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1 Chemistry 05 B Practice Exam Dr. Jessica Parr First Letter of last Name PLEASE PRINT YOUR NAME IN BLOCK LETTERS Name: Last 4 Digits of USC ID: Lab TA s Name: Question Points Score Grader Total 00 Please Sign Below: I certify that I have observed all the rules of Academic Integrity while taking this examination. Signature: Instructions:. You must show work to receive credit. 2. If necessary, please continue your solutions on the back of the preceding page (facing you). 3. YOU MUST use black or blue ink. (No pencil, no whiteout, no erasable ink.) 4. There are 9 problems on pages. Please count them before you begin. A periodic table and some useful equations can be found on the last page. 5. Good luck!! =)

2 . (8 pt) Circle the correct answers for the following questions. i. The catalyzed pathway in a reaction mechanism has a activation energy that causes a reaction rate. a) higher, lower b) higher, higher c) lower, higher d) lower, steady e) higher, steady ii. Which of the following statements best describes the condition(s) needed for a successful formation of a product according to the collision model. a) The collision must involve a sufficient amount of energy, provided by the motion of the particles, to overcome the activation energy. b) The relative orientation of the particles has little or no effect on the formation of the product. c) The relative orientation of the particles has an effect only if the kinetic energy of the particles is below some minimum value. d) The relative orientation of the particles must allow for formation of the new bonds in the product. e) The energy of the incoming particles must be above a certain minimum value and the relative orientation of the particles must allow for formation of new bonds in the product. iii. For a particular system at a particular temperature there equilibrium constant(s) and there equilibrium position(s). a) are infinite, is one b) is one, are infinite c) is one, is one d) are infinite, is one e) none of these iv. Which of the following is true for a system with a relatively small equilibrium constant? a) It will take a short time to reach equilibrium b) The equilibrium lies to the left c) It will take a long time to reach equilibrium d) The equilibrium lies to the right e) Two of these 2

3 v. Equilibrium is reached in chemical reactions when: a) the rates of the forward and reverse reactions become equal. b) the concentrations of reactants and products become equal. c) the temperature shows a sharp rise. d) all chemical reaction stop. e) the forward reaction stops. vi. Given the reaction A (g) + B (g) C (g) + D (g). You have the gases A, B, C, and D at equilibrium. Upon adding more of gas A, the value of K: a) increase because by adding A, more products are made, increasing the product to reactant ratio. b) decreases because A is a reactant so the product to reactant ratio decreases. c) does not change because A does not figure into the product to reactant ratio. d) does not change as long as the temperature remains constant. e) depends on whether the reaction is endothermic or exothermic. 3

4 2. (4 pt) Answer the following questions using the following data for the reaction: Time (s) [HBr] (mol/l) HBr (g) + O 2 (g) 2 H 2 O (g) + 2 Br 2 (g) (a) What is the half-life for this reaction? (b) (c) What is the concentration of H 2 O after 30 s? What is the concentration of Br 2 after 80 s? (d) If the initial concentration of O 2 is.0 M, what is the concentration of O 2 after 40 s? (e) The rate of disappearance of HBr is times the rate of disappearance of O 2. (f) The rate of appearance of H 2 O is times the rate of disappearance of O 2. (g) The rate of appearance of Br 2 is times the rate of disappearance of O 2. 4

5 3. (9 pt) Consider the following data concerning the equation: H 2 O I H + I H 2 O Experiment [H 2 O 2 ] [I - ] [H + ] rate 0.00 M 5.00 x 0-4 M.00 x 0-2 M 0.37 M/s M.00 x 0-3 M.00 x 0-2 M M/s M.00 x 0-3 M.00 x 0-2 M M/s M.00 x 0-3 M 2.00 x 0-2 M.084 M/s i. The rate law for the reaction is: a) rate = k[h 2 O 2 ][I - ][H + ] b) rate = k[h 2 O 2 ] 2 [I - ] 2 [H + ] 2 c) rate = k[i - ][H + ] d) rate = k[h 2 O 2 ][H + ] e) rate = k[h 2 O 2 ][I - ] ii. The average value of the rate constant k (without units) is a) 270 b) 2.74 x 0 4 c) 37 d) 08 e) none of these iii. Two mechanisms are proposed: I. H 2 O 2 + I - H 2 O + OI - II. H 2 O 2 + I - +H + H 2 O + HOI OI - + H + HOI HOI + I - + H + I 2 + H 2 O HOI + I - + H + I 2 + H 2 O I 2 + I - - I 3 I 2 + I - - I 3 Which of the following describes a potentially correct mechanism? a) Mechanism I with the first step the rate determining step. b) Mechanism I with the second step the rate determining step. c) Mechanism II with the first step the rate determining step. d) Mechanism II with the second step the rate determining step. e) None of the above is correct. 5

6 4. (0 pt) For a reaction aa products, [A] o = 6.0 M, and the first two half-lives are 56 and 28 minutes, respectively. a. Determine k, with appropriate units. b. Calculate [A] at t = 99 minutes. 6

7 5. (9 pt) A first order reaction with an initial concentration of.0 M is found to have a half-life of.26 s at 57.0 o C, and a half-life of s at 99 o C. What is the activation energy for this reaction? 7

8 6. (7 pt) Consider the reaction: 4 PH 3 (g) P 4 (g) + 6 H 2 (g). If, in a certain experiment, over a specific time period, mol PH 3 is consumed in a 2.0 L container each second of reaction, what are the rates of production of P 4 and H 2 in this experiment? 7. (2 pt) For the reaction: H 2 (g) + Br 2 (g) 2 HBr (g) K p = 3.5 x 0 4 at 495 K. What is the value of K p for the following reactions at 495 K? a. HBr (g) ½ H 2 (g) + ½ Br 2 (g) b. 2 HBr (g) H 2 (g) + Br 2 (g) c. 2 H 2 (g) + 2 Br 2 (g) 4 HBr (g) 8

9 8. (0 pt) The following equilibrium pressures were observed at a certain temperature for the reaction: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) : P NH3 = 0.03 atm, P N2 = 0.85 atm, P H2 = atm. a. What is the value of K p for this reaction? b. If P N2 = atm, P NH3 = atm, and P H2 = atm, does this represent a system at equilibrium? 9

10 9. ( pt) At 327 o C, the equilibrium concentrations are [CH 3 OH] = 0.5 M, [CO] = 0.24 M, and [H 2 ] =. M for the reaction: CH 3 OH (g) CO (g) + 2 H 2 (g). Calculate K p at this temperature. 0

11 H He Li Be B C N O F Ne 20.8 Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc (99) 44 Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po (209) 85 At (20) 86 Rn (222) 87 Fr (223) 88 Ra Ac Rf (26) 05 Db (262) 06 Sg (263) 07 Bh (262) 08 Hs (265) 09 Mt (268) Lanthanides 58 Ce Pr Nd Pm (45) 62 Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 75 Actinides 90 Th Pa U Np Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (25) 99 Es (252) 00 Fm (257) 0 Md (258) 02 No (259) 03 Lr (26) R = L*atm/K*mol = 8.34 J/K*mol PV = nrt k = A exp(-e a /RT) = 2 2 ln T T R E k k a ] 0 [ ] [ A kt A + = 0 ] ln[ ] ln[ A kt A + = ] 0 [ ] [ A kt A + =

02/05/09 Last 4 Digits of USC ID: Dr. Jessica Parr

Chemistry 05 B First Letter of PLEASE PRINT YOUR NAME IN BLOCK LETTERS Exam last Name Name: 02/05/09 Last 4 Digits of USC ID: Dr. Jessica Parr Lab TA s Name: Question Points Score Grader 2 2 9 3 9 4 2