2005/06 Spring CHE 107 108 total points
+ 100 points
Exam 4
Name: ____________________________ Section: 5701 Date: 4/20/2006
Directions: Answer the following questions completely. For multiple choice questions, circle the correct answer(s). For calculations, be sure to show all your work (including units) for full credit. If you show no work, you will receive zero points. The symbols to the left of each number indicate the chapter-concept area-letter that the question is testing over; concepts you'll have to remember from CHE 105 have not been listed. The last page of this exam has a periodic table and other possibly useful information that you may rip off to use during the exam.
15-III-d,e 15-IV-a,b 15-V-a,b,c
1. A student collected some data for the reaction shown below. The student observed the reaction for two minutes and measured the concentration of compound [Q] in M. The student then graphed the data to determine the order of the reaction. Q 3R+P a. (1 point) Look at the student's graphs below and determine the order of the reaction. The order of the reaction is:
What order is m y reaction? 0 -1 0 ln [Q] -2 -3 -4 -5 -6 time (m in) 0.025 0.5 1 1.5 2 2.5 [Q] (M) 0.02 0.015 0.01 0.005 0 0 0.5 1 1.5 2 2.5 tim e (min) y = -0.4141x - 3.9628 R2 = 0.9812 y = -0.0055x + 0.0187 R2 = 0.9359 What order is m y reaction?
b. (2 points) What is the value of the rate constant for this reaction with proper units?
1/[Q] (M1)
What order is m y reaction? 150 100 50 0 y = 32.644x + 50.345 R2 = 0.9996 0 0.5 1 1.5 2 2.5
c. (1 point) Now, write the rate law for this reaction. Be sure to include the value of the rate constant you just found.
tim e (m in)
d. (3 points) What is the half-life for this reaction?
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15-III-a,c
2. (5 points) Multiple choice: Which statement concerning relative rates of reaction is correct for the decomposition of phosphine? 4 PH3(g) P4(g) + 6 H2(g) a. The rate of disappearance of PH3 is 6/4 the rate of appearance of H2. b. The rate of appearance of P4 is equal to the rate of appearance of H2. c. The rate of disappearance of PH3 is 1/4 the rate of appearance of P4. d. The rate of appearance of H2 is 6 times the rate of appearance of P4. e. The rate of disappearance of PH3 is 4 times the rate of appearance of H2. 3. (1 point) What is the name given to a substance that increases the rate of a chemical reaction but is not itself consumed? 4. (7 points) Given the initial rate data for the reaction, A + B C, determine the rate expression (including the value of the rate constant) for the reaction. [A] (M) [B] (M) rate (M/s) 0.334 0.134 4.11109 0.334 0.187 8.00109 0.668 0.134 4.11109
15-II-a,e 15-VII-d
15-III-a,c,d,e 15-IV-a
So, the rate law is: rate =
15-III-d,e
5. (3 points) For the reaction below, write a rate law such that it is second-order with respect to NO and third order overall. 2 NO(g) + 2 H2(g) N2(g) + 2 H2O(g)
15-II-a,c,e 15-VII-d
6. (3 points) Multiple choice: For the reaction in the previous question, which of the factor(s) listed below will affect the value of the rate constant for this reaction? Circle all that apply. a. increasing the concentration of hydrogen b. adding a catalyst c. increasing the temperature 7. (5 points) The decomposition of phosphine, PH3, follows first-order kinetics. The half-life for the reaction at 550C is 81.3 seconds. How many seconds does it take for 89.0% of a phosphine sample to decompose? Note, reaction at top of page.
15-III-e 15-V-c
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15-III-a,c
8. Ozone decomposes to oxygen according to the balanced chemical equation below. 2 O3(g) 3 O2(g) a. (2 points) Write the rate of reaction in terms of reactant and product concentrations. rate = b. (4 points) Now, If the rate of disappearance of ozone is 7.2104 M/s, what is the rate of formation of oxygen?
15-VII-a,b,c
9. Nitrogen monoxide reacts with chlorine to produce NOCl. 2 NO(g) + Cl2(g) 2 NOCl(g) A proposed mechanism for this reaction is step 1: 2 NO(g) N2O2(g) step 2: N2O2(g) + Cl2(g) 2 NOCl(g) a. (1 point) Circle the reaction intermediate(s) in the above reactions. b. (1 point) Which step is rate-determining? c. (3 points) Write the rate equation for each elementary step. Step 1: Step 2: d. (1.5 points) What is the molecularity of each step? Step 1: Step 2:
fast, equilibrium slow
e. (3 points) Is this mechanism chemically reasonable? Briefly explain why or why not.
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15-VI-a,b
f. points) (2 What is the rate law for this reaction? 10. (5 points) Calculate the activation energy, Ea, for the reaction below given k (at 25.0 C) is 3.46105 s1 and k (at 45.0 C) is 5.79104 s1. N2O5(g) 2 NO2(g) + O2(g)
20-II-a,b
11. Balance the following equation (all states are aqueous except where noted otherwise) by answering the following questions: SnSO4 + K2Cr2O7 + H2SO4 Sn(SO4)2 + K2SO4 + Cr2(SO4)3 + H2O() a. (2 points) What is the unbalanced complete ionic equation?
b. (1 point) Cross out the spectator ions in your answer to part "a" so that it is now an unbalanced net ionic equation. c. (1.5 points) Note, no partial credit on this part! In the equation, what... is oxidized is reduced gains electrons loses electrons is the oxidizer is the reducer
d. (5 points) Write the half equations that describe this reaction; be sure to label whether the half reaction is the oxidation or the reduction half reaction! Also, be sure to completely balance them so you can do part "e"!
e. (2 points) Now, add your half reaction back together and add back in the spectators appropriately so that you can finally add in the proper stoichiometric coefficients to balance the equation! Simply write the appropriate numbers below; please write the number one if needed.
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____ SnSO4 + ____ K2Cr2O7 + ____ H2SO4 ____ Sn(SO4)2 + ____ K2SO4 + ____ Cr2(SO4)3 + ____ H2O() 12. (5 points) Multiple choice: Of the choices below, look at the chart of standard reduction potentials and determine which one contains only elements or ions that will oxidize Sn(s)? a. Cu(s) and Al3+(aq) c. F2(g) and Cu2+ e. Al3+(aq) and Na+(aq) b. F (aq) and Cu(s) d. Al(s) and Na(s) f. none of these 13. (5 points) Use the chart of standard reduction potentials to write a spontaneous reaction between cadmium and tin. Show the two half reactions and then the overall reaction. Finally, calculate its standard cell potential.
20-II-a 20-III-e
20-II-b 20-III-a,d,e
20-III-a,d 20-IV-a
14. (5 points) Calculate the cell potential, at 25C, based upon the overall reaction below if [Cu2+] = 0.75 M and [Al3+] = 0.0010 M. 3 Cu2+(aq) + 2 Al(s) 3 Cu(s) + 2 Al3+(aq)
20-II-a,c
15. (5 points) All of the following statements concerning voltaic cells are true except: a. Reduction occurs at the cathode. b. The anode is the positively charged electrode. c. They can be used as a source of energy to power things. d. Electrons flow from the anode to the cathode to complete a circuit. e. The two half-cells are somehow connected to remain charge neutral. 16. (6 points) Answer the following questions given the following standard reduction potentials. Fe2+(aq) + 2 e Fe(s) E = 0.440 V...