Chemistry 317
Exam 2
1Name
201.
There is an additional, separate take-home component
Inorganic Chemistry
November 4, 2013
Molecular orbital diagrams for trihydrogen. Show the diagrams for parts (a) and (b) below in the
standard “three-column” form, where the fragments from which you construct the diagram are the
right and left column, and the resulting diagram is in the center. (This is the way you’ve always
done it, I’m just saying “show your work.”) Draw pictures of what all the orbitals look like (all
three columns), showing relative phases and nodes.
(a) Draw a molecular orbital diagram for linear H3 using the molecular orbitals of a stretched H2 and
the atomic orbital of a single H atom as the two starting diagrams.
(b) Draw a molecular orbital diagram for “equilateral-triangle” H3. Use a “slightly stretched” H2 and an
H atom brought in from the side.
Exam 2
page 2
92.
Some questions about acids.
•
HBr and HI are equivalently strong acids in water due to solvent leveling. Explain.
•
What is a solvent in which the relative strengths of HBr and HI could be determined? (Show, using a
chemical equation.)
•
Which of the two acids is stronger in acetic acid? Explain the difference in strength of the two acids.
93.
More acid-base questions.
In terms of hard-soft acid-base theory, which end of the SCN– anion would coordinate to:
(a)
Cr3+
(b)
Pt2+
Briefly explain your choices.
Is Keq > 1 or Keq < 1 for the following reaction? Explain your answer in the context of HSAB theory.
Et3P–BBr3 + Me3N–BF3 ⇌ Et3P–BF3 + Me3N–BBr3
Exam 2
94.
page 3
Beachley and Maloney studied (Organometallics, 1997, 16, 4016-4019) the following Lewis acidbase reaction:
Keq
!
!
!
GaR3! +!
Ph2PH!
!
R
Keq
CH2CMe 3
Mes
Et
CH2SiMe 3
Me
CH2CMe 2Ph
25
<<1
1.25 × 103
20
104
20
!
R3Ga—P(H)Ph2
Me !=
Et !=
Ph !=
!CH3
!CH2 CH3
!C6 H5
Me
Mes !=
Me
Me
(a) Rank the triorganogallium species in order from strongest to weakest Lewis acid toward Ph2PH.
(b) Rationalize the observed trend in Lewis acidity.
Exam 2
125.
Give
page 4
a clear well-labeled molecular orbital diagram for the HCl molecule. Pay attention to the
relative energies of the orbitals used to make the diagram. Valence orbital energy levels are given in
the supplemental material. Include pictures of all orbitals (atomic and molecular).
Exam 2
page 5
Valence orbital potential energies:
Orbital potential energy (eV)
Atomic
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Element
1s
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
–13.6
–24.5
2s
2p
–5.5
–9.3
–14
–19.5
–25.5
–32.4
–46.4
–48.5
–8.3
–10.7
–13.1
–15.9
–21.6
–21.6
3s
3p
–5.2
–7.7
–11.3
–15
–18.7
–20.7
–25.3
–29.3
–6
–7.8
–10
–12
–13.7
–15.9
Exam 2
page 6
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Chemistry 317
Exam 2
TAKE HOME COMPONENT Inorganic Chemistry
November 4, 2013
Name
Ground rules:
1. Complete the exam in a single “sitting.” Time started:_______________ Time completed: ______________
2. You may use textbooks, notes, and on-line resources.
3. You may not use the assistance of any other person.
4. To earn full credit, the work steps to obtain an answer must be shown.
256.
In a step-by-step fashion, construct a molecular orbital diagram for the valence electrons of trigonal
bipyramidal PH5. Your diagram should include symmetry labels and sketches of all the orbitals.
One suggested approach is as follows:
(a) make a molecular orbital diagram for equilateral-triangle H3 by inspection. Check for degeneracy of
orbitals by Frost circle or by reducing a reducible representation of three 1s orbitals arranged in a
triangle under D3h symmetry. (Your diagram should have two orbitals on one side and one orbital on
the other side mixing to give a total of three orbitals in the center. This diagram needs symmetry
labels ONLY for the resultant diagram in the center.)
(b) make a molecular orbital diagram for trigonal planar PH3 in point group symmetry D3h. Use the
diagram from (a) above as one side, and use the phosphorus atom as the other side. Assign all
orbitals (starting orbitals and the resultant orbitals) symmetry labels according to the D3h character
table. Take care to align the principal rotation axis as the z-axis.
(c) make a molecular orbital diagram for trigonal bipyramidal PH5 in point group symmetry D3h. Use the
diagram from (b) above as one side, and a stretched out H2 as the other side. Assign all orbitals
(starting orbitals and the resultant orbitals) symmetry labels according to the D3h character table.
Take care to align the principal rotation axis as the z-axis.
•
•
populate the PH5 molecular orbital diagram with valence electrons.
mark the HOMO and the LUMO
Discussion question:
Interpret your diagram to determine a bond order for the P—H bonds in the molecule. Think about the
pictures of orbitals you drew, and whether they should be considered bonding or not. Your answer
should be just a paragraph.
(Do work and answer discussion question on your own paper and attach.)
Exam 2
page 8
157.
Acid–Base questions.
(a) In the following reactions, identify what system of acid–base classification applies. Then, label the
acid and base species in the equation according to the system you selected.
(i)
PCl5 + ICl → PCl4+ + ICl2–
(ii)
Pt + XeF4 → PtF4 + Xe
(iii)
NOF + ClF3 → NO+ + ClF4–
(b) Using the Drago–Wayland E, C parameters, determine which of Et3N, Et2O, and PMe3 is the
strongest base towards the Lewis acid GaMe3. The parameters are used to determine the enthalpy of
adduct formation (A + B → A–B) according to this equation:
–ΔH = EAEB + CACB
Here is a Table of E, C parameters:
acid
EA (kJ/mol)1/2 CA (kJ/mol)1/2
base
EB (kJ/mol)1/2 CB (kJ/mol)1/2
BMe3
11.84
3.21
py
2.66
13.68
AlMe3
35.43
1.92
NH3
3.03
6.79
AlEt3
28.39
2.88
Me3N
2.43
22.91
GaMe3
28.29
0.82
Et3N
2.64
22.15
GaEt3
25.63
0.39
Me2O
2.21
5.97
InMe3
26.98
0.76
Et2O
2.21
6.30
SbCl5
16.12
10.41
THF
2.17
8.43
BF3
24.93
1.66
Me2S
1.17
14.67
Et2S
1.13
15.14
Me3P
2.27
13.32
Me2Se
1.00
16.38
(For part (b), do work and answer discussion question on your own paper and attach.)