Chemistry 6 10:00 Section Time limit = 3 hours Spring There are two sections to this exam. Please read the instructions carefully.

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1 Chemistry 6 10:00 Section Final Exam Time limit = 3 hours Spring 2005 Name There are two sections to this exam. Please read the instructions carefully. In the first section, there are 13 multiple choice questions, each is worth 8 points. YOU MUST ANSWER QUESTIONS 1 THROUGH 3 AND THEN ANY 7 OF YOUR CHOICE. IF YOU ANSWER MORE THAN 7, THE FIRST 7 WILL BE GRADED. CROSS OUT THOSE YOU DO NOT WISH GRADED OR LEAVE THEM BLANK. You do not need to show any work for these problems. In the second section, there are 13 short answer problems and each is worth 12 points. YOU MUST ANSWER QUESTIONS 1 AND 2 AND THEN ANY 8 OF YOUR CHOICE. IF YOU ANSWER MORE THAN 8, THE FIRST 8 WILL BE GRADED. CROSS OUT PROBLEMS YOU DO NOT WISH TO BE GRADED. Show all work for partial credit. All submitted work must be your own effort. The Honor Principle applies to your submission of this exam. The last 2 pages contain equations and other potentially useful information. Feel free to tear them off to use during the exam. Part 1. Part TOTAL 1

2 PART 1. For these multiple-choice problems, each correct answer is worth 8 points. It is not necessary to show any work. Circle the correct answer. YOU MUST ATTEMPT I-1 THROUGH I-3. Then, do 7 problems of your own choosing. I-1. Arrange in order of decreasing bond length: NF NF + NF - a. NF > NF + > NF - b. NF + > NF > NF - c. NF - > NF + > NF d. NF + > NF - > NF I-2. What is the hybridization of each carbon atom in acetic acid? H 3 C-C-OH O a. C:sp C:sp 3 b. C:sp 2 C:sp c. C:sp 3 C:sp 3 d. C:sp 3 C:sp 2 I-3. Arrange in order of increasing boiling point: NaF Ar CH 4 NO a. CH 4 < Ar < NO < NaF b. NaF < NO < Ar < CH 4 c. Ar < CH 4 < NO < NaF d. NaF < CH 4 < NO < Ar I-4. What is the wavelength of an electron whose energy is equal to 8.00 x J? a. 0.55nm b. 248nm c. 1.2 x10 15 nm d. 3.0 x10 8 nm I-5. Which one of the following correctly lists the elements in order of increasing electronegativity? a. F<Cl<Br<I b. Cl<Br<I<F c. Cl<Br<F<I d. I<Br<Cl<F I-6. The reaction N 2 O 5 2NO 2 + ½ O 2 has a rate constant of 5.7 x 10-2 s -1. If at the start of the reaction, the pressure of N 2 O 5 is 2.13atm, what will the pressure of N 2 O 5 be after 18.7s? a. 0 atm b atm c atm d atm I-7. Arrange in order of increasing lattice energy: MgO NaF KCl CaS a. CaS<KCl< MgO<NaF b. CaS<MgO< KCl<NaF c. KCl<NaF<CaS<MgO d. KCl<CaS<MgO<NaF I-8. Which of the following transition metal ions would be expected to yield colorless complexes? La 3+ Ti 3+ Co 2+ Fe 3+ a. Ti 3+ Co 2+ b. La 3+ Ti 3+ c. Ti 3+ Fe 3+ d. La 3+ Fe 3+ I-9. How many unpaired electrons are in CoF 6 3-? a. 0 b. 2 c. 3 d. 4 I-10. How many nodes are there in the angular part of the 4p wavefunction for hydrogen? a. 1 b. 2 c. 3 d. 4 I-11. The wavelength for the n=3 -> n=2 transition in a particular one-electron ion is 72.2nm. What is the identity of the ion? a. Li 2+ b. He 2+ c. Be 3+ d. F 8-2

3 I-12. The gas phase reaction 2NO 2 + F 2 2NO 2 F has a rate law Rate = k[no 2 ][F 2 ]. Which one of the following sets of reactants will appear in the rate determining step for the mechanism consistent with this rate law? a. 2NO 2 and F 2 b. NO 2, NO 2 F and F c. NO 2 and F 2 d. 2NO 2 F I-13. Consider a particle in a 3-dimensional cubic box. What the degeneracy for the first excited state of this system? a. 0 b. 1 c. 2 d. 3 PART 2. For these short-answer problems, each correct answer is worth 12 points. Show all work. YOU MUST ATTEMPT II-1 AND II-2. Then, do 8 (and only 8) problems of your own choosing. II-1. Consider the effect of forming an octahedral complex on the p orbitals of a metal atom. Determine the energy splitting of the p orbitals (if any) and sketch a crystal field diagram analogous to that we sketched in class for the d orbitals. II-2. Which of the following molecules would be stabilized by addition of an electron to form an anion? Which would be stabilized by ionization to form a cation? Briefly explain. C 2 N 2 CN NO II-3. For each of the following molecules/ions: draw a Lewis dot structure determine the orbital hybridization of the central atom indicate the ideal geometry (counting non-bonding electrons as bonds ) indicate the actual geometry (the geometry you observe) The central atom for each molecule is written first. XeF 4 BrF 5 SF 4 II-4. Consider the hypothetical reaction A B + ½ D, which occurs via the mechanism A B + C, k 1 A + C B + D, k 2 Use the steady state approximation to write an expression for [D]. II-5. Cyanate ion, NCO -, and isocyanate ion, CNO -, are isomers. Draw resonance structures for each of these ions and indicate which one of the resonance structures makes the most important contribution to the observed geometry. II-6. For the reaction [Co(NH 3 ) 5 Cl] 2+ + H 2 O [Co(NH 3 ) 5 H 2 O] 3+ + Cl -, the rate constant at 25 C is 3.06 x 10-3 min -1 and at 30 C, 5.65 x 10-3 min -1. Find the activation energy. II-7. Consider a He + atom. Two transitions with the same n i are observed. The high energy transition terminates at n f = 1 and the other at an unknown quantum level. The wavelengths of these transitions are nm and 23.4 nm. What is the value of the unknown quantum number? (Do not assume a value for n i.) 3

4 II-8. For each of the following choose the molecule or atom with the higher transition temperature. Give a one-line explanation of your choice. (a) Which has the higher boiling point PH 3 or CH 4? (b) Which has the higher boiling point NH 3 or PH 3? (c) Which has the higher boiling point Xe or Ar? (d) Which has the higher boiling point CF 4 or HF? (e) Which has the higher boiling point H 3 C-O-CH 3 or H 3 C-CH 2 -O-H? (f) Which has the higher melting point CaO or KCl? II-9. What is the wavelength associated with a He atom at 298K? II-10. The cyclopentadienyl anion, C 5 H 6 -, has the structure shown below (a ring of five carbon atoms, each of which is also bonding to a hydrogen atom. Describe the hybridization at each carbon atom. Is it possible to draw resonance structures for this ion? If so, how many? (You do not need to draw them). II-11. The dipole moment of gaseous CsF is 7.88D and the bond length is 0.255nm. What is the percent ionic character of the Cs-F bond? II-12. What are the electronic configurations of OF, OF + and OF -? Compare the bond energies, bond lengths and magnetic properties of these species. 4

5 Possibly useful information: k Boltzmann = 1.38 x J/K R = 8.31 J/mole K 1nm = 10-9 m m electron = 9.11 x kg [A] = [A] o e -kt N A = 6.02 x mole -1 1/[A] = 1/[A]o+ kt h = 6.62 x J s k = Ae- Ea/RT a 0 = 0.530Å k = P πd 2 v e -Ea/RT average velocity = (8RT/πM) 1/ 2 t 1/2 = 0.693/k t 1/2 = 1/[A] o k E = hν 1Å = 10-8 cm p = h/λ x p h/4π p = mv c = λ/ ν E = 2.18 x J Z 2 /n 2 c = 3 x 10 8 ms -1 E = R Z 2 ( 1/n 2 f - 1/n 2 i ) K.E. = 1/2mv 2 = p 2 /2m E = φ + 1/2(mv 2 ), where φ is the threshold energy or the work function e = 1.60 x Coulombs R (Rydberg constant) = 2.18 x J = cm -1 1J = 1kg m 2 s -2 1eV = 1.6 x J 1D = x C m µ = Q d #" #" # # 5

6 Spectrochemical series (partial list) F - < H 2 O < NH 3 < CN - red orange violet yellow blue green 6