exam_3_132_2005_solutions

exam_3_132_2005_solutions - Chemistry 132 Hour Examination...

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Chemistry 132 Hour Examination 3 April 21, 2005 Name ( PRINT)____ SOLUTIONS ____________ Student number____________________________ Workshop Leader__________________________ 1. (30 points ) ____________________ 2. (20 points ) ____________________ 3. (25 points ) ____________________ 4. (35 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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Name_____________________________ 2 1. (30 points) The hydroperoxide radical, HO 2 , is a highly reactive species that plays an important role in atmospheric chemistry, via the following reaction to produce hydrogen peroxide: H O 2 (g) + HO 2 (g) H 2 O 2 (g) + O 2 (g) The following initial rate data are used to determine the order of the reaction at 10 ° C: [HO 2 ] 0 , M Initial rate, M s -1 1.0 × 10 -8 2.8 × 10 -7 2.0 × 10 -8 11.2 × 10 -7 3.0 × 10 -8 25.2 × 10 -7 4.0 × 10 -8 44.8 × 10 -7 (a) From the data shown above, determine the order of the reaction. Doubling concentration quadruples the rate. Therefore, second order (b) From the data, determine the rate constant. Initial rate = k[HO 2 ] 0 2 k = initial rate/[HO 2 ] 0 2 = 2.8 × 10 -7 M s -1 /(1.0 × 10 -8 ) 2 M 2 = 2.8 × 10 9 M -1 s -1 (c) Using the appropriate integrated rate law, calculate the time required for an HO 2 concentration of 2.0 × 10 -8 M to decay by a factor of two, to 1.0 × 10 -8 M. 1/[HO 2 ] t = 1/[HO 2 ] 0 + 2kt for second order kinetics. Substitute concentrations. 1/ (1.0 × 10 -8 ) = 1/ ( 2.0 × 10 -8 ) + 2(2.8 × 10 9 )t 10 8 = 0.5 × 10 8 + 2(2.8 × 10 9 )t Solving, we find t = (0.5 × 10 8 ) / [2(2.8 × 10 9 )] s = 0.0089 s
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Name_____________________________ 3 2. (20 points) Air bags are based on the fact that sodium azide, NaN 3 , decomposes rapidly through the following reaction: 2NaN 3 ( s ) 2Na( s ) + 3N 2 ( g ) (1) The reaction is initiated by an impact-driven detonator that heats the NaN 3 rapidly to about 200 ° C , allowing it to decompose via reaction (1). In commercial applications, a 100 g charge of NaN 3 can produce about 30 L of nitrogen at room temperature in about 40 milliseconds. You are a chemist working for General Motors Research Laboratory developing better technology for air bags. You wish to speed up the decomposition of
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exam_3_132_2005_solutions - Chemistry 132 Hour Examination...

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