Unformatted text preview: Chemistry 6B: General Chemistry C. Hoeger University of California, San Diego Winter 2005 6B Midterm 1 PRACTICE 120 pts
This is an ACTUAL midterm given in Winter of 2005; Student had 80 minutes to complete it. Pretend it is your real exam and take it as if it were one! This will be the best practice you can have. Please note: NO answers for this will be provided. VERSION: A Page 2 of 4 Part 1. Calculation/Short Answer Problems (Values as shown). All work for the following must be done in THIS TEST BOOKLET. Answers with no work will receive zero points. Your work must be clear and logical AND SHOWN to receive maximum (OR ANY) credit. A) (25 pts) Consider a 1.00 mol sample of acetylene (C2H2) is allowed to undergo the following two step, reversible process: the gas is a) first heated at constant volume until its pressure increases from 2.5 atm to 3.2 atm and then b) cooled at constant pressure until its volume changes from 4.0 L to 2.2 L. Given that for acetylene that CP,m = 43.93 Jmol1 K1 determine the following: i) the work for step a) of the process (in kJ) ii) the work for step b) of the process (in kJ) iii) the heat associated with step a) of the process (in kJ) iv) the heat associated with step b) of the process (in kJ) v) the change in internal energy (U) for the entire process (in kJ) B) (15 pts) Acetylene can also undergo what is known as a "Diels-Alder" Reaction: * * CH CH * HC * CH + CH2 CH CH CH2 CH2 CH2 CH CH
(for the sake of clarity asterisks have been used to mark the original two carbon atoms of acetylene). Using bond enthalpies, predict Hrxn for this reaction. (BOND ENTHALPIES (kJ/mol): CH: 412; CC: 348; C=C 612; CC 837) C) (15 pts) Calculate S for the following transition: acetone (1.5 mol, liq, 27 C) acetone (1.5 mol, gas, 27 C) Some pertinent information: CP,m = 124.7 Jmol1 K1 (liquid); CP,m = 85.3 Jmol1 K1 (gas); Hvap = 29.1 kJmol1; Tb = 56.4 C; MMacetone = 58 g/mol Part 2. Multiple Choice Problems (5 Points each). Select the best answer from the choices given and mark it on your scantron! DON'T SPEND TOO MUCH TIME ON THESE...THEY ARE NOT WORTH THAT MUCH! 1. Use Trouton's constant to estimate the enthalpy of condensation of diethyl ether, which boils at 309 K. A) -26 kJmol-1 D) -85 kJmol-1 B) +26 kJmol-1 E) +85 kJmol-1 -1 C) -275 kJmol 2. The molar entropy of silver at 298 K is equal to the area under the curve obtained by plotting (from T = 0 to T = 298 K) A) Cp versus T. D) Cp/T versus T. B) lnCp versus T. E) Cp versus 1/T. C) ln(Cp/T) versus T. VERSION: A 3. Calculate Ssurr at 298 K for the reaction H2(g) + F2(g) 2HF(g) H r = -546 kJmol-1, Sr = +14.1 JK-1mol-1 A) +14.1 JK-1mol-1 B) +1820 JK-1mol-1 C) +1830 JK-1mol-1 D) -1830 JK-1mol-1 E) -14.1 JK-1mol-1 Page 3 of 4 4. Estimate the minimum temperature at which magnetite can be reduced to iron by graphite. Fe3O4(s) + 2C(s) 2CO2(g) + 3Fe(s) Sr = +351.44 JK-1mol-1 The standard molar enthalpies of formation of magnetite and CO2(g) are -1118.4 and -393.51 kJmol-1, respectively. A) 670C B) 787C C) 943C D) 1790C E) Magnetite cannot be reduced by carbon at any temperature. 5. Which of the following statements about spontaneous processes at constant temperature and pressure is not true? a. b. c. d. e. A spontaneous process having negative H and negative S is enthalpy driven. A spontaneous process having positive H and positive S is entropy driven. A process having positive H and negative S cannot occur spontaneously at any temperature. A process having negative H and positive S can occur spontaneously at any temperature. All of these statements are true. 6. For the reaction, X + Y A + B, Gr is 1324 kJ mol1. Which one of the following statements is not valid concerning this reaction? a. b. c. d. e. The reaction is thermodynamically favorable. The reaction is spontaneous as written. The products are more stable than the reactants. The reaction will proceed rapidly from left to right. All of these are valid. 7. In which of the following phase changes should the difference between H and E be larger? a. b. c. d. e. fusion vaporization The difference should be the same for both processes. There is no difference between H and E for these processes. This cannot be answered in a general way but must be considered on a case-by-case basis. 8. Given the following thermochemical equations, calculate the standard enthalpy of formation for propane, C3H8(g). C(s) + O2(g) CO2(g) 1 H2(g) + 2 O2(g) H2O(l) C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) a. 3131 kJ mol1 d. 1773 kJ mol1 b. 129 kJ mol1 e. none of these Hr = 393 kJ mol1 Hr = 286 kJ mol1 Hr = 2452 kJ mol1 c. 4775 kJ mol1 VERSION: A 9. Calculate the lattice enthalpy of barium oxide from the following data. (note: all signs are correct as shown) Hf Ba(g): 180 kJmol1 Hf O(g): 249 kJmol1 Hf BaO(s): 554 kJmol1 a) 2310 kJmol1 b) 1466 kJmol1 c) 2046 kJmol1 d) 3403 kJmol1 e) 3154 kJmol1 IE1 Ba(g): 502 kJmol1 IE2 Ba(g): 966 kJmol1 EA1 O(g): 141 kJmol1 EA2 O(g): 844 kJmol1 Page 4 of 4 10. Estimate the normal boiling point of Br2(l), given the molar enthalpy and molar entropy of vaporization of 30.7 kJmol1 and 93.0 JK1mol1, respectively. a) 303C b) 57C c) 30C d) 330C e) 100C 11. For the following reaction CuO(s) + H2(g) Cu(s) + H2O(g) at 298 K, Hr = 87.0 kJ and Sr = 47.0 JK1. Calculate Gr at 400 K. a) 18.9 kJ b) +106 kJ c) 106 kJ d) +18.9 kJ e) 68.2 kJ ...
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