exam_3_132_2007_solutions

exam_3_132_2007_solutions - Chemistry 132 Hour Examination...

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Chemistry 132 Hour Examination 3 April 19, 2007 Name ( PRINT)____ SOLUTIONS ___________ Student number____________________________ Workshop Leader__________________________ 1. (25 points ) ____________________ 2. (25 points ) ____________________ 3. (30 points ) ____________________ 4. (30 points ) ____________________ 5. (20 points ) ____________________ TOTAL (130 points ) Show your work neatly and circle your answers . Give correct units for answers requiring them. The last two pages of the exam consist of a sheet of potentially useful formulas and a Periodic Table. You may detach these sheets.
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2 1. (25 points) For many years, the reaction of H 2 + I 2 2HI was considered a textbook example of a bimolecular process. However, in 1967, J. W. N. Sullivan showed that atomic iodine was involved in the reaction. This was demonstrated by shining ultraviolet light into the reaction mixture, thereby increasing the rate. The following mechanism has been proposed for the reaction: I 2 = 2I fast prior equilibrium, K 1 I + H 2 = IH 2 fast prior equilibrium, K 2 I + IH 2 ⎯→ 3 k HI + HI slow a) (5 points) Verify that the steps add up to the overall reaction Product I atoms in first reaction cancel out with reactants in reactions 2 and 3. IH 2 product in second reaction cancelled out in reaction 3. Net reaction: H 2 + I 2 2HI b) (15 points) Write down an expression for the rate law, in terms of [H 2 ] and [I 2 ], and identify the observed rate constant in terms of constants in the mechanism. Rate = k 3 [ I ][ IH 2 ] Get [ IH 2 ] from second prior equilibrium: [ IH 2 ] = K 2 [ I ][ H 2 ] Get [ I ] from first prior equilibrium: [ I ] = (K 1 ) 1/2 [ I 2 ] 1/2 Thus [ IH 2 ] = K 2 (K 1 ) 1/2 [ I 2 ] 1/2 [ H 2 ] So, rate = k 3 [ I ][ IH 2 ] = k 3 (K 1 ) 1/2 [ I 2 ] 1/2 K 2 (K 1 ) 1/2 [ I 2 ] 1/2 [ H 2 ] = k 3 K 1 K 2 [ H 2 ][ I 2 ] c) ( 5 points) Explain why the Sullivan mechanism is consistent with earlier beliefs that the reaction was bimolecular. The rate appears to be first order in each of the reactants, but
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This note was uploaded on 04/16/2008 for the course CHM 132 taught by Professor Farrar during the Spring '08 term at Rochester.

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exam_3_132_2007_solutions - Chemistry 132 Hour Examination...

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