IMF problems to be thinking about . . .
1.) Which of the following pure substances (meaning that is the only species present!)
exhibit H-bonding?
O
C2H6 , CH3OH ,
H3C
C
O
NH2
,
H3C
C
H
Answer for Q1 and 2: we are examining whether or not a species (and only that species) will
participate in H-bonding with itself, we need to have an F-H, O-H, or N-H bond AND we
need a lone pair on an O, N, or F. In the first species there are no F-H, N-H, or O-H bonds in
the molecule. So C2H6 will not participate in H bonding. Methanol and the amide will.
Methanol has an O-H and the O has 2 sets of lone pair electrons. So H-bonding is possible.
In the amide, there is an N-H (2 of them!) so the H-bonding will occur between the H and
the N and also the O has lone pairs of electrons, so the N-H will be able to hydrogen bond
with those lone pair electrons as well.
2.) Where would the substance H-bond . . .
3.) Would the answer be the same if we mixed these species in water? Where is H-bonding
possible in this solvent?
The answer is the same, except now the alcohol and the amide can not only H-bond with
themselves but also with the O-H in the water molecule as well.
4.) For each substance, list the DOMINANT IMF’s present and indicate which species will
have the higher b.p. (boiling point)
a. MgCl2 vs NCl3
In MgCl2 the predominant IMF is the ion-ion interaction between the Mg+2 and the Cl1. In NCl3, the molecule is polar (there is a lone pair on the N, making the molecule
asymmetrical). The dominant IMF in a polar molecule is dipole-dipole. Both species
also have London forces, but that is not the dominant IMF present. Since ion-ion is
stronger than dipole-dipole, MgCl2 should have the higher b.p.
b. CH3NH2 vs CH3F
Both substances are polar, so both are subject to dipole-dipole forces. The first species
(the amine) also has H-bonding possible (the second substance does NOT, since there
is no F-H bond in the molecule, and C-H is not good enough!). The added IMF of
hydrogen bonding in the amine gives it the higher b.p.
c. C2H6 vs C3H8
Both substances are non-polar. There are only C-H bonds present. Both species only
have London forces. Both species contain a chain of carbons, so the larger and longer
the C chain, the stronger the London force is. This means that C3H8 has stronger
London forces so it will have the higher b.p.
5.) How do IMFs affect the viscosity of a substance?
The IMFs tend to hold molecules/atoms/ions together. The more species hold onto one
another, the harder it is for them to “flow”. Viscosity is defined as a species resistance to
flow. The easier the species flows, the fewer IMFs there are holding the
molecule/atoms/ions to one another. If there are a lot of IMFs, then the substance will be
more viscous (and therefore does not flow easily).
6.) Explain why methanol and water have such different b.p.
Both species are polar, so they have dipole-dipole forces. Both species have London forces,
both species have H-bonding. BUT, methanol is only able to form a maximum of 3
hydrogen bonds per molecule while water can (and does!) form 4 hydrogen bonds per
molecule. This extra attraction means that more IMFs must be overcome in order to cause
water to change from a liquid to a gas. The more IMFs there are in a molecule (or the strong
the IMFs are) the higher the temperature needed to cause the phase change. Since water has
an extra H-bond, it will have the higher boiling point.
7.) Which species would you expent to have the higher b.p. F2 or Br2 - explain why based on
the type and strength of the IMFs in each molecule
Both F2 and Br2 are non-polar species, thus the only force that both species are subject to
is the London force. The larger the species, the more polarizable the electron cloud is, so
the greater the partial charge that develops. The greater the partial charges that are
created, the stronger the molecules attraction to one another. Since Br2 is SO much bigger
than F2, its London forces are stronger so it has a higher b.p.
8.) Why are gases more easily compressed than liquids?
Gases are more easily compressed than liquids because they are mostly empty space.
Why are they mostly empty space? Because the molecules/atoms are not attracted to one
another (they do not have IMFs). Since there is a lot of space between the species, they
can be shoved closer (thus they can be compressed) compared to atoms/molecules that
are already close together (the molecules/atoms) that are in the liquid phase.
9.) Based on IMFs, explain why liquids flow and solids do not
Liquids flow because the molecules have IMFs, but the IMFs have not locked the species
in a location. In the solid phase, the molecules/atoms/ions are locked in place due to the
IMFs (the IMFs keep the molecules “stuck” together). Because the IMFs keep the
molecules stuck in place, the IMFs do not allow the molecules/atoms/ions to move out
of their location, so the substance does not flow. In liquids, there are IMFs, but the
species are not locked in place. The IMFs keep the species attracted to one another, but
the IMFs are just weak enough to allow the species to move, and new IMFs are formed
within the substance. The molecules/atoms/ions tumble around which allows the
substance to flow.
10.)
Define/explain polarizability
Polarizability is the creation of a dipole moment in a species as the electrons move
asymmetrically around the nucleus. Not all electrons are located exactly opposite one another.
If electrons are located more on one side of the molecule than another at a particular instant in
time, a dipole moment is created. This will then induce a dipole in a neighboring molecule.
Polarizability increases with the size of the species. The larger the species, the larger the
electron cloud. The larger the electron cloud, the more likely the electrons will not be
symmetrical in the molecule, the more likely a dipole will be created. We see induced dipoles
in non-polar molecules. We see induced dipoles due to a polar molecule or an ion coming near
a polar or non-polar molecule.
11.)
Define/explain polarity
Polarity results from the permanent asymmetrical location of electrons in a molecule that
results in one part of the molecule having “extra” electrons in a certain location on the
substance (near the most electronegative atom that “sucks” the electrons towards itself)
which results in a partial negative charge on that area of the substance. We see the partial
positive charge on the least electronegative atom as that atom “loses” its electrons to the
more electronegative atom in the molecule. Polar molecules and polarity comes from a
molecule that has atoms with different electronegativity values and an asymmetrical
“shape” such that the atom(s) can pull the electron density towards itself creating a dipole
moment.
12.)
What type(s) of IMFs result from polarizability?
Induced dipoles and London forces are the result of polarizability
13.)
What type(s) of IMFs result from polarity?
Dipole-dipole forces result from polarity (as well as dipole-induced dipole forces result
from a polar molecule coming near a non-polar molecule)
14.)
Pentanol (mm = 88.15 g/mole) has nearly the same molar mass as hexane (mm =
86.27 g/mole) but pentanol is more than 12 times as viscous at 20oC. Explain why
Pentanol has a region on the molecule that has an O-H bond. That area of the molecule will
have a dipole, and the O-H bond can participate in H-bonding. So pentanol has London, dipole,
and H-bonding IMFs present. Hexane is a non-polar molecule, so the only IMF that is has is
London forces. Since pentanol has more and stronger IMFs, its molecules are held together
more tightly than hexane. The stronger the species are held on to one another, the more
resistance to flow there is, the more viscous the substance.
15.)
Warm blooded bodies have a narrow range of body temperature because of the
high water content – explain why we have this homeostasis
Since most of the body is made up of water, and water is a polar molecule and also has Hbonding, these are IMFs that must be overcome in order for the water to turn from a liquid
to a gas. There are so many and such strong IMFs in water, it takes a LOT of energy to
cause a phase change, meaning that you can increase the temperature, but nothing really
happens!
16.)
A drooping plant will “pop back up” if it is watered, explain how this happens,
how can a plant get water all the way to the top of its branches/leaves?
Because of the IMFs in water, it has capillary action, and the water molecules can “climb”
up one another pulling itself up the walls of the plant, allowing it to reach the branches
and leaves.