Chem 103L
Answers to Problem Set #1
F
F
P
F
F
1. a) What is the dipole moment of PF5 (structure at right)? Explain your answer.
F
The dipole moment is zero. The reason is because the molecule has high symmetry so that, despite liarge
dipoles for each individual bond, when all the bond dipole vectors are considered together, the vectors all
cancel.
b) Which has a larger dipole moment, PH3 or ammonia? Explain your answer.
Both PH3 and NH3 are polar molecules. However, there is a much larger difference in electronegativity
between N and H compared to P and H, therefore the bond dipoles are larger and sum to a larger
molecular dipole moment for ammonia.
2. Philip Ball in his book “Life’s Matrix: A Biography of Water” offers the drawings below to describe the
interactions between water—here depicted as elvish characters— in liquid (left) and in ice (right).
a) Specify what aspects in his drawings correlate with the molecular structure of water as you know it.
That is, how do the elves correctly mimick water molecules? What are their hands? Their feet?
There is, of course, no one correct way to view this. The elves’ bodies are the oxygen atoms of water. I
thought the hands represented the lone pair electrons on O and the hand “grabbed” the feet, i.e. H’s of
adjacent elves (oxygen atoms. That’s because I tend to think of electrons as ‘causing’ the interactions.
But some of you thought the hands were H’s and the feet were lone pairs. In any case, there are two of
each for each elf/oxygen. The picture on the right shows the specific ways elves’ hand/feet interaction
would cause the 6-membered ring in ice.
3. Ball describes one ( of many!) essential roles of water on our planet as a “conveyor belt” for materials
to be circulated throughout the oceans, all over the globe. A related idea is presented in problem 1.63 on
page 65 of your text.
Identify those properties of water that allow the conveyor belt to function to distribute materials—i.e.,
nutrients to feed aquatic life — over the entire planet.
Those features of water that contribute to the circulation of nutrients—chemical—dissolved in water are
largely due to the densities differences of water and ice at various temperatures. So as ice on the surface
melts in warm regions, the water sinks, perhaps going to the bottom just as the slightly warmer but more
dense 4 deg C water did in class. Warmer water would rise and eventually freezer if it moved to frigid
areas. Of course the chemicals dissolved in the water could contribute too to density differences.
4. At the bottom of the ocean are these thermal vents called black smokers where all kinds of crazy
critters live. There’s a whole population of hyperthermophiles, bacteria that live in (hold on to your hats!)
water at 100 deg C. That’s right: they live in boiling water.
Speculate on how the DNA composition, specifically the C-G and A-T content, of hyperthermophilic
bacteria might differ from the bacteria living in your bathroom.
Based on what you learned about H-bonds between DNA bases, one expects that bacteria unexpectedly
stable at higher temperature would have a larger fraction of C-G pairings in DNA over A-T pairing, since
the CG pairs have three H-bonds which contributes to greater stability.
5. Arrange these molecules, all of which contain 4 C atoms, in order of increasing b.p. Explain your
arrangement.
The order, from low to high boiling points, is:
CH3CH2CH2CH3
b.p. -12 deg C.
CH3CH2OCH2CH3
b.p. 35 deg C.
CH3CH2C(=O)CH3
b.p. 80 deg C.
CH3CH2CH2C(=O)OH
b.p. 163.5 deg C.
HOCH2CH2CH2CH2OH
b.p. 230 deg C.
The reasons are: butane is non-polar and has only London dispersion forces attracting molecules
together; diethylether is only slightly polar; butanone has a =O group that increases polarity and the
dipole moment, butyric acid is highly polar and has one –OH to make H-bonds between molecules and
the dihydroxybutane has two –OH groups and so can make more H-bonds, contributing to very strong
interactions between molecules.
1.71(a) Cotton clothing gets wrinkled because it is easy to break some H-bonds and makes others, while
the garment is distorted in shape on the wearer—or in a pile on the floor! When the forces are no longer
acting, the new H-bonds hold the fibers in the positions they had taken during the time the clothing was
worn.
(b) The heat of the iron helps to make the H-bonds break more readily. The mass of the iron forces the
fabric flat so that H-bonds that are remade are now holding the fabric flat. Thus the wrinkles are
removed and the “press” restored.
(c) To make the fabric “permanent press”, you want to eliminate the easily broken H-bonds. One way is
to use reagents that react with —OH groups that are close to one another to form a permanent bond
holding them together.
(d) Since the softness of cotton comes from the ability of the cellulose chains and fiber made from them
to change shape easily, preventing these easy changes will make the fabric less soft. Thus, bonding the
chains together, as suggested in part (c) would probably make the fabric less soft.
1.101(a) Water needs heat from your skin to evaporate. While supplying the heat for water to change
from a liquid to a gas, you feel a cooling effect on your skin.
(b) The molecules in liquids are attracted to one another and stay close together, which is similar to the
way they behave in solids. But describing a liquid as a "disordered" solid could be a bit misleading,
because the molecules in liquids are free to move around from place to place anywhere in the volume
they occupy, while still staying close together.
(c) The fact that the molecules in a liquid can move around from place to place makes them similar to
gases. The molecules in gases are very far apart, moving essentially independently of one another. Since
there are many more molecules in a given volume of a liquid compared to the same volume of a gas,
liquids might be described as "dense” gases. This could be a bit misleading, if we are led to think that
the molecules in the liquid are as independent of one another as they are in the gas phase. The
attractions between molecules in the liquid have to be reasonably large, in order to keep them in the
liquid phase.
(d) Solid water (ice) floats on liquid water because the density of liquid water is greater than the density
of ice. The relatively open structure of the hydrogen-bonded network of water molecules in ice occupies
a larger volume than the same molecules in the liquid phase where some of the hydrogen bonds have
broken.
(e) During condensation of a gas to a liquid, a great deal of energy is released to the surroundings. This
is why you can be badly burned by steam, if it condenses to water on your skin.
(f) When lakes freeze during the winter, ice covers the top of the liquid water. Since ice is less dense than
liquid water, ice floats and does not fall to the bottom of the lake. Thus, to form more (thinker) ice heat
must leave the layer of water just beneath the surface ice. Solid ice is a pretty good thermal insulator, so
this is a slow process.
(g) When water freezes in pipes, the ice expands due to the larger volume of ice compared to liquid water
[see part (d)]. This causes the pipes to break.
1.106 (a) The (FHF)– Lewis structure,
, shows the H atomic core apparently sharing two
pairs of electrons equally with two different F atomic cores. The H atomic core usually shares only one
pair of electrons with another atomic core, or, in other H bonds, the sharing is with a second pair that is
different and weaker than in its covalent bond.
(b) In (FHF)–, the hydrogen atomic core is equidistant from the two electronegative fluorine atomic
cores which it hydrogen bonds together with a rather high bond energy. In the hydrogen bond between
two water molecules, the bond from the H atomic core to one O atomic core is a strong covalent bond
with the bonding pair shared somewhat equally by the O and H atomic cores. The bond to the other
oxygen is about twice as long and quite weak, with the nonbonding pair of electrons still largely
associated with the O atomic core. The (FHF)– ion is likely to be linear, as we have said the strongest
O–H---O bonds are as well. In water the formation of the hydrogen bond stretches the covalent bond
from 94 to 100 pm and the H bond to the other oxygen has a bond length of 180 pm. In the (FHF)– ion,
the covalent bond from F to H in HF is stretched from 93 to 113 pm, which is quite a bit more than for
the water molecule. The length of the bond to the second F atomic core, however, is also 113 pm, which
is substantially shorter than the corresponding bond in water and is associated with the high bond
energy for formation of this bond between HF and F–. The distance between the second period atomic
centers in the water hydrogen bond is 280 pm. The corresponding distance in (FHF)– is 226 pm, which is
very much smaller and is, again, a reflection of the high bond energy. Bonding in the (FHF)– ion
certainly does blur the distinction between a hydrogen bond and a covalent bond. Since the bonds
between the H atomic core and the F atomic cores are identical in this ion, there is no way to tell which
is (or was) the covalent bond and which is the hydrogen bond. The distinction makes no sense for this ion
and suggests that there is a continuum of bonding interactions from weak induced dipole interactions to
strong covalent bonds.