eng-08spring-CHM2046C-e102x03-key

eng-08spring-CHM2046C-e102x03-key - Exam 3 answer key The...

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Exam 3 answer key 15 Mar 08 The test version is listed after before problem #1, after #20 and next to #21. PART I (60 pts total, each question is 3 pts) The “date covered” is the lecture date for that material. “Expected score” is the percent from a previous semester. There was a typo on #18, so everyone will get credit for that problem. PART II (45 pts possible) Lewis structures problem. (8pts) Use Lewis structures to help explain why nitric acid, HNO 3 , is a stronger acid than nitrous acid, HNO 2 . To understand the strength of acids, we must look at the stability of their conjugate bases (Mar 9 th lecture). The conjugate base of nitrous acid is the nitrite ion The nitrite ion has two resonance forms. The conjugate base of nitric acid is the nitrate ion The nitrate ion has three resonance forms . Because there are more resonance forms (and because of the inductive effects of the oxygens), the charge will be more delocalized around the anion. Thus, the nitrate ion is more stable; hence, nitric acid is more willing to donate a proton to form the nitrate ion. 6 pts Lewis structures of conjugate bases (only 2 pts if only the acids are drawn), 2 pts reasonable explanation # Question Date covered Expected Score Percent Correct η -3- A η -3- B ζ -3- B 1 equilibrium concentration 2/25 37.5 66.7 e c a 2 finding K c from known conc. 2/25 41.7 47.6 ab ab b 3 understanding Q 2/27 41.7 64.3 c b a 4 Δ G from equilibrium constant 2/27 & 29 33.3 54.8 a a c 5 K p and K c (with a solid in the reaction) 2/25 41.7 31.0 e c a 6 Le Châtelier’s principle: pressure 2/27 66.7 38.1 b c a 7 Le Châtelier’s principle: temperature 2/27 58.3 83.3 b c a 8 Le Châtelier’s principle: concentration 2/27 83.3 76.2 b a a 9
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This note was uploaded on 07/10/2011 for the course CHM 2045 taught by Professor Eng during the Fall '10 term at UNF.

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eng-08spring-CHM2046C-e102x03-key - Exam 3 answer key The...

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