Chem2080 2010 Prelim1 key - Page 1 of 6 Chemistry 2080 -...

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Page 1 of 6 Chemistry 2080 - Spring 2010 Prelim #1 Tuesday March 2nd, 7:30 to 9:00 pm Name: _____________________________________ 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 6 pages in this exam, and there is also a separate equation sheet. There are 5 questions; the points for each are indicated below. 1. _____________/20 2. _____________/20 3. _____________/20 4. _____________/20 5. _____________/20 Total _____________/100
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of 6 Page 2 of 6 1) (20 points) (a) Write a balanced equation for the combustion of liquid cyclohexane (C 6 H 12 ) in oxygen to give gaseous carbon dioxide and liquid water. (b) The standard enthalpy change for this combustion reaction at 25°C and 1 atm pressure is – 3,920 kJ/mol of cyclohexane. Use this figure to calculate the standard enthalpy of formation ( Δ f ) of liquid cyclohexane at 25°C and 1 atm pressure. Note that Δ f for H 2 O(l) = –285.8 kJ/mol, Δ f for H 2 O(g) = –241.8 kJ/mol, and Δ f for CO 2 (g) = –393.5 kJ/mol, all at 25°C. _______________________________________________________________________________ (a) C 6 H 12 (l) + 9O 2 (g) –––––––> 6CO 2 (g) + 6H 2 O(l) Δ rxn = – 3,920 kJ (b) Δ rxn = Σ ν p Δ f (products) Σ ν r Δ f (reactants) = – 3,920 kJ i.e. Δ rxn = [6x Δ f (CO 2 (g)) + 6x Δ f (H 2 O(l))] - [1x Δ f (C 6 H 12 (l)) + 9x Δ f (O 2 (g))] So: Δ rxn = – 3,920 kJ = [6molx(-393.5kJ/mol) + 6molx(-285.8kJ/mol)] - [1molx Δ f (C 6 H 12 (l)) + 9x0] Thus: – 3,920 kJ = [-2,361 kJ - 1,714.8 kJ] – [ Δ f (C 6 H 12 (l))] Hence: Δ f (C 6 H 12 (l)) = 3,920 kJ 4,076 kJ = – 156 kJ for 1 mol of cyclohexane. i.e. Δ f (C 6 H 12 (l)) = – 156 kJ/mol
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of 6 Page 3 of 6 2) (20 points) Acetoacetic acid, CH 3 COCH 2 COOH, a reagent used in organic synthesis, decomposes in acidic solution, producing acetone and CO 2 (g). CH 3 COCH 2 COOH(aq) –––––––> CH 3 COCH 3 (aq) + CO 2 (g) This first-order decomposition has a half-life of 144 minutes. (a) How long will it take for a sample of acetoacetic acid to be 65% decomposed? b) How many liters of CO 2 (g), measured at 24.5°C and 748 Torr, are produced as a 10.0 g sample of CH 3 COCH 2 COOH decomposes for 575 min? [Ignore the aqueous solubility of CO 2 (g).] _______________________________________________________________________________
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of 6 Page 4 of 6 3) (20 points) Imagine a slab of water ice of surface area 1,203 square meters and uniform thickness of 15.00 cm.
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This note was uploaded on 03/02/2011 for the course CHEM 2080 taught by Professor Davis,f during the Spring '07 term at Cornell University (Engineering School).

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Chem2080 2010 Prelim1 key - Page 1 of 6 Chemistry 2080 -...

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