Intermolecular Associations

Intermolecular forces or intermolecular associations (IMAs)


Inter≠Intra
Intra-molecular hold the molecule together, like the C-C bonds in sugar or the H-O bonds within the water molecule. These are very strong. However, the strength of the intra-molecular bond has nothing to do with that of the intermolecular interaction (the thing holding the water molecules together in your glass). Intermolecular forces will determine things like the melting point and boiling point. The greater the intermolecular forces, the higher the melting and boiling points.
Intermolecular forces are much weaker than covalent bonds:
This is important...meaning I’m going to ask about it: Ionic crystals are different from covalent solids, in general. In an ionic compound, the inter-ionic forces also hold the crystal together. So, for ionic solids, the lattice energy does correlate with melting point, etc.
For covalent compounds (except for some special cases like diamonds), the covalent bonds do not hold the crystal together. Weaker forces do. So
• Covalent bond strength is not related to melting point or boiling point and
• melting points and boiling points are generally lower for covalent compounds than for ionic solids...because the intermolecular forces are weak.

Types of forces:


We are only dealing with a limited number of these: Dipole with another dipole; dipole with an ion (practically the same thing); Hydrogen bonds (a special form of dipole-dipole) and dispersion forces. So, really there are only two things: dipole-based forces and dispersion forces. Dispersion forces are temporary dipoles induced either by dipoles, ions, or just random fluctuations. So,
really, we are just dealing with one thing: dipoles.

How do these work?

Dipole:


To have a permanent dipole, you need two things: a polar bond; and an asymmetric geometry. Any time there is a difference in the electronegativity, there is a polar bond.
However, you need to do the Lewis structure complete with the bond angle estimates, to determine whether the charges balance out, or end up with electrons on only one side of the molecule. For example, compare three molecules with tetrahedral sites we have drawn: CH
4; NH3 and H2O.

Of these, CH4 is non polar because the electrons are all being drawn to the C and there is no negative side of the molecule. NH
3 and to an even greater extent, H2O are polar.
These things line up with their negative ends against their positive ends. This leads to a fairly strong strong interactions.
If you mix an ion with any of these polar compounds, then the negative side of, say, water lines up with the positive ion and the positive (hydrogen end) lines up with the negative ion. This is an “ion-dipole.”
If I mix water and NH3, they will pair up with the negatives lined up with the positives in the mixture.

Induced dipole


This can occur whenever you put a dipole or ion next to a non-polar molecule. That would be then either a dipole-induced or an ion-induced dipole. These are temporary.

Random dispersive forces, also called London Dispersion forces, occur just when there are random fluctuations in the electron distribution that leads to other neighboring molecules polarizing. This is what I described as electrons "sloshing" around.
These are very fleeting. However, in a population of molecules, you can have significant induced dipoles.
These generally correlate with the size of the molecule. The more electrons and the more dispersed the “cloud,” the more you can get random fluctuations and temporary dipoles.
As a result, if you look at melting points of the di-atomic halogens F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid. This is because with increasing electron cloud, you get stronger dispersive forces and therefore higher melting point and boiling points.
The other thing that affects this is the surface area.
What forces do you think hold CO2 together? Is it polar? If not, what is left?
Can you explain why N2 has a lower boiling point than O2?

The Lab:



We are going to do a graphing lab. There is no "pre-lab."
The exercise here was intended to look for patterns.