130 points on 6 pages + a useful page 7

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1 Name KEY Chemistry 350 Spring 2012 Exam #2, March 30, minutes 130 points on 6 pages + a useful page 7 1. Circle the element/compound most likely to have the desired property. Briefly explain your choice (12 pts) a) Softest base N 3- Se 2- S 2- P 3- Small charge with big radius b) Strongest acid in water H 3 AsO 3 H 3 AsO 4 H 3 AsO 2 most non-h O s, c) Strongest acid in water [V(OH 2 ) 6 ] 2+ [V(OH 2 ) 6 ] 4+ [V(OH 2 ) 6 ] 5+ highest charge d) Most soluble in water LiF LiCl LiBr LiI Li hard, I soft 2. Using Hard/Soft-Acid/Base theory, determine whether these reactions have equilibrium constants, K, greater than or less than 1. Designate each ion as hard (H) or soft (S). (9 pts) a) 2 PbS + PbO 2 2 PbO + PbS 2 K < 1 SS HH SH HS b) Nb 2 S HgSe Nb 2 Se HgS K < 1 HH SS HS SH c) BaF 2 + HgI 2 BaI 2 + HgF 2 K < 1 HH SS HS SH /21

2 Chem 350, Spring 2012, Exam 2 CCM, Page 2 3. Fill in the blanks with the proper element symbols: (3 pts) Si P S Ge As Se Sn Sb Te 4. Sketch the overlap regions of the following combinations of orbitals, all with co-linear z axes, i.e. the two atoms share a common z axis. Classify the interactions as bonding, anti-bonding or non-bonding (no interaction). If the interaction is bonding or anti-bonding indicate if the interaction is, or. (9 pts) a) d xy and d xy b) d xz and p x Bonding, Bonding, d) Briefly comment on the relative strengths of sigma, pi, and delta overlaps Sigma bonds are formed by orbitals overlapping along the axis between two nuclei. As such, they face directly at one another and have very strong overlaps and produce very strong bonds. Pi and delta bonds are formed by orbitals that do not face one another, so they are weaker. One would think that delta bonds should be stronger than pi, but remember that d orbitals are often more spatially extended so the overlap is not as strong. So is how we usually write it 5. Draw portions of two close-packed layers and show the relative positions and numbers of octahedral and tetrahedral holes. Clearly indicate the number of octahedral and tetrahedral holes per close-packed atom. (6 pts) /18

3 Chem 350 Fall 2009, Exam 2 CCM, Page 3 6. We talked about a number of definitions of acids and bases in class. For the solvent system SbF 5, consider the following. (12 pts) a) Write a balanced chemical equation for the auto-ionization equilibrium by fluoride exchange. Be sure to clearly identify the acidic and basic species. 2 SbF 5 SbF SbF 6 Acid Base b) Briefly comment on how the solvent system description of acids and bases compares to Lewis acid/base theory and Bronsted-Lowry acid/base theory. Bronsted-Lowry acid/base theory relates to proton transfer. Acids donate H+ and bases accept H+. Solvent system acid/base theory relates to cations and anions. Something that generates more cations in the system is acidic and something that generates more anions is basic. In water, these two systems agree (and are really the Arrhenius system!). Lewis theory is broader and includes both B-L and SS theories. In Lewis theory acids are electron pair acceptors and bases are electron pair donors. c) In this solvent system silver fluoride would be considered (circle one) acidic or basic. d) Write a balanced chemical equation to provide evidence for your choice in c). AgF + SbF 5 Ag + + SbF 6-7. Shown at right is a unit cell for the mineral cuprite: (12 points) a) Based on the metrics shown, to which of the seven crystal systems does this unit cell likely belong? cubic b) What is the formula unit for this unit cell in terms of Cu and O 1 / 8 x 8 + ½ x 6 = 4 Cu Cu 4 O 2 Cu 2 O 1 x 2 = 2 O c) How many of these formula units are there per unit cell? 2 d) I can describe the nickel arsenide structure type as a hexagonal close-packed arrangement of anions with cations in all of the octahedral holes. Please describe cuprite in similar terms. Cuprite is a cubic close-packed arrangement of cations with anions in 1 / 4 ( 2 / 8 ) of the tetrahedral holes. /24

4 Chem 350 Fall 2009, Exam 2 CCM, Page 4 8. The molecule ICl 3 has C 2v symmetry. a) Analyze the x, y, and z coordinates of ICl 3 to determine the reducible representation,, for the atoms in the molecule ICl 3. (4 pts) = b) Using the character table provided, determine which irreducible representations are normal vibrational modes. (12 pts) (Note: If you have no answer for part a), use = ) Order = 4. 3N=12. 3N-6=6 A 1 = 1 / 4 [(12)(1)(1) + (-2)(1)(1) + (4)(1)(1) + (2)(1)(1)] = 4 A 2 = 1 / 4 [(12)(1)(1) + (-2)(1)(1) + (4)(1)(-1) + (2)(1)(-1)] = 1 B 1 = 1 / 4 [(12)(1)(1) + (-2)(1)(-1) + (4)(1)(1) + (2)(1)(-1)] = 4 B 2 = 1 / 4 [(12)(1)(1) + (-2)(1)(-1) + (4)(1)(-1) + (2)(1)(1)] = 3 overall, = 4 A 1 + A B A 2 = 12 modes C 2v E C 2 (z) v(xz) v(yz) A z x 2, y 2, z 2 A R z xy B x, R y xz B y, R x yz Of these 1 A 1 +1 B B 2 are translations (z, x, y) and 1 A B B 2 are rotations (R z, R y, R x ) this leaves only 3 A B 1 + B 2 as normal vibrational modes c) and which of those modes are IR- and/or Raman-active. Be sure to indicate how many total bands are expected to be observed. (4 pts) IR: A 1, B 1, and B 2 are active, 6 bands Raman: A 1, B 1, and B 2 are active, 6 bands assuming no overlap! 9. Given the Cartesian coordinates xyz consider the following matrix transformations: (6 pts) For each, i) provide the new Cartesian coordinates after transformation, ii) identify the symmetry operation that was performed, and iii) determine the character of the 3 x 3 matrix a) b) i) x y z -x y z ii) xz C 2 (around z) iii) 1-1 /26

5 Chem 350 Fall 2009, Exam 2 CCM, Page Consider the hyponitrite ion, NO -. (20 pts) a) In the box provided, generate a well-labeled molecular orbital diagram for NO -. useful info? Orbital potential energy (ev) element 2s 2p N O E LUMO HOMO ON YOUR DIAGRAM b) Clearly indicate which orbitals are the HOMO and/or LUMO. It needn t look exactly like this, but the rules must be followed c) Indicate whether NO - is paramagnetic or diamagnetic (circle one) d) Using the diagram you generated in a, determine the likely bond order of i. NO - ii) NO iii) NO + ½(6-2) = ½(6-1) = ½(6-0) = e) What is the most likely position for adding H + to the NO - ion? Explain your answer. H+ is most likely added to the N because the larger electron density lobe is on N in the HOMO/LUMO the shapes of the lopes and orbitals are much like those generated in CO, displayed on page 59 of S&A, our text. The N also carries the negative formal charge. /20

6 Chem 350 Spring 2012, Exam 2 CCM, Page Generate the MO diagram for the theoretical linear molecule BH 2 by showing the following: a) Determine the point group of the theoretical linear molecule BH 2. (2 pt) D h b) Using the character table provided (it may or may not be for a point group of the same symmetry as part a) generate the group orbitals of H 2 and indicate the symmetry (i.e. irreducible representation label) of each. Please use z as the bonding axis. (4 pts) E C 2 (z) C 2 (y) C 2 (x) i (xy) (xz) (yz) A g x 2, y 2, z 2 B 1g R z xy B 2g R y xz B 3g R x yz A u B 1u z - B 2u y - B 3u x - c) Indicate the relevant atomic orbitals and their symmetries for the central atom, B. (4 pts) d) In the box provided, generate a well-labeled molecular orbital diagram --be sure to label the orbital levels appropriately and to indicate whether each orbital is bonding, anti-bonding, or non-bonding. (11 pts) E Orbital potential energy (ev) element 1s or 2s 2p B H bonding: a g b 1u non-bonding: b 2u b 3u anti-bonding: a g * b 1u * 1s /21

7 Chem 350 Spring 2012, Exam 2 CCM, Page 7 Some of the structure types you know Perovskite Nickel Arsenide Sphalerite Wurtzite Rock salt/halite Hexagonal Close-packed Body-Centered Cubic Anti-fluorite Fluorite Diamond Cesium Chloride Simple Cubic Face-centered Cubic Cubic Close-packed A useful table? r + /r - CN