Chem 143 2018 Final Exam Solns.pdf - Chem 143 2018 Final...

Chem 143 2018 Final Exam Solutions
Ia.) CH
2
F
2
> CF
2
Cl
2
> CF
4
.
The geometry of each molecule is based on a tetrahedral structure, as shown
below. CF
4
has strict tetrahedral symmetry and there are a variety of non-
coincident proper rotation axes, reflection planes that do not contain some of the
axes, and improper rotations. Thus the molecular dipole moment of CF
4
is exactly
zero. Both CH
2
F
2
and CF
2
Cl
2
have a C
2
rotation axis (which bisects the F-C-F angle),
and two reflection planes containing the C
2
axis, one containing the carbon and
fluorines, and one containing the carbon and the other two atoms. Thus both
CH
2
F
2
and CF
2
Cl
2
may have a dipole moment constrained to lie along the C
2
axis.
The molecular dipole moment represents the sum of the bond dipoles, the
magnitudes of which are dependent on differences in electronegativity. The
electronegativity differences predict that C-F bonds should have the largest bond
dipoles, while C-H has the smallest. C-Cl bonds have large bond dipoles, but they
are smaller than the C-F bond dipoles. In CH
2
F
2
, the C-H bond dipoles and the C-
F bond dipoles add to result in the largest dipole. In CF
2
Cl
2
the bond dipoles are
more similar and almost cancel to give a significantly lower molecular dipole.
A
A
F
F
C
2
σ
σ
F
F
F
F
C
2
C
3
C
3
A = H, Cl
Ib) Br > Cl > F.
The standard enthalpies of the reactions are given by
∆ H
o
rxn
= 2 ∆ H
f
o
(HX)
- ∆ H
f
o
(H
2
)
- ∆ H
f
o
(X
2
)
For F
2
(g) and Cl
2
(g), ∆ H
f
o
(X
2
)
= 0 since these represent the elemental states of fluorine
and chlorine, and ∆ H
o
rxn
= 2 ∆ H
f
o
(HX)
= -542 kJ mol
-1
for F and -184 kJ mol
-1
for Cl.
For Br
2
(g), ∆ H
o
rxn
= 2 ∆ H
f
o
(HBr)
- ∆ H
f
o
(Br
2
,g)
= -103 kJ mol
-1
, thus establishing the
ordering given above.

Ic) CH
4
> BH
3
> ClF
3
.
The structures and the C
3
axes are shown below.
H
H
C
H
H
C
3
C
3
C
3
C
3
CH
4
is tetrahedral and there is a C
3
axis along each C-H bond for a
total of four C
3
axes.
H B
H
H
C
3
BH
3
is trigonal planar and there is only
one C
3
axis perpendicular to the
plane of the molecule.