CHEMISTRY 202 Name ______________________________ Hour Exam III December 1, 2016 Signature ___________________________ Dr. D. DeCoste T.A. _______________________________ This exam contains 23 questions on 12 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 21, 22, and 23.

1-20 (60 pts.) _________

21 (16 pts.) _________

22 (21 pts.) _________

23 (23 pts.) _________

Total (120 pts) _________

Useful Information:

PV = nRT R = 8.314 J/Kmol = 0.08206 Latm/molK

k = Ae–Ea/RT ln(1

2

kk ) =

REa [

1

1T

– 2

1T

]

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 1

1. How many of the following statements are true? I. Substance A is composed of polar molecules and substance B is composed of nonpolar

molecules. We can conclude that substance A has a higher boiling temperature than substance B at the same conditions of atmospheric pressure.

II. N2 is expected to act more like an ideal gas than CO at the same conditions of pressure and temperature.

III. Ionization energies for neutral atoms and positive ions are always positive. IV. The larger the hydrocarbon (a compound consisting solely of carbon and hydrogen), the

higher the vapor pressure.

a) 0 b) 1 c) 2 d) 3 e) 4 2. Which of the following has the lowest boiling point?

a) C6H14 b) H2O c) CH3CH2OH d) CH3OCH3 e) C3H8 3. The skeletal structure for CH3NCO, methyl isocyanate, is

Methyl isocyanate has resonance structures, but one of these minimizes formal charge. For the resonance structure that minimizes formal charge, what is the expected C–N–C bond angle?

a) 90° b) 109° c) 120° d) 180°

4. Consider the following Lewis structure where E is an unknown element:

Which of following could be the identity of element E?

a) O b) S c) Br d) Xe e) Two of a-d could be the identity of E.

5. How many of the following molecules are polar?

SF4 NH3 CH4 H2O SO2 OCl2 OF2 CF4 XeCl4

a) 2 b) 3 c) 5 d) 6 e) 7

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 2

6. Which of the following pairs of molecules/ions can be described with the same shape (molecular structure) although they have different geometries (electron-pair arrangements) around the central atom? a) H2O and SO4

2– b) SO2 and XeCl2 c) SF4 and CF4 d) CO2 and I3

– e) At least two of the above (a-d) have the same shape but different geometries.

7. How many of the following statements help to explain why MgO forms an ionic compound

with Mg2+ and O2– ions?

I. The first ionization energy of Mg is positive, which tells us that Mg+ is not stable as an isolated ion.

II. The second ionization energy for Mg is more endothermic than the first ionization energy. III. The second ionization energy for Mg is exothermic. IV. While the first electron affinity for O is exothermic, the second electron affinity is

endothermic. V. While the first electron affinity for O is endothermic, the second electron affinity is

exothermic.

a) 0 b) 1 c) 2 d) 3 e) 4 8. For which of the following mixtures is ΔHsoln expected to be the most negative?

a) C6H14 and C7H16 b) H2O and CH3OH c) CH3CH2OH and CH3OH d) C7H16 and H2O e) (CH3)2CO and H2O

9. Consider the hypothetical reaction A + 2B → C for which the following data were

collected [A]0 [B]0 Initial Rate (d[C]/dt)

0.10 M 0.20 M 1.8 × 10-4 Ms-1 0.10 M 0.10 M 1.8 × 10-4 Ms-1 0.20 M 0.20 M 7.2 × 10-4 Ms-1

Determine the value for the rate constant, k, is (units of M and s).

a) 9.0 × 10-3 b) 1.8 × 10-2 c) 4.5 × 10-2 d) 9.0 × 10-2 e) 0.45

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 3

10-12. Recall the hydrogen chloride cannon demo from lecture in which a mixture of hydrogen gas and chlorine gas was initiated by a burning magnesium strip to produce hydrogen chloride gas.

10. Given the following bond energies, determine ΔHf° for HCl(g). Bond Bond energy

H–H 432 kJ/mol Cl–Cl 339 kJ/mol H–Cl 427 kJ/mol

a) –344 kJ/mol b) 41.5 kJ/mol c) 83.0 kJ/mol d) –83.0 kJ/mol e) –41.5 kJ/mol

11. The activation energy for the uncatalyzed reaction is 339 kJ/mol HCl formed. Suppose we introduce a catalyst and find that the rate of the reaction increases by a factor of 5.00 × 103 at 25°C. Determine the activation energy of the catalyzed reaction in units of kJ/mol HCl formed. Assume the catalyzed and uncatalyzed reactions have the same form and orders, and the pre-exponential factor, A, is the same for both.

a) 39.8kJ/mol b) 211 kJ/mol c) 318 kJ/mol d) 335 kJ/mol e) 360. kJ/mol

12. How does ΔH for the catalyzed reaction compare to ΔH for the uncatalyzed reaction?

a) ΔHcatalyzed has the same sign and magnitude as ΔHuncatalyzed. b) ΔHcatalyzed has the same sign but a different magnitude as compared to ΔHuncatalyzed. c) ΔHcatalyzed has a different sign but the same magnitude as compared to ΔHuncatalyzed. d) ΔHcatalyzed has a different sign and a different magnitude as compared to ΔHuncatalyzed. e) With the data given, we cannot tell how ΔHcatalyzed and ΔHuncatalyzed compare.

------------------------------------------------------------------------------------------------------------------------------ 13. Consider the following data when studying the reaction aA → Products:

Time (seconds) [A] (M) 0 9.500 20.00 9.000 40.00 8.500 60.00 8.000

Determine [A] at t = 147.3 seconds. a) 5.742 M b) 5.818 M c) 5.963 M d) 6.379 M e) 6.742 M

14. The reaction aA → Products for which [A]0 = 5.00 M is 14.0% complete after 19.0 minutes

and 31.4% complete after 42.7 minutes. Determine the rate law constant, k, for this reaction (using M and minutes).

a) 1.71 × 10-3 b) 7.94 × 10-3 c) 0.0368 d) 0.103 e) 0.226

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 4

15. The reaction NO(g) + O3(g) → NO2(g) + O2(g) is known to be first order with respect to O3(g). An experiment is run at 25°C in which [O3] = 5.00 M and the following data were collected:

Time (seconds) [NO] (M) 0 10.0 × 10–2 20.0 6.67 × 10–2 40.0 5.00 × 10–2 60.0 4.00 × 10–2 80.0 3.33 × 10–2 100.0 2.86 × 10–2 120.0 2.50 × 10–2

Determine the value of the rate constant, k, for the reaction at 25°C (using M and seconds).

a) 5.00 × 10–4 b) 2.50 × 10–3 c) 0.0173 d) 0.0500 e) 0.250

16. The following mechanism is proposed for the decomposition of A2B(g) to A2(g) and B2(g):

1. A2B(g) A2(g) + B(g) 2. A2B(g) + B(g) → A2(g) + B2(g)

Using rate = dt

d ][B2 , use the steady-state approximation to determine which of the

following best represents the rate law constant for the proposed mechanism at extremely high concentrations of A2B.

a) k1 b) k–1 c) k1k2 d) 1

21

−kkk e)

1

1

−kk

17. Consider the following three reactions:

aA → Products rate = k bB → Products rate = k[B] cC → Products rate = k[C]2

Assume you begin each of the reactions at the same time with the same initial concentration ([A]0 = [B]0 = [C]0 = 1.00 M). If the value of the rate constants (k) is the same for each reaction, determine the order from highest to lowest concentration when [C] = 0.70 M. a) [A] > [B] > [C] b) [B] > [C] > [A] c) [A] > [C] > [B] d) [C] > [B] > [A] e) We cannot tell without an actual value for the rate law constant, k.

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 5

18. The reaction 2A + B → C has the following proposed mechanism. Determine the rate law. Note: the rate law constant given in the choices could be a combination of the individual rate law constants of the elementary steps of the mechanism.

Step 1: A + B D (fast equilibrium) Step 2: D + B E (fast equilibrium) Step 3: E + A → C + B (slow)

a) k[A] b) k[A]2[B] c) k[A][B]2 d) k[A][B] e) k[A]2[B]2

--------------------------------------------------------------------------------------------------------------------------- 19-20. Choose the best graph for the plots described below. A graph can be chosen once,

more than once, or not at all.

19. A plot of 1/[A] vs. t for a reaction of the type aA → products which is second-order in A.

[A] 20. A plot of [A] vs. t for a reaction of the type aA → products which is second-order in A.

[B]

a)

e) d)

b) c)

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 6

21. Consider the formation of N2O(g) (skeletal structure = N–N–O) from N2(g) and O2(g).

a. Using all possible resonance structures and formal charge arguments, estimate ΔHf° for N2O(g) given the bond energies below. Show all work and justify/explain your answer. [12 pts.]

Bond Bond energy

O=O 495 kJ/mol N–O 201 kJ/mol N=O 607 kJ/mol N≡O 814 kJ/mol N–N 160 kJ/mol N=N 418 kJ/mol N≡N 941 kJ/mol

ANS: Between ~46 kJ/mol and ~164 kJ/mol, closer to 46 kJ/mol; see text and videos for explanations.

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 7

21. (con’t) b. The actual value of ΔHf° for N2O(g) is 82 kJ/mol. Compare this answer to your

answer from part “a”. How does it support your choice of most favorable resonance structures based on formal charge? Why is the answer (82 kJ/mol) not exactly the same as any one of your calculations in part “a”? Explain. [4 pts.]

ANS: 82 kJ/mol is between 46 kJ/mol and 164 kJ/mol, and is closer to 46.5 kJ/mol, see text and videos for explanations.

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 8

22. We can use the periodic table to predict the formula for magnesium chloride. This allows us to determine the balanced chemical equation for the reaction of magnesium metal and hydrogen chloride gas to produce magnesium chloride and hydrogen gas. How can we prove this formula? Thermodynamics of course! How, for example, can we differentiate among the following choices?

I. The ions in magnesium chloride are Mg+ and Cl–. II. The ions in magnesium chloride are Mg2+ and Cl2–. II. The ions in magnesium chloride are Mg2+ and Cl–.

a. Given the following data, determine ΔH for each of the three possible reactions between

magnesium metal and hydrogen chloride gas as described above. Report your answer in units of kJ/mol of magnesium chloride formed. Show all work. [12 pts.]

Enthalpy of sublimation of Mg(s) (kJ/mol): 150. Successive ionization energies of Mg(g) (kJ/mol): 735, 1445 Successive electron affinities of Cl(g) (kJ/mol): –349, 1232 Bond energy of HCl(g) (kJ/mol): 427 Bond energy of H2(g) (kJ/mol): 432 Lattice energy for MgCl (kJ/mol): –918 (Mg+ and Cl–); –3652 (Mg2+ and Cl2–) Lattice energy for MgCl2 (kJ/mol): –2493

I. –171 kJ/mol

II. –228 kJ/mol

III. –439 kJ/mol

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 9

22. (con’t) b. Estimate the signs of ΔS for each of the three reactions and justify/explain your

answers. [6 pts.]

ΔS for each of the three reactions = negative; see text and videos for explanations. c. Given your answers to parts “a” and “b” above, explain how your answers show that

the ion charges as predicted by placement on the periodic table are the most thermodynamically preferred. [3 pts.]

See text and videos for explanations.

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 10

23. On the Friday lecture before break we looked at the so-called “Magic Genie” demonstration, in which hydrogen peroxide decomposes to water and oxygen gas with the aid of a catalyst. In this demonstration we used 100.0 mL of 30.00% by mass aqueous H2O2, which has a concentration of 9.790 M. The balanced chemical equation for the reaction is:

2H2O2(aq) → 2H2O(l) + O2(g) rate = dt

d ]OH[- 22

a. When we run the demonstration with the catalyst we could monitor the reaction by

collecting the oxygen gas. Suppose we collect the oxygen gas in a 10.00-L container at 25°C and obtain the following data:

Time (seconds) Pressure O2 (atm) 0 0 2.00 0.4276 4.00 0.7025 6.00 0.8792 8.00 0.9927 10.00 1.066

Determine the differential rate law for the catalyzed reaction and value of k (include units, in terms of M H2O2 and seconds). Show all work/explain your answer. [Note: more space is provided on the next page if needed.] [9 pts.]

---------------------------------------------------------------------------------------------------------------------

ANS: rate = k[H2O2]; k = 0.221 s–1.

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 11

23. (con’t)

a. Use the space below to continue working on part “a” if needed.

b. We carry out the demonstration again with 100.0 mL of the 9.790 M H2O2(aq) and collect the oxygen gas in a 10.00-L container, but this time we carry out the reaction at 15°C. After 10.00 seconds the pressure of oxygen is noted to be 0.8187 atm. Determine the activation energy, Ea, of the catalyzed reaction in units of kJ/mol. Assume the frequency factor, A, to be constant. Show all work/explain your answer. [6 pts.]

Ea = 41.8 kJ/mol

CHEMISTRY 202 Fall 2016 Hour Exam III Page No. 12

23. (con’t) c. We carry out the reaction at 25°C again, but this time uncatalyzed. Again, we start

with 100.0 mL of 30.00% by mass solution (9.790 M H2O2). Because the reaction is so much slower, we can easily obtain [H2O2] vs. time data. The data obtained are:

Time (days) [H2O2] (M) 50.00 7.662 150.00 5.340 250.00 4.098 350.00 3.325 450.00 2.797 550.00 2.414

Determine the differential rate law for the uncatalyzed reaction and value of k (include units, in terms of M H2O2 and days). Show all work/explain your answer. [4 pts.]

rate = k[H2O2]2 k = 5.67 × 10–4 M–1day–1.

d. Propose a two-step mechanism for the uncatalyzed decomposition of hydrogen

peroxide. Use Lewis structures for all species in the reaction and explain how it matches your rate law as determined in part “c”. [4 pts.]

Answers can vary; see text and videos for explanations.