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Inorganic Chemistry 411/511
Midterm Exam # 2 KEY
60 minutes. Show all work, partial credit will be given. Supplemental information is provided
after the test questions.
1.
What are the point groups for:
(a) NH2Cl [7 pts]
(b) BrF4– [7 pts]
(a) Cs
(b) D4h
(c) Do symmetry rules require that either (a) or (b) are non-polar? Explain. [6 pts]
(a) can be polar, (b) must be non-polar (it belongs to a D group)
2.
(a) The phosphate (PO43-) anion dimerizes at pH = 12 to form P2O74– . Write a balanced
chemical reaction for the dimerization of phosphate. [7 pts]
2 PO43- (aq) + H2O

P2O74- (aq) + 2 OH- (aq)
(b) Use Pauling’s equation to estimate the pKa’s of nitric and perchloric acids. [7 pts]
pka ≈ 8 – 5p HNO3 has p = 2 and pka ≈ -2, HClO4 has p = 3 and pka ≈ -7
(c) Given your results in 2(b), will there be a significant difference in the acidity of nitric
and perchloric acids in aqueous solution? Explain. [6 pts]
No significant difference due to solvent leveling for any acid with pKa < 0
2
3.
(a) Sketch a benzene molecule, clearly showing all the mirror symmetry planes. [6 pts]
Three mirror planes contain 2 para C atoms, three bisect opposite C-C bonds, and the last
one is the horizontal plane that contains the molecule. The figure shows the edges of the
first 6 planes, and shows the horizontal plane as a rectangle.
(b) Which Cl-substituted benzene C6HnCl6-n has exactly four planes of symmetry? [6 pts]
C6H3Cl3 with Cl in non-adjacent positions has 4 σ planes, 3 σv and 1 σh
(c) BCl3 has a symmetric stretching vibration that involves simultaneous movement of all
ligands away from the central B (illustrated below). Explain whether this A1’
vibrational mode will be activated in IR and/or Raman spectroscopy, and how you
know. A D3h is provided at the end of the test. [7]
Raman active, not IR active, because it has x2+y2 and z2 (both quadraticr) as
basis functions, not x, y, or z
4.
(a) Sketch all the possible isomers for the Oh complex [IrCl3F3] [7 pts]
There are two possible isomers, fac and mer
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5.
Given the Pourbaix diagram for Sm, plotting
solution potential vs pH, write a balanced halfreaction for the net REDOX reaction that occurs
when Sm metal is placed in 1 M HCl [6 pts]
Sm(s) + 3 H+(aq) → Sm3+(aq) + 3/2 H2(g)
HMnO4
6.
0.90
H2MnO4
1.28
HMnO3
2.9
MnO2
Part of the Latimer diagram for Mn at pH=0 is given above.
(a) Write a balanced reduction half-reaction for HMnO4 / MnO2 at pH=0 [6 pts]
3e- + 3H+ + HMnO4
→
MnO2 + 2 H2O
(b) Calculate the potential for the half-reaction in 6(a) at pH = 0 [6 pts]
[1 (0.90 V) + 1 (1.28) + 1 (2.9 V)] / 3 = +1.69 V
(c) Calculate the potential for the half-reaction in 6(a) at pH = 3
Nernst equation: E = E0 – (0.059 V / n) log Q
[8 pts]
E = +1.69 V – (0.059 V / 3) log (1/[H+]3)
E = +1.69 V – (0.059 V) ΔpH
E = +1.69 V – (0.059 V) 3 = +1.51 V
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Circle the one best answer for each. [4 pts each]
7.
Which of the following is the strongest acid?
(a) Ti(H2O)62+
(b) Cr(H2O)62+
(c) Fe(H2O)62+
(d) Ni(H2O)62+
(f) Ni(H2O)6+
(g) all have similar pKa’s
(e) Ca(H2O)62+
(h) solvent leveling means all are effectively identical in aqueous solutions
8.
Using the Frost diagram for O provided, which statement
is TRUE?
(a) water is more easily oxidized in acid than in base
(b) the most thermodymically stable species for O in
aqueous base is HO2(c) the peroxide oxidation potential is independent of pH
(d) H2O2 cannot dissociate in acid or in base
(e) O2 is a stronger oxidant in acid than in base
(f) O redox potentials are essentially pH independent
Character table for D3h point group
linear,
E 2C3 3C'2 σh 2S3 3σv
quadratic
rotations
x2+y2, z2
A1’ 1 1
1
1
1
1
A2’ 1 1
-1
1
1
-1
Rz
E' 2 -1
0
2 -1
0
(x, y)
A1’’ 1 1
1
-1 -1 -1
A2’’ 1 1
-1 -1 -1
1
z
E'' 2 -1
0
0
(Rx, Ry)
-2 1
(x2-y2, xy)
(xz, yz)
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