Download Problem Set 9 Solutions

Problem Set 9 Solutions
1.
For each of the following molecules, provide a Lewis structure, including resonance and formal
charge. Predict the electron-domain geometry, the molecular geometry, and the hybridization of
the central atom. For each nonionic species, state whether it will be polar or nonpolar.
(2 pts each, 8 pts total)
Lewis Structure
a)
O
Se
O
O
Se
O
F
Xe
e–-Domain
Geometry
Molecular
Geometry
Hybridization
Polarity
trigonal planar
bent
sp2
polar
octahedral
square planar
sp3d2
nonpolar
trigonal
bipyramidal
trigonal
bipyramidal
sp3d
nonpolar
trigonal
bipyramidal
linear
sp3d
N/A
b)
F
F
F
c)
Cl
Cl
P
Cl
Cl
Cl
d)
F
Br
F
2.
a)
Consider methyl cyanide, H3CCN. (8 pts total)
Draw the best possible Lewis structure of methyl cyanide. (2 pts)
H
H
C
C
N
H
b)
Predict the geometry and hybridization of each carbon atom. (2 pts)
left C: tetrahedral, sp3
right C: linear, sp
c)
Describe the bonding in this molecule in terms of sigma (σ) and/or pi (π) bonds and the orbitals
involved in bond formation. (4 pts)
There are three σ bonds made between left C sp3 hybrid orbitals and H s-orbitals.
There is one σ bond made between left C sp3 hybrid orbital and right C sp hybrid orbital.
There is one σ bond made between right C sp hybrid orbital and N sp hybrid orbital.
There are two π bonds made between right C unhybridized p-orbitals and N unhybridized porbitals.
3.
a)
(10 pts total)
Draw an energy-level diagram for the valence molecular orbitals of CO. Include the relevant
atomic orbitals in your diagram. Clearly label each molecular orbital (e.g σ2s, π2p, etc.) and fill in
the appropriate number of electrons. Use the same energy order as the neutral O2 molecule. (3 pts)
σ2p*
π2p*
2p
π2p
σ2p
σ2s*
2s
C
2s
atomic
orbitals
O
σ2s
atomic
orbitals
CO
molecular
orbitals
b)
Is CO paramagnetic or diamagnetic?
What is the bond order of CO?
c)
Draw a Lewis structure for CO. Does this match the predicted bond order? (2 pts)
C
d)
O
diamagnetic
3
(2 pts)
Yes, it has a triple bond (bond order of 3)
CO can react with OH– to form the formate ion, HCO2–. Draw the best possible Lewis structure for
the formate ion. Include any non-zero formal charges and resonance if appropriate. Describe the
bonding in terms of sigma (σ) and/or pi (π) bonds, discussing the orbitals involved in bond
formation, and describing any delocalization if appropriate. (Note: In the formate ion, C is the only
central atom; the H is bonded to C.) (3 pts)
O
H
C
O
H
O
C
O
There is one σ bond made between C sp2 hybrid orbital and H s-orbital.
There are two σ bonds made between C sp2 hybrid orbitals and sp2 hybrid orbitals on each O.
There is one delocalized π bond made between C unhybridized p-orbital and the two O’s
unhybridized p-orbitals.
4.
(4 pts total)
a)
The polyatomic ion [HCNXeF] + has all the atoms bonded in the listed order, and its molecular
geometry is entirely linear. Provide a Lewis structure that is consistent with this molecular
geometry. (2 pts)
H
C
N
Xe
F
Note that the C and N are linear, with sp hybridization. The Xe is linear, with sp3d hybridization
and three (equatorial) lone pairs!
b)
In the molecule XeO2F2, the Xe–O bonding electrons take up slightly more space around the xenon
atom than the Xe–F bonding electrons do. Given this information, predict the geometry of the
XeO2F2 molecule. Draw a sketch of how this would appear, including the lone pair, and explain
your reasoning. (2 pts)
F
2+
Xe
O
O
F
In the trigonal bipyramid case, the equatorial electron pairs have more room than the axial electron
pairs. Thus the lone pairs (which are bigger) always go in the equatorial positions. Extending this
reasoning, the oxygen bonding pairs (which are bigger than the fluorine bonding pairs) would
likewise prefer the equatorial positions.