CHEMISTRY 202 Practice Hour Exam III Fall 2016 Dr. D. DeCoste Name Signature T.A. This exam contains 23 questions on 10 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 (15 pts.) 22 (20 pts.) 23 (25 pts.) Total (120 pts) Useful Information: R = 8.314 J/Kmol = 0.08206 Latm/molK k = Ae -Ea/RT k 2 E ln( ) = a 1 [ k1 R T 1 1 T ] 2
Practice Hour Exam III Page No. 1 1. Which of the following best completes the following sentence concerning trends on the periodic table? While there can be exceptions, in general a) smaller atoms have larger ionization energies, and smaller electronegativity values. b) smaller atoms have smaller ionization energies, and smaller electronegativity values. c) smaller atoms have smaller ionization energies, and larger electronegativity values. d) smaller atoms have larger ionization energies, and larger electronegativity values. e) There are no general trends among atomic radius, ionization energy, and electronegativity. 2. Given that the first two ionization energy values for Rb(g) = 403 kj/mol and 2633 kj/mol, respectively, determine the electron affinity of Rb(g). a) 403 kj/mol b) 2230 kj/mol c) 2633 kj/mol d) 3036 kj/mol e) The electron affinity of Rb(g) cannot be determined with these data. 3. Consider a compound formed from a Group 2 element (such as Be or Mg and designated as M below) and a Group 6 element (such as O or S and designated as X below). Which of the following is the most reasonable statement? a) An ionic compound with the ions M 2+ and X 2- is the only stable form of the compound. b) An ionic compound with the ions M + and X - could be thermodynamically favorable, but an ionic compound with the ions M 2+ and X 2- is more thermodynamically favorable. c) The compound between M and X will be in a 1:1 ratio but it is best to think of the bond as polar covalent since atoms do not want to lose electrons (thermodynamically speaking). d) The bond between M and X is polar covalent as explained in c above, but we cannot determine the molecular formula without a molar mass. e) An ionic compound will form but we cannot predict the charges with any degree of accuracy. 4. Consider the following reaction: C 2 H 4 (g) + X 2 (g) CH 2 XCH 2 X(g) ΔH = 549 kj Estimate the C X bond energy given that the C H bond energy is 413 kj/mol, the C C bond energy is 347 kj/mol, the C=C bond energy is 614 kj/mol, and the X X bond energy is 154 kj/mol. Note: you will need to draw Lewis structures for the reactants and products to determine the possible presence of multiple bonds. a) 198 kj b) 352 kj c) 485 kj d) 704 kj e) 970 kj
Practice Hour Exam III Page No. 2 5. Consider the following molecules: SF 4 ICl 5 XeF 4 NF 3 For how many of the molecules above are the geometry (electron-pair arrangement) and shape (molecular structure) the same? a) 0 b) 1 c) 2 d) 3 e) 4 6. How many of the following statements is/are false? I. When the difference in electronegativity between two atoms is very large, the bond most likely to form is an ionic bond. II. Covalent bonding results from the sharing of valence electrons between two atoms. III. The valence electrons in a polar bond are found nearer (on the average) to the more electronegative atom in the bond. IV. VSEPR theory states that the central atom in a molecule has the bonded atoms and lone pairs arranged so to minimize electron-electron repulsions. V. If a molecule has polar bonds it is a polar molecule. VI. It is possible for a molecule with polar bonds to have no overall dipole moment. a) 0 b) 1 c) 2 d) 3 e) 4 7. For which of the following mixtures is ΔH soln expected to be the most positive? a) C 6 H 14 and C 7 H 16 b) H 2 O and CH 3 CH 2 OH c) (CH 3 ) 2 CO and H 2 O d) CH 3 CH 2 OH and CH 3 OH e) C 7 H 16 and H 2 O 8. When table salt (solid NaCl) is dissolved in water, the overall process is slightly endothermic (the change in enthalpy is 4 kj/mol). The lattice energy for NaCl is 787 kj/mol. Determine the enthalpy of hydration. a) 783 kj/mol b) 791 kj/mol c) 783 kj/mol d) 791 kj/mol e) 4 9. Consider two pure gaseous substances A and B each made of molecules of approximately the same size. Substance A consists of molecules which are more polar than those of substance B. How many of the following statements is/are true? I. Substance A has a higher vapor pressure than substance B. II. Substance A has a higher boiling point than substance B. III. Substance A is a more ideal gas than substance B IV. The bonds in molecule A must be more polar than the bonds in molecule B. a) 0 b) 1 c) 2 d) 3 e) 4
Practice Hour Exam III Page No. 3 10. Choose the correct molecular geometry, shape, and polarity for sulfur tetrafluoride (sulfur is the central atom) GEOMETRY SHAPE POLARITY a) trigonal bipyramid see-saw polar b) tetrahedral tetrahedral polar c) tetrahedral tetrahedral non-polar d) octahedral square planar non-polar e) trigonal bipyramid trigonal pyramid polar 11. For a reaction: aa Products, [A] 0 = 4.00 M, and the first two half-lives are 48 and 24 minutes, respectively. Calculate [A] at t= 81 minutes. a) 0.48 M b) 0.62 M c) 1.24 M d) 1.49 M e) 2.81 M --------------------------------------------------------------------------------------------------------------------- 12-13. Consider the following data concerning the equation: H 2 O 2 (aq) + 3I (aq) + 2H + (aq) I 3 (aq) + 2H 2 O [H 2 O 2 ] [I ] [H + ] rate I 0.100 M 5.00 x 10-4 M 1.00 x 10-2 M 0.137 M/sec II. 0.100 M 1.00 x 10-3 M 1.00 x 10-2 M 0.268 M/sec III. 0.200 M 1.00 x 10-3 M 1.00 x 10-2 M 0.542 M/sec IV. 0.400 M 1.00 x 10-3 M 2.00 x 10-2 M 1.084 M/sec 12. The average value for the rate constant (units with M and sec) is a) 108 b) 137 c) 2.71 x 10 3 d) 2.74 x 10 4 e) 3.14 x 10 7 13. 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 rate determining. d) Mechanism II with the second step rate determining. e) Neither mechanism supports the correct rate law. ----------------------------------------------------------------------------------------------------------------
Practice Hour Exam III Page No. 4 14. How many of the following are true concerning catalysts? I. The catalyzed reaction has a different rate constant from the uncatalyzed reaction. II. The catalyzed reaction has a different value of ΔH rxn from the uncatalyzed reaction. III. The catalyzed reaction has a different equilibrium constant from the uncatalyzed reaction. IV. The catalyzed reaction has a different activation energy from the uncatalyzed reaction. a) 0 b) 1 c) 2 d) 3 e) 4 ---------------------------------------------------------------------------------------------------------------- 15-16. Consider the following reaction: The following mechanism is proposed: 2NO(g) + H 2 (g) N 2 O(g) + H 2 O(g) 1. 2NO(g) N 2 O 2 (g) 2. N 2 O 2 (g) + H 2 (g) N 2 O(g) + H 2 O(g) Answer the following questions. Note: the k terms in the rate laws takes into account the rate law constants for the elementary steps and are not necessarily equivalent in the choices below. 15. Assuming the first step is a fast equilibrium step and the second step is rate-determining, which of the following best represents the rate law? a) rate = k[h 2 ] b) rate = k[no][h 2 ] c) rate = k[no][h 2 ] 2 d) rate = k[no] 2 [H 2 ] e) rate = k[no] 2 [H 2 ] 2 d [H ] 16. Using rate = 2, use the steady-state approximation to determine the rate law for the dt proposed mechanism and choose the correct statement below. a) If [H 2 ] is very low, the rate law appears to be the same as the rate law you determined in #23. If [H 2 ] is very high, the rate law appears to be rate = k[no] 2. b) If [H 2 ] is very high, the rate law appears to be the same as the rate law you determined in #23. If [H 2 ] is very low, the rate law appears to be rate = k[no] 2. c) If [NO] is very low, the rate law appears to be the same as the rate law you determined in #23. If [NO] is very high, the rate law appears to be rate = k[h 2 ]. d) If [NO] is very high, the rate law appears to be the same as the rate law you determined in #23. If [NO] is very low, the rate law appears to be rate = k[h 2 ]. e) The rate law appears the same as what you determined in #23 regardless of [NO] or [H 2 ]. ------------------------------------------------------------------------------------------------------------------------
Practice Hour Exam III Page No. 5 17. 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: H 2 (g) + Cl 2 (g) 2HCl(g) Suppose the activation energy for the reaction is changed from 239 kj/mol to 155.0 kj/mol at 298K by the introduction of a catalyst. Calculate the ratio of rate(catalyzed):rate(uncatalyzed). Assume the pre-exponential factor, A, is the same for the catalyzed and uncatalyzed reactions. a) 0.967 b) 1.03 c) 1.54 d) 2.30 x 10 7 e) 5.30 x 10 14 18. Recall the activated complex demonstration performed in lecture (the one which reacted to form a green transition state, and then turned pink to reveal the original catalyst). Suppose we run two trials, one at 55 C and one at 65 C. The times for the reaction are 85 seconds and 41 seconds, respectively. Determine the activation energy for this reaction. Assume the frequency factor, A, to be constant. a) 261 J/mol b) 663 J/mol c) 48.5 kj/mol d) 67.2 kj/mol e) 98.3 kj/mol --------------------------------------------------------------------------------------------------------------------------- 19-20. Choose the correct graph for the plots described below. A graph can be chosen once, more than once, or not at all. a) b) c) d) e) 19. A plot of [A] vs. time for reaction type aa products which is zero-order in A. 20. A plot of t ½ vs. [A] for reaction type aa products which is first-order in A.
Practice Hour Exam III Page No. 6 21. We discussed in lecture how compounds can have the same chemical formula but different structures, and therefore different properties. Four different compounds exist with the chemical formula C 3 H 9 N. One such compound is isopropyl amine which is used in chemical weapons. Isopropyl amine has the following Lewis structure: a. Draw the three other distinct Lewis structures with the chemical formula C 3 H 9 N. For each of these structures you should minimize formal charge. Can you choose a best Lewis structure based on formal charge? If so, rank the structures. If not, explain why not. [6 pts.] b. Of the three compounds in part a, one is a gas at room conditions and the other two are liquids. Label which is the gas and which are liquids, and rank the two liquids in terms of boiling point (which boils at the higher temperature?). Support your answer with a discussion of VSEPR theory, explain the nature of any relevant intermolecular force, and explain the relationship between intermolecular forces and boiling points. [9 pts.]
Practice Hour Exam III Page No. 7 22. In the future, you are visiting friends at their 10 th College Reunion (not UIUC). They are looking through an old chemistry notebook when they come across the following chemical equation: H 2 (g) + Cl 2 (g) 2HCl You all notice that there is no phase for HCl. I think it is a solid, states one of your friends. It is made up of H + and Cl ions. Another friend replies, I think it is a gas. The bond between hydrogen and chlorine is covalent not ionic. They are both ready to call off their lifelong friendship when you smile, take your Chemistry 202 textbook from your backpack, and tell them that thermodynamics can solve this problem. So, which friend is correct?; that is, which is the correct chemical equation for the reaction of hydrogen and chlorine gases? Your choices are: H 2 (g) + Cl 2 (g) 2HCl(s) or H 2 (g) + Cl 2 (g) 2HCl(g) Use the data below to provide quantitative support to your answer; that is, provide numbers and calculations along with your discussion. A complete answer will include a discussion of ΔH, ΔS, ΔS surr, and ΔS univ along with calculating any of these values you can while estimating the sign (with explanation) for those you cannot calculate. NOTE: Assume the lattice energy of HCl(s) = 1125 kj/mol
Practice Hour Exam III Page No. 8 22. Provide your answer in the space below. See the previous page for what is required. [20 pts.]
Practice Hour Exam III Page No. 9 23. Pure N 2 O was placed in a rigid vessel and allowed to decompose. The following data were collected in an experiment at a constant temperature of 298K. Note that this is total pressure. Assume the rate of the reverse reaction is negligent in this problem. 2N 2 O(g) 2N 2 (g) + O 2 (g) rate = d( P 2 dt N O ) P total (atm) Time (min) 1.000 atm 0 1.125 atm 10.00 1.218 atm 20.00 1.288 atm 30.00 1.341 atm 40.00 1.381 atm 50.00 a. Determine the rate law for the reaction including the order of the reaction and the value (with units) for the rate constant. Show all work. [5 pts.] b. Determine the partial pressure of N 2 (g) at 110.0 min. Show all work. [4 pts.] c. The reaction is carried out again at 303K. The first half-life is noted to be at 4.58 min. Determine the activation energy of the reaction. Show all work. [4 pts.]
Practice Hour Exam III Page No. 10 23. (con t) d. The formula of the reactant, N 2 O, doesn t tell us if the skeletal structure is N-O-N or N-N-O, nor does it tell us if there is a best Lewis structure. Your goal is to find the best Lewis structure for the compound, using formal charge as the primary basis. Define formal charge in your answer. Justify your answer completely. [7 pts.] e. Using the bond energy below and your answers to parts a, c, and d above, develop a mechanism for the reaction and support your answer. [5 pts.] Bond Bond energy (kj/mol) N=O 607 N O 201 N=N 418 N N 941