CHEMISTRY 201X
September 10-14, 2007
Practice Problems
(35)
Chlorine, bromine, and iodine form pentafluorides.
At 273 K, one of these is a solid, one
is a liquid and one is a gas.
Specify which is which, and explain your reasonings.
Without knowing anything about the structures, we can assume that London
dispersion forces will dominate.
Then, at 273 K, IF
5
should be a solid
, BrF
5
should
be a liquid
, and ClF
5
should be a gas.
Melting Point (K) Boiling Point (K)
IF
5
283
371
BrF
5
212
313
ClF
5
170
260
(36)
How do expect the boiling point of hydrogen peroxide, H
2
O
2
, to compare with those of
fluorine, F
2
, and hydrogen sulfide, H
2
S, which are two substances with molar masses
comparable with that of hydrogen peroxide?
H
2
O
2
(MW = 34 g/mol) can have hydrogen-bonding, so we can expect a higher
boiling point (150.2
°
C) than for F
2
(MW = 38 g/mol) (
−
188
°
C) and H
2
S (MW = 34
g/mol) (
−
60.2
°
C).
(37)
The van der Waals constant,
b
(L/mol), is related to the volume per molecule in the
condensed (liquid) state, and thus to the size of a molecule.
The following combination
of van der Waals constants,
a
/
b
, has units of L
⋅
atm/mol.
1 L
⋅
atm (= 101.325 J) is a unit
of energy, so
a
/
b
is proportional to the
strength of the attractive forces
between
molecules.
Use the van der Waals constants below to rank the attractive forces from
strongest to weakest.
Gas
a
(L
2
⋅
atm/mol
2
)
b
(L/mol)
a
/
b
(L
⋅
atm/mol)
H
2
0.2444
.02661
9.185
N
2
1.390
.03913
35.52
HCl 3.667
.04081
89.86
SO
2
6.714
.05636
119.13
So, the attractive forces increase in strength from H
2
, N
2
, HCl, SO
2
.
This can be
explained by both London dispersion forces and the effect of dipole-dipole forces for
HCl and SO
2
over the nonpolar molecules H
2
and N
2
.
(38)*
Since the van der Waals constant
b
is related to the volume excluded per mole of
molecules, it should be proportional to
N
A
σ
3
, where
N
A
is Avogadro’s number
(molecules/mole) and
σ
is the distance of closest approach in the Lennard-Jones
potential.
(a)
Use the following table of information and plot
b
vs.
N
A
σ
3
.
Do you see any
correlation between the two values?
Can you write a mathematical equation based on
the graph?