH 3 OH, H 2 4. HF, CH 4 Explanation: For a molecule to be involved in hydrogen bonding, the molecule must have a hydrogen atom and a highly electronegative atom such as F, O, N, or Cl. The only two molecules that satisfy these requirements are HF and CH 3 OH. 004 10.0 points Based on the information in the table, Species Δ H 0 f S 0 kJ/mol J/mol · K CHCl 3 ( ) 134 . 5 202 . 0 CCl 4 ( ) 135 . 4 216 . 4 Cl 2 (g, 1 atm) 0 . 0 223 . 0 HCl (g, 1 atm) 92 . 31 186 . 8 what are Δ H and Δ S for the reaction CHCl 3 ( ) + Cl 2 (g, 1 atm) CCl 4 ( ) + HCl (g, 1 atm)? 1. Δ H = +93 . 2 kJ/mol Δ S = +82 . 8 J/mol · K 2. Δ H = 362.2 kJ/mol Δ S = +82 . 8 J/mol · K 3. Δ H = 362.2 kJ/mol Δ S = 21.8 J/mol · K 4. Δ H = 93.2 kJ/mol Δ S = +21 . 8 J/mol · K
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Version 174 – Exam 4 – Sutcliffe – (53770) 2 5. Δ H = 93.2 kJ/mol Δ S = 21.8 J/mol · K correct Explanation: Δ H 0 rxn = summationdisplay n Δ H 0 f prod summationdisplay n Δ H 0 f rct = [( 135 . 4 kJ / mol) +( 92 . 31 kJ / mol)] [( 134 . 5 kJ / mol)] = 93 . 21 kJ / mol rxn Δ S 0 rxn = summationdisplay nS 0 f prod summationdisplay nS 0 f rct = [216 . 4 J / mol · K +186 . 8 J / mol · K] [202 J / mol · K +223 J / mol · K] = 21 . 8 J / mol · K 005 10.0 points When 2.00 kJ of energy is transferred as heat to nitrogen in a cylinder fitted with a piston at an external pressure of 2.00 atm, the nitro- gen gas expands from 2.00 to 5.00 L against this constant pressure. What is Δ U for the process? 1. 2 . 61 kJ 2. +1 . 39 kJ correct 3. +2 . 61 kJ 4. 0 . 608 kJ 5. 0 Explanation: q = 2 kJ P = 2 atm V 0 = 2 L V f = 5 L For expansion against a constant external pressure, w = P ext Δ V = ( 2 atm)(5 L 2 L) × (101 . 325 J · L 1 · atm 1 ) = 607 . 95 J = 0 . 60795 kJ .
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