Download Inorganic Chemistry 412 / 512

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Inorganic Chemistry 412 / 512
NAME: ___________________________________
Midterm Exam #1 solutions
100 pts total, 60 minutes.
Please show all work, partial credit may be awarded.
1.
(a) Use Wade’s rules to determine the number of cluster-bonding electron pairs and the
structure type (closo, nido, or arachno) in B4H10 . [7 pts]
4 B-H + 6 H = 4 (2) + 6 = 14 cluster bonding electrons, or 7 electron pairs. The
number of vertices, n, is the number of B atoms (4), sp this is an (n+3) or arachnotype structure.
(b) Sketch the geometry of B4H10. (just show the arrangement of B atoms) [5 pts]
See the butterfly structure in Figure 12.11
(c) Give the point group for the closo [B6H6]2- ion and describe the expected 11B NMR
spectrum (ignore 11B- 11B coupling, but include 11B-1H coupling). [6 pts]
[B6H6]2- has Oh symmetry and all B are equivalent. The spectrum will therefore
have only 1 B peak, however, this is split into a doublet due to 11B – 1H coupling.
2.
(a) The mineral beryl has composition Be3Al2Si6O18. Sketch the silicate anion in this
mineral. [8 pts]
The silicate anion is a cyclic hexamer (Si6O182-)12- .
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3. A Pourbaix diagram (E0 vs pH) is shown below for Pb.
(a) Give a balanced reaction that occurs when PbCl4 is added to 1 M HCl (aqu). [6]
2H+ + PbCl4 + H2O

Pb2+ (aqu) + 4 HCl (aqu) + O2 (g)
(b) Is Pb4+ a stronger oxidant in acidic or basic solution, or is it oxidizing potential pH
independent? Explain. [5]
Stronger in acidic solution. See the downward slope of the Pb(IV)/Pb(II)
equalibria in the above diagram.
(c) Pb(IV) is a much stronger oxidant than Ge(IV). Explain. [6]
The inert pair effect. Pb forms weaker bonds than Sn, and the highest oxidation
state is therefore less stable.
4.
Give an example of the reaction of an alkali metal with anhydrous ammonia. [6 pts]
Ex:
5.
Na
 Na+(solv) + e-(solv) blue solution
CF4, SiF4, and GeF4 are all gasses at STP. On the other hand, SnF4 is a refractory solid
stable up to about 600 C. Explain. [8 pts]
Sn and Pb tetrafluorides have Oh coordination about the metal ions, rather than
tetrahedral. This is part of a general trend where larger congeners in the p-block
often have higher coordination. The Oh coordination requires M-F-M bridging and
therefore a covalent network rather than discreet molecules.
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6.
Give a synthetic reaction for preparing an organometallic compound containing an alkali
metal. [7 pts]
Ex: RX + 2 Li ---> RLi + LiX (halide exchange)
7.
What is being manufactured in the process illustrated below? Also, what is being
continually input in order to obtain the desired product. [8 pts]
This is the Hall-Heroult process for making Al metal. Bauxite (Al2O3) is added to
the cryolite (Na3AlF6) flux and energy is input in the form of electricity (an
electrolytic process at about 5V potential).
8.
Explain why LiH has greater thermal stability than the other Group 1 hydrides but Li2CO3
is the least stable of the group 1 carbonates. [8 pts]
See assigned exercise 11.7 solutions
9.
Multiple choice, 4 points each. Circle the ONE best answer.
a.
Why is BF3 a relatively weak Lewis acid?
(a) it is a strong oxidizing agent
(b) it is unstable towards hydrolysis
(c) it is colorless
(d) it has no ionic bonding
(e) pi-bonding stabilizes the trigonal planar geometry in BF3
(f) the entropy of the B-F bond is very small compared to most bonds
b.
Which of the following does NOT have a layered structure?
(a) Graphite (b) KC8 (c) quartz (c) hexagonal BN (d) MgB2 (e) mica
c.
Which is not a chelating ligand?
(a) 18-crown-6 ether
(b) 15-crown-5 ether
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(c) 2.1.1-cryptand
(d) triphenylphosphine
(e) EDTA
d.
Which is NOT true about zeolites?
(a) they generally decompose in the presence of water and O2
(b) they often have anionic aluminosilicate frameworks
(c) the negative charge on a framework is due to Al3+ substitution for Si4+
(d) zeolites have micropores with dimensions ranging from 0.4 to 2 nm
(e) they can be synthetic or naturally-occurring compounds