Chem 121 Winter 2016: Section 03, Sample Problems
Solutions: Answers
Problems, with some modifications and additions, from Chemistry - The Central Science, 3rd edition.
1.
Wine consists predominantly of ethanol (ethyl alcohol, C2H5OH) and water. What type(s) of
intermolecular forces exist between the molecules?
Dispersion forces, which arise because of the transient presence of instantaneous dipoles, exist
between all molecules, so must exist in an ethanol-water mixture. Dipole-dipole forces operate
between molecules that contain permanent dipoles; both ethanol and water contain such dipoles.
Finally, we can anticipate H-bonding, since both molecules contain oxygen directly bonded to a
hydrogen atom.
H-bonded interactions are stronger than dipole-dipole interactions. For light molecules, dispersion
forces are lower than both H-bond forces and dipole-dipole forces. However, for large molecules
dispersion forces may become significant.
2.
(a)
What is meant by the vapour pressure of a liquid?
The vapour pressure of a liquid is the partial pressure of the vapour that exists above a liquid that is
free to evaporate, once equilibrium is established. Note that the vapour pressure is not dependent
on the presence or absence of any other type of molecule in the vapour phase (though
experimentally it is more readily measured if there are no other gases present).
(b)
How is the vapour pressure of a liquid affected by (i) the presence of molecules of
another species in the vapour phase? (ii) the presence of molecules of another species in the liquid
phase?
(i) Not at all. Every (ideal) gas behaves as though there are no other gases present i n the
volume it occupies, so, assuming that the vapour we are interested in behaves nearly ideally, the
presence of other gases is of no consequence.
(ii) Molecules can only evaporate when they are at the surface of a liquid. If a second
material is present in a liquid, some of the molecules of the second material will occupy positions on
the surface, thus reducing the number of solvent molecules there, and hence lowering the vapour
pressure.
3.
Rationalize the difference in boiling points for each pair of substances:
(i) HF (20oC) and HCl (-85oC)
F is more electronegative than Cl, so the dipole moment in HF should be greater than that in HCl.
Consequently, the dipole-dipole forces in HF should be stronger than those in HCl. Since these
intermolecular forces must be overcome in order that a liquid molecule can evaporate, we expect
that HF molecules will evaporate less readily and therefore will need to be heated to a higher
temperature to boil.
Alternatively, we could argue that HF will show H-bonding, whereas in HCl there will be only dipoledipole interactions; H-bonding is stronger than dipole-dipole forces, so again HF should have the
higher boiling point.
(ii) CHCl3 (61oC) and CHBr3 (150oC)
We’d expect CHCl3 to have a slightly higher dipole moment than CHBr3, since chlorine is more
electronegative than bromine. However, the boiling points are the wrong way round for this to be
the explanation, which suggests that another factor is more important. This second factor is
presumably dispersion forces, since bromine has many more electrons than chlorine, with
correspondingly greater dispersion forces between them.
(iii) Br2 (59oC) and ICl (97oC).
The two molecules have virtually identical molecular weights, so we’d expect the dispersion forces
to be similar. (Remember, though, that dispersion forces depend upon how many electrons are in
the molecules and not directly on the weight of the molecule. Here we are using the molecular
weight as a kind of proxy for the number of electrons.) However, since ICl combines two different
atoms there must be a non-zero dipole moment in the molecule, so dipole-dipole forces exist. No
such forces are present in the Br2 molecule.
4.
Explain the following observations:
(a) as the temperature increases oil flows faster through a narrow tube
Viscosity falls as the temperature rises. This is because an increasing temperature makes the
molecules move faster. They therefore spend less time interacting with each other and are more
able to break the intermolecular forces that make them stick to one another, so the overall amount
of interaction falls as temperature rises and the viscosity, which is a measure of the amount of
intermolecular interaction, falls also.
(b) the surface tension of CHBr3 is greater than that of CHCl3
Must be the greater dispersion forces once again. The greater the intermolecular forces, the more
the molecules tend to stick together and the greater the surface tension.
(c) small raindrops resting on a waxed surface, such as the body of a new car, are nearly
spherical, whereas oil droplets on the same surface are nearly flat.
Adhesive forces between water and a waxed surface are much lower than the cohesive forces that
exist between water molecules, so the molecules try to gather themselves into the smallest possible
volume, which is a sphere. The reverse is true for oil.
5.
(a)
vaporization?
What relationship exists between the enthalpies of melting, sublimation and
Hsub = Hmelt + Hvap
(b)
changes.
Comment on the corresponding entropy and Free energy changes for these phase
Similar relationships exist for entropy and free energy. Both these properties are state functions: it
doesn’t matter how we get from solid to vapour – the overall change in state function is the same
whether or not the process goes through the liquid phase.
6.
(a)
diagrams?
What is the difference between the triple point and the critical point in phase
At the triple point three phases can co-exist. (Often these three phases are liquid, solid and vapour,
but that need not be the case. There are many examples of phase diagrams in which a couple of
solid phases can co-exist with a liquid phase, or where three different solid phases are in
equilibrium). The critical point is the highest temperature at which it is possible to condense a
vapour by increasing the pressure.
(b)
Why do the phase diagrams of some materials contain several triple points, but only
one critical point?
Because for almost every material there is only one liquid phase (helium is an exception), and for
every material (with no known exceptions) there is only one vapour phase. The critical point marks
the end of the equilibrium line between a vapour and a liquid, and it is not possible to draw a phase
diagram in which two such points exist.
7.
A flask of water is connected to a vacuum pump and the pump is turned on. After a short
time the water starts to boil; after a further period the liquid starts to freeze. Explain both
observations.
Water boils when its vapour pressure equals the external pressure, in other words, the pressure
above the liquid. If this pressure is reduced below the vapour pressure of the water at whatever the
temperature of the water is, the liquid will boil. The vapour pressure of water at room temperature
is around 25 mmHg, a pressure easily achieved by a pump, so it is a simple matter to make room
temperature water boil.
Boiling is endothermic, so as the liquid boils it cools. If the vacuum pump is good, the liquid will
continue to boil even though it is losing heat, because the pressure above the liquid remains less
than the vapour pressure of the liquid; it therefore continues to cool. Eventually this cooling may
bring the temperature of the water down to 0oC, at which point it will freeze.