Name: _________________________________________
Date:____________________________
Required Paper Homework 2
1.) Given the following types of IMF’s, give a short definition of the IMF, list what is “needed” for that
IMF to occur.
this is the interaction of a charged particle (e.g. a cation)
with another charged particle (e.g. an anion). It is the
strongest of all possible IMF’s.
a. Ion-ion:
An example of this in action would be liquid, or molten
sodium chloride
this is the interaction that happens between a charged
particle (a cation or an anion) with a polar molecule. Two
different types of species must be present.
b. Ion-dipole:
An example of this is dissolving sodium chloride (ionic
species to create ions) in water (a polar species)
this is an interaction that occurs when polar
molecules interact. It can be a pure substance (all the same
type of molecule, e.g. all HCl) or it could be a mixture of polar
molecules (e.g. HF and HCl)
c. Dipole-dipole:
An example of this is a solution of hydrochloric acid (HCl)
is an interaction that occurs when you have an O-H,
N-H, F-H bond in a molecule AND also an O, N, or F with
lone pairs of electrons. This is a special subset of dipoledipole IMFs because you only get H-bonding with polar
molecules
d. H-bonding:
An example of H-bonding occurs in water.
this is an interaction that occurs when you have
a charged particle (the ion, + or -) and a non-polar molecule.
The ion induces, or creates a dipole in the non-polar
molecule, thus creating an attraction between the two species.
e. Ion-induced dipole:
This type of interaction is seen in your body with hemoglobin
for instance
f.
this is an interaction that occurs when you
have a polar molecule and a non-polar molecule. The polar
molecule induces, or creates, a dipole in the non-polar
molecule, thus creating an attraction between the two species.
Dipole-induced dipole:
This type of attraction is seen when you have dissolved O2
gas in water. O2 is non-polar, water is polar.
this type of attraction is
seen in all molecules but is the only IMF present in non-polar
species. Size of the molecule comes into play- with the larger
molecules being more polarizable (easier to create regions of
partial positive and partial negative charges) than smaller
molecules. Shape is also important- the more points of
contact there are, the stronger the two molecules interact with
one another.
g. London/dispersion (induced dipole-induced dipole):
This type of attraction is seen in long chain organic molecules
that contain only C’s and H’s.
2.) Why are dipole-dipole interactions weaker than ion-dipole interactions?
Dipole-dipole forces are made up of partial charges (less than 1). While ion-dipole
forces are made up of an ion of full charge and a partial charge. The full charge partial
charge is a stronger attraction than a partial partial. Also, the full charge will tend to
distort the dipole even MORE so creating a stronger attraction
3.) Two molecules – one polar the other nonpolar – have similar molecular masses. Which one is more
likely to have the higher boiling point? WHY?
If the two molecules are of similar size, they will have London forces of similar strength.
For a non-polar molecule, London is the ONLY force that the molecule has. A polar
molecule, however, will have both London and dipole-dipole IMFs. This extra/stronger
attraction will keep the molecules more tightly together requiring more heat to disrupt
leading to a higher boiling point
4.) Why do the strengths of London (dispersion) forces generally increase with increasing molecular
size?
London forces generally increase with increasing size because larger electron
clouds are more polarizable (easier to distort). The larger electron clouds are
farther from the nuclei, so they are less “tightly” bound and more easily distorted
creating a dipole much easier than smaller molecules.
5.) You have two compounds: NH3 and CH3OH. Draw Lewis structures for BOTH molecules. List
every IMF that each species has – and then determine which species has the highest boiling point.
Write a brief explanation for your choice.
Ammonia: Has the ability to H-bond in 4 total places (is polar)
Has LDF forces (mm = 17.01 grams/mole)
Methanol: Has the ability to H-bond in 3 total places
Has LDF forces (mm = 32.05 grams/mole)
Even though ammonia has 4 total places it could H-bond – compared to the three
locations on methanol, there is no guarantee it can use all 4. And the size of
methanol is much greater (nearly twice the size) which means that methanol will
be subject to significantly stronger LDF’s than ammonia. Given the similarities
with the H-bonding IMF with the discrepancy lying with the LDF force, methanol
should have the higher b.p. due to the much larger LDF’s present in the molecule