1
Inorganic Chemistry 411/511
Midterm Exam # 2
60 minutes. Show all work, partial credit will be given. Supplemental information is provided on
the last page.
1.
(a) Using VSEPR theory, draw the structures for the following molecules and determine
their point groups. [8 pts each]
PF5
H3PO4
F
O
4pts
4pts
F
F
P
P
F
F
D3h
OH
HO
OH
4pts
C3V
4pts
(b) Are the above molecules polar or non-polar, and are they chiral or achiral? Explain
briefly. [6 pts]


2.
PF5 is non-polar, since it is in D group; H3PO4 is polar since it is in C
group. (3 pts)
Both molecules are achiral since they have mirror planes. (3 pts)
What are the common names of HClO4, HClO3, HClO2, and HClO ? (8 pts)
Perchloric acid, chloric acid, chlorous acid and hypochlorous acid, respectively.
2
(a) Explain why the Fe3+(aq) ion is acidic (pKa ≈ 3), using the dimerization concept of iron
aqua-complex ions. Give a balanced reaction equation in your answer. [6 pts]
3.
3+
2 [Fe(OH2)6] (aq)



5+
+
 [Fe2(OH2)10OH] (aq) + H3O (aq)
Show dimerization: 2 pts
Show releasing hydronium: 2 pts
Balancing: 2 pts.
(b) Is the pKa of Fe2+(aq) lower or higher than that of Fe3+(aq)? Explain. [4 pts]


4.
Higher (2 pts)
Inductive effect (2 pts)
Use Pauling’s rules to estimate the pKa of the following acids. Show your work. [9 pts]
HNO3, (3 pts)
p is 2, pKa = 8 – 5p = -2
H3PO3 (3 pts)
p is 1, pKa = 8 – 5p = 3
H3BO3 (3 pts)
p is 0, pKa = 8 – 5p = 8
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5.
At 298 K: Nernst equation E = E0 – (0.059 V/n) log Q
At pH = 0, Latimer Diagram
H2MnO4
(a)
+1.28
HMnO3
+2.9
+0.95
MnO2
Mn3+
+1.5
Mn2+
Will Mn3+ spontaneously disproportionate in an acidic aqueous solution (pH=0)? Explain
your answer. (5 pts)
Yes. 2 pts.
Because, 1.5 is bigger than 0.95. 3 pts.
(b)
Identify specie(s) from the above Latimer diagram that can form as the product of
comproportionation reaction at pH = 0. Explain your answer. (5 pts)
MnO2. 2 pts.
Because, 2.9 is bigger than 0.95. 3 pts
(c)
Calculate the potential for the half-reaction reduction of H2MnO4 to MnO2 at pH = 0. [5
pts]
Weighted average: (1*1.28 + 1*2.9)/2 = 2.09 V
(d)
Write a balanced half-reaction for the reduction of H2MnO4 to MnO2 at pH = 0. [8 pts]
H2MnO4 + 2H3O+ + 2e- MnO2 + 4H2O
H2MnO4 + 2H+ + 2e- MnO2 + 2H2O
or
 Use hydroniums or protons in the reactant 2 pts.
 Mass balanced: 3 pts.
 Charge balanced: 3 pts
(e)
Will the potential for the reduction half-reaction in part (d) become higher or lower if the
pH of the reaction solution increases? Explain your answer. [6 pts]
4
o Lower: 2 pt.
o Show the Nernst equation 2 pts.
o Use the concentration of hydronium or proton to explain 2 pts.
6.
Based on the following information
Donor Number Acceptor Number
THF
20
8
H2O
18
55
Is NH3 more soluble in THF or H2O? Explain your answer. (6 pts)


In water. (3 pts)
Due to a high AN of H2O. H2O is a better acid than THF. (3 pts)
7.
Which one of the following is the least soluble in water? [4 pts]
(a) NaCl
(b) NaBr
(c) LiI (d) HgI2 (e) KI
8.
Which of the following is the strongest acid in aqueous solution? [4 pts]
(a) HCl
(b) HNO3
(c) HClO4
(d) HI
(e) H2SO4
(f) all of these have similar acidities in water due to solvent leveling
Which of the following acids is the strongest? [4 pts]
9.
a
b
c
10. Which of the following has an inversion center? [4 pts]
(a) SF6
(b) CH4 (c) PF5, (d) NH3 (e) CH3Cl
d
(f) cis-SF4Cl2
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