Chem2080 2011 Prelim1 key

Chem2080 2011 Prelim1 key - Page 1 of 8 Chemistry 2080 -...

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Page 1 of 8 Chemistry 2080 - Spring 2011 Prelim #1 Tuesday March 1st, 7:30 to 9:00 pm Name: ______ KEY____________________________ CU ID: ____________________ Last First M.I. Lab: _______________________________ Lab TA Name: __________________ Day Time Instructions: Answer all questions in the spaces provided. You have 90 minutes. You must show all work for full credit. There are 8 pages in this exam, and there is also a separate equation sheet. There are 6 questions; the points for each are indicated below. 1. _____________/15 2. _____________/20 3. _____________/20 4. _____________/15 5. _____________/15 6. _____________/15 Total _____________/100
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of 8 Page 2 of 8 1) (15 points) Under certain conditions, the experimental rate law for the gas-phase reaction of molecular hydrogen with molecular bromine: H 2 (g) + Br 2 (g) –––––––> 2HBr (g) is given by: rate = k[H 2 ][Br 2 ] 1/2 Show that the unusual reaction order of one half for Br 2 can be explained by the following mechanism: k 1 Step 1: Br 2 2Br . (rapid equilibrium) k-1 k 2 Step 2: Br . + H 2 ––––––––> HBr + H . (slow) k 3 Step 3: H . + Br 2 ––––––––> HBr + Br . (fast) ____________________________________________________ Assume an equilibrium in Step #1: k 1 [Br 2 ] = k -1 [Br . ] 2 . . . . . . (4) The RDS is Step #2 so Rate = k 2 [Br . ][H 2 ] But from equation #4, [Br . ] = (k 1 [Br 2 ]/ k ) 1/2 So, Rate = k 2 [H 2 ] (k 1 [Br 2 k ) 1/2 = k obs 2 ][Br 2 ] 1/2 Q.E.D.
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of 8 Page 3 of 8 2) (20 points) Calculate the standard enthalpy of combustion at 298.15K per mole of a gaseous fuel that contains C 3 H 8 and C 4 H 10 in the mole fractions of 0.62 and 0.38, respectively. (Assume that the water produced in the combustion is liquid, not gas). Here are values of Δ f in kJ per mole (i.e. standard molar enthalpies of formation) at 298.15K, for some compounds: CO 2 (g) -393.5 CH 4 (g) -74.81 C 3 H 8 (g) -103.8 H 2 O (l) -285.8 C 2 H 6 (g) -84.68 C 4 H 10 (g) -125.6
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of 8 Page 4 of 8 3) (20 points) Given the following standard enthalpies of reaction: H 2 (g) + F 2 (g) –––––––> 2HF(g) Δ H° = –537 kJ . . . . . (a) C(s) + 2F 2 (g) –––––––> CF 4 (g) Δ H° = –680 kJ . . . . . (b) 2C(s) + 2H 2 (g)
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This note was uploaded on 04/29/2011 for the course CHEM 2080 taught by Professor Davis,f during the Spring '07 term at Cornell.

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Chem2080 2011 Prelim1 key - Page 1 of 8 Chemistry 2080 -...

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