Chemistry 1311
Problem Set 2
1. Draw structures of the following molecules. Your structures should: i) depict the
geometry of the species, ii) show the location of stereochemically active, lone pairs of
electrons on the central atom (i.e., non-bonding electron pairs that are involved in the
determination of the geometrical basis for the structure), iii) indicate the correct number
of lone pairs, if any, on all terminal atoms.
a) CO2
b) ClF3
c) O3
d) SnCl2
e) CH2O
f) SOCl2
Hint: When in doubt choose the least electronegative atom as the “central atom” in
compounds such as f).
C
O
F
O
Cl
- O
F
O
+
O
F
Cl
Sn
H
Cl
Cl
C O
S
Cl
H
O
2. Draw structures of the following species. Your structures should: i) depict the
geometry of the species, ii) show the location of stereochemically active, lone pairs of
electrons on the central atom (i.e., non-bonding electron pairs that are involved in the
determination of the geometrical basis for the structure), iii) indicate the correct number
of lone pairs, if any, on all terminal atoms and, iv) show formal charges on individual
atoms consistent with the number of valence shell electrons assigned to each and
consistent with the overall charge on the species.
a) CO3 2-
-
b) NO2&
c) SO3 2-
O
C O
O
N
d) SnCl3&
O -
- O
Sn
Cl
Cl
Cl
H
P+
H
H
H
e) PH4+
-
O S O
O
-
3. For each of the species in problems 1 and in 2 indicate what set of hybrid orbitals
would be used by the central atom to accommodate its valence shell electron pairs.
1-a 2 electron pairs, sp
2-a 3 electron pairs, sp2
1-b 5 electron pairs, sp3d
2-b 3 electron pairs, sp2
1-c 3 electron pairs, sp2
2-c 4 electron pairs, sp3
1-d 3 electron pairs, sp2
2-d 4 electron pairs, sp3
1-e 3 electron pairs, sp2
2-e 4 electron psirs, sp3
1-f 4 electron pairs, sp3
4. For species a-c in problem 2 experiment indicates that the E-O bond distances
within each ion are equivalent? Indicate how this is accounted for using resonance
structures and determine the bond order of the E-O interaction for each species.
The model here is resonance between equivalent structures which permute the multiple
bond across all locations. These are:
-
O
-
O
C O
C O
- O
O S O
O
-
O S O
O
-
O
O
-
O
C O
- O
- O N O
-
-
N
O -
O S O O
-
5. In which of the following molecules are the bonds to hydrogen most polar?
BH3, CH4, OH2, HF
Bond polarity is directly related to difference in electronegativity for the connected
atoms. The greatest difference in electronegativity is for hydrogen and fluorine;
therefore, the bond between hydrogen and fluorine will be most polar.
6. Which of the molecules given in problem 5 have molecular dipole moments?
Only OH2 and HF have molecular dipole moments as the x, y, and z components of the
bond dipoles exactly cancel in both BH3 and CH4.
7. First ionization energies and electron affinities for elements X, Y and Z are given
below. Which pair of elements will form the least polar covalent bond? Give a one to
two sentence explanation of the basis for your choice.
Element
X
Y
Z
Ionization Energy
1680 kJ mol-1
1090 kJ mol-1
1255 kJ mol-1
Average % χ
1004
606
906
Electron Affinity
328 kJ mol-1
122 kJ mol-1
349 kJ mol-1
In the Mulliken definition, electronegativity is proportional to the average of the
ionization energy and the electron affinity. A column of such values has been added.
The difference in the averages for X & Z is smallest so that these atoms will form the
least polar bond.
8. Predict the compositions of the possible phosphorus fluoride molecules, PFn and
anions, PFnx&.
Based upon the available singly occupied orbitals in ground state P (three) and the
excited form in which one of the s electrons has been moved to a d orbital (five) one
would expect PF3 and PF5 to be the neutral molecules. To the extent that each of these
is acidic and can utilize a d orbital, or another d orbital in the case of PF5 to bind to
fluoride ion, then one might expect PF4& and PF6&. The latter is known, but the former is
not. The reason can be traced to the repulsion between the phosphorus lone pair and
the fluoride ion charge in PF4&. Also realize that the oxidation state of P is +3 in PF3 and
+5 in PF5. The charge on phosphorus and correspondingly its acidity are much higher
in PF5, hence the greater affinity for fluoride ion.
9. Monoprotonation of the sulfite ion (problem 2c), yields hydrogen sulfite ion (also
known as the bisulfite ion). Draw resonance structures of the ion that properly
represent the fact that two of the S-O bonds are equivalent and shorter than the third.
What is the bond order of the equivalent S-O bonds? What is the oxidation state of
sulfur in the bisulfite ion?
This is accounted for by resonance between two equivalent HOSO2& Lewis structures.
The bond order of the terminal S-O bonds is 1.5 and the oxidation state of sulfur is +4.
H
O S O
O
-
H
O S O
O
10. a) Indicate the type of hybridization each of the numbered atoms in the drawing
below should have.
1 (oxygen) will be sp3 hybridized as there are four electron pairs in the valence shell
2 (three coordinate carbon) will be sp2 hybridized, which is always the case when
carbon is attached to three atoms in a trigonal planar fashion.
3 (carbon) will be sp hybridized, which is always the case when carbon is attached to
two atoms in a linear fashion.
4 (carbon) will be sp3 hybridized, which is always the case when carbon is attached to
four atoms.
b) What orbital is used by carbon atom 2 for forming the pi (π) bond?
All carbon atoms use p atomic orbitals in forming pi bonds. Any double bond involving
carbon will consist of a sigma and a pi bond. A triple bond consists of a sigma and
two pi bonds.
1
2
H 3C O
H
O
H2
C
..
..
.. ..
C
C C
H
C
H2
3
C
4
C
CF3