Chem 6 Sample exam 3 (final) (200 points total) NAME:

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1 Chem 6 Sample exam 3 (final) (200 points This is a closed book exam to which the onor Principle The last three pages contain equations, physical constants, a periodic table and other good stuff; you can detach them for easy Please write clearly and SOW YOUR WORK. If you need to write on the back of the exam paper, please indicate this Some questions are more challenging than others. Allot your time accordingly, and try to answer EVERY Please put your name both on this page and on page 2. NAME:

2 2 NAME: FOR GRADING USE ONLY Question 1 (20 pts)... Question 2 (20 pts)... Question 3 (20 pts)... Question 4 (20 pts)... Question 5 (20 pts)... Question 6 (20 pts)... Question 7 (20 pts)... Question 8 (20 pts)... Question 9 (20 pts)... Question 10 (20 pts)... TOTAL (200 pts)...===========

3 3 1. (20 pts) The figure below represents part of the emission spectrum for a gasphase hydrogen atom. All the lines result from electronic transitions from excited states to a state with unknown quantum number n. I have not shown all the lines in the spectrum between lines A and B, but line A has the shortest wavelength in the series, 3280 nm. Find the quantum number n and the wavelength for line B. Put your answers in the boxes below and show your work. n A B wavelength for line B wavelength

4 2. (20 pts) Consider the reaction A B + C with the proposed mechanism 4 A + A k 1 k 1 A* + A k 2 A* B + C where A* represents a molecule of A with enough energy to surmount the activation energy barrier. (a, 10 pts) Find the rate law for the production of B using the steady-state approximation for the intermediate A*. (b, 10 pts) In the gas phase, this reaction obeys first-order kinetics, but at very low pressures of A, second-order behavior is observed. Explain this observation in terms of your result from part (a) and give a physical interpretation of these observations in terms of the relative rates of the two steps.

5 5 3. (20 pts, 10 each). The highest occupied molecular orbital (OMO) for a molecule is the highest-energy MO which contains electrons. (a) Identify the OMO of F, and sketch the combination of atomic orbitals used to form this MO. [If the OMO is actually a degenerate set of MO's, say so, and just sketch one of the set.] Your sketch should show the relative contributions of the atomic orbitals to the MO, where appropriate. Note: this should be a picture of the orbital, not the MO energy level scheme. (b) The ionization energy of a molecule can be defined in the same way as for an atom. The ionization energy of F is i. greater than that of F ii. the same as that of F iii. less than that of F Circle your choice and explain your answer.

6 6 4. (20 pts) (a, 4 pts) The atom connectivity for a molecule is shown below. Complete the Lewis structure (using the octet rule), including π-bonds and lone pairs as needed. 1 C C C C C 4 C C 5 C N C O (b, 7 pts) Fill in the table to assign the correct hybridization to each atom. If none of the atoms has a given hybridization, say so. hybridization sp sp 2 sp 3 dsp 3 C atom numbers (don't forget to include N and O, too) (b, 3 pts) Give the bond angles at carbons numbers 1, 2, and 4. carbon Bond angles (c, 6 pts) Consider the C2-C3 and C9-O bonds. For each one, explain which atomic and/or hybrid orbitals are used to make the bonds. Put your answers in the boxes and include a brief explanation below. bond C2-C3 orbitals used to make bonds C9-O

7 7 5. (20 pts) The figure below plots the magnitude of the lattice energies vs the metal M for some M(II) dihalides, MX 2. Assume that the factors controlling lattice energy for MX 2 are similar to the ones discussed in class for MX, and that the Madelung constants for these MX 2 compounds are all the same. (a, 10 pts, 5 each) Explain why (for a given halide) the lattice energy generally increases in magnitude as you go across the row, from Ca to Zn. Explain why (for a given metal) the lattice energy increases in magnitude as you vary the halide; smallest for I and largest for F.

8 (b, 10 pts) The lattice energy generally increases in magnitude as you go across the row from Ca to Zn (see the dotted lines). owever, only the compounds where M is Ca, Mn, or Zn give data points which lie on these lines. The other molecules have lattice energy which is larger in magnitude than expected from these lines. Explain the origin of this extra stabilization and why it is missing for Ca(II), Mn(II) and Zn(II). Note: the solid-state structure of these compounds features octahedral coordination at the metal center; they have high-spin electron configurations. 8

9 9 6. (20 pts) Explain these experimental observations. (a, 6 pts) CO 2 is a gas under standard conditions, while CS 2 is a liquid. (b, 8 pts) The boiling points of compounds 1-3 increase as shown. O C C O O 1 2 O 3 C C C C O Cl C O increasing boiling point (c, 6 pts) The shape of the meniscus of water in a glass tube is different from that of mercury in a glass tube. mercury in glass water in glass

10 10 7. (20 pts, 10 each) For this question, use this MO energy level ordering for CO: σ 2s < σ* 2s < π 2p < σ 2p < π* 2p < σ* 2p (a) The LUMO (lowest unoccupied molecular orbital) of CO is defined as the lowest-energy MO which doesn't contain electrons. Identify the LUMO and sketch the combination of atomic orbitals used to form this MO. [If the LUMO is actually a degenerate set of MO's, say so, and just sketch one of the set.] Your sketch should show the relative contributions of the atomic orbitals to the MO, where appropriate. Note: this should be a picture of the orbital, not the MO energy level scheme. (b) Consider CO as a ligand to a transition metal M. The z-axis is the M-CO axis. M C O z Show how a π-bonding MO for this molecule can be formed by sketching the appropriate combination of orbitals (your LUMO and an appropriate metal d- orbital). Note: this should be a picture of the orbital, not the MO energy level scheme. Be sure to say which metal d-orbital you choose, and include the coordinate axes and nodes, where appropriate, in your picture.

11 11 8. (20 pts) Consider the hypothetical solid-state ionic compound e + Cl. Assume that its Madelung constant M = 1.75 (same as in NaCl). We discussed lattice energy qualitatively, but you can also find it quantitatively using this equation: lattice energy = MN A Q 1 Q 2 4πε o d Use a Born-aber cycle to find the maximum e-cl distance in e + Cl for which formation of e + Cl from e(g) and Cl 2 (g) at standard conditions is thermodynamically favorable. ere's some data for this problem: o f (Cl(g)) = 122 kj/mol N A = 6.02x10 23 mol 1 Ionization energy (e) = 2377 kj/mol IQI = 1.60x10 19 C Electron affinity (Cl) = 355 kj/mol ε o = 8.854x10 12 C 2 J 1 m 1 Don't just manipulate these numbers. Explicitly show the Born-aber cycle that you use. Put your answer in the box below. Circle your choice to say if it seems physically reasonable and briefly explain your reasoning. maximum e-cl distance (in Å, please) Physically reasonable? too short too long just right

12 9. (20 pts) An unknown sample of a metal complex could be either [Fe(CN) 6 ] 3 or [FeBr 6 ] 3. Explain how you could use observations of the magnetic properties, including measurement of the magnetic moments, and the absorption spectrum (color) of the complex to identify the sample. 12

13 (20 pts, 4 each) Multiple choice; circle your answers, no explanation needed. (a) One (or more) of these molecules has a dipole moment. Which one(s)? (i) XeF 2 (ii) XeF 3 + (iii) XeF 4 (iv) C 2 4 (ethylene) (b) Pick the molecule with the lowest bond order. (i) 2 (ii) CN (iii) O 2 (iv) e 2 + (c) Pick the paramagnetic molecule(s). There may be more than one. (i) O 2 (ii) N 2 (iii) WCl 6 (iv) Cr(CO) 6 (d) Pick the false statement(s) about water, 2 O. There may be more than one. (i) The -O- bond angle is less than (ii) The boiling point of water is higher than expected (in comparison to 2 S, 2 Se, and 2 Te) because of the greater importance of hydrogen bonding in 2 O than in these analogues. (iii) Increasing the pressure on a sample of ice (at constant temperature) raises its melting point. (iv) Ice, liquid water and steam can coexist under appropriate conditions of temperature and pressure. (e) The minimum uncertainty in the energy for a particle of mass m in a onedimensional box of length L is: (i) h 2 /8mL 2 (ii) h/4πl (iii) h 2 /32π 2 ml 2 (iv) h 2 /16π 2 ml 2

14 14 Equations, Constants, etc. rate = k[a] [A] = [A] 0 e kt ln[a] = ln[a] 0 kt t 1/2 = 0.693/k rate = k[a] = + kt [A] [A] 0 t 1/2 1 = k[a] 0 k = Ae Ea/RT lnk = lna E a RT rate = k 2[E] 0 [S] k -1 + k 2 where K [S] + K M = M k 1 R = Jmole 1 K 1 = Latmmole 1 K 1 T( C) = T(K) λ = h/mv = h/p pv = nrt KE = mv 2 /2 = hν hν 0 N 0 = 6.02x10 23 mole 1 E = mc 2 h = 6.626x10 34 Js x p h/2 c = 3x10 8 ms 1 h = h/2π 1 nm = 10 9 m e = 1.60x10 19 C 1 Å = m kinetic energy = (1/2)mv 2 = p 2 /2m λν = c mvr = nh/2π E = hν = hc/λ electron mass = 9.11x10 31 kg p = mv 1J = 1kgm 2 /s 2 E n = Z2 e 4 m e 8ε 2 o n 2 h = 2 (2.18x10-18 J) Z2 n 2 E = 2.18x10 18 JZ 2 2 (1/n f 1/n i 2) a o = 0.529Å ν = 3.29x10 15 s 1 Z 2 2 (1/n f 1/n i 2) r n = (n 2 /Z)a o

15 15 V = (4/3)π r 3 p = mv Ψ n = A sin( n π x L ) E n = n2 h 2 8 m L 2 Total Energy = K.E. + P.E. Quadratic formula: For ax 2 + bx + c = 0, x = [ b ± (b 2 4ac) 1/2 ]/2a Electronegativity Info atom Electronegativity B 2.04 C 2.55 N 3.04 O 3.44 F 3.98 Cl 3.16 ENERGY ORDERING -- DIATOMIC MO SCEME for Li-N: σ 2s < σ 2s < π 2p < σ 2p < π 2p < σ 2p for O-Ne: σ 2s < σ 2s < σ 2p < π 2p < π 2p < σ 2p U ~ MZ + Z /d spectrochemical series: Br < NCS < Cl < F < O < 2 O < N 3 < NO 2 < CN < CO weak-field strong-field (high-spin) (low-spin) µ s = [n(n+2)] 1/2 B.M.

16 16 PERIODIC TABLE Li Na K Rb Cs Fr 87 (223) Be Mg Ca Sr Ba Ra 21 Sc Y La Ac Ti Zr f Unq (261) V Nb Ta (262) Unp Cr Mn Mo Tc (98) W Unh Re Uns (263) (262) Fe Ru Os Uno (265) 27 Co Rh Ir Une (266) Ni Pd Pt Cu Ag Au Zn Cd g B Al Ga In Tl C 6 N Si P Ge As Sn Sb Pb Bi O Se Te Po 84 (209) S F 9 2 e Ne Cl Ar Br I At (210) 85 Kr Xe Rn (222) Ce Pr Nd Pm Sm Eu Gd Tb Dy o (145) Th Pa U Np Pu Am Cm Bk Cf Es Er Tm Yb Lu Fm Md No Lr (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)

17 11 8. (20 pts) Consider the hypothetical solid-state ionic compound e + Cl. Assume that its Madelung constant M = 1.75 (same as in NaCl). We discussed lattice energy qualitatively, but you can also find it quantitatively using this equation: lattice energy = MN A Q 1 Q 2 4πε o d Use a Born-aber cycle to find the maximum e-cl distance in e + Cl for which formation of e + Cl from e(g) and Cl 2 (g) at standard conditions is thermodynamically favorable. ere's some data for this problem: o f (Cl(g)) = 122 kj/mol N A = 6.02x10 23 mol 1 Ionization energy (e) = 2377 kj/mol IQI = 1.60x10 19 C Electron affinity (Cl) = 355 kj/mol ε o = 8.854x10 12 C 2 J 1 m 1 Don't just manipulate these numbers. Explicitly show the Born-aber cycle that you use. Put your answer in the box below. Circle your choice to say if it seems physically reasonable and briefly explain your reasoning. maximum e-cl distance (in Å, please) Physically reasonable? too short too long just right

Chem 6 Sample exam 2 (150 points total) NAME:

Chem 6 Sample exam 2 (150 points total) NAME: hem 6 Sample exam 2 (150 points total) @ This is a closed book exam to which the onor Principle applies. @ The last page contains equations and physical constants; you can detach it for easy reference.