final-2a-2008-practice-key2

# final-2a-2008-practice-key2 - Your name(Last(First Your...

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Unformatted text preview: Your name (Last)______________ (First)__________ Your section _______Your TA______________ Date ____________ Student ID ______________ Signature ______________ Practice Final Friday, March 21, 2008 6:00 PM – 8:00 PM CHEM 2A, Winter 2008 Professor Guo This is a closed book exam. The only thing you may consult with is a pocket calculator or similarly functional math tool. Problem points 1 /10 (Mid II) 2 /20 (Mid I) 3 /20 (Mid I) 4 /20 (Mid II) 5 /10 (Mid II) 6 /40 7 /60 8 /60 Total /240 Please complete as much as possible because these are all partial credit problems Question 1 (10 points) 2 L of oxygen at 1.5 atm is mixed with a 0.5-L chamber filled with hydrogen at 2.0 atm at 0 °C. What is the total pressure right before the reaction? [5 points] 2 L/0.5 L = 4, so Pressure of O 2 is 4 x1.5 atm = 6 atm. Ptotal = 6 + 2 = 8 atm Calculate the energy released from this reaction, in kJ. (The bond energy for O-H is 464 kJ/mol, O=O is 498 kJ/mol, and H-H is 436 kJ/mol.) [5 points] nO 2 = 0.134 mol nH 2 = 0.0446 mol The energy released per 2 mol of H 2 O is 4x464 kJ/mol – 498 – 2x 436 = 486 kJ/mol So per mol of H 2 O the amount of energy released is 243 kJ/mol And 0.0446 x 243 = 10.83 kJ Question 2 (20 points) CrO 4 2- and S 2 O 4 2- undergo a redox reaction in acidic conditions to form Cr(OH) 3 (s) and SO 3 2- . Assign an oxidation state to each element in these four compounds. Balance the overall reaction using half reactions (reduction and oxidation). Clearly show each step of how you balance the equations. [10 points] Cr 6+ O 2- to Cr 3 + and O 2- S +3 O 2- to S +4 O 2- CrO 4 2- > Cr(OH) 3 (s) S 2 O 4 2- > SO 3 2- CrO 4 2- + 5H + > Cr(OH) 3 (s) + H2O S 2 O 4 2- + 2H2O> 2SO 3 2- +4H + CrO 4 2-...
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