Intermolecular Forces

Outline

• How Molecules Exert Forces on Each Other
  • Electrical Forces...Again
  • Permanent Charges
  • Permanent Dipoles
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

How Molecules Exert Forces on Each Other

We've been looking at gases, where we've assumed that the molecules mostly ignore any electromagnetic or gravitational forces from each other. As a result, we were able to use the ideal gas law to estimate the behavior of the gases as a whole, assuming that the only thing we had to worry about was kinetic energy exchanges in elastic collisions.

Electrical Forces...Again

Now we look at situations where the molecules of a substance do interact: in liquids (where the interactions are constantly changing) and in solids (where the kinetic energy is minimized). The force in any of the situations below depends on Coulomb's law:

$$F_e=\frac{k{q}_1{q}_2}{r^2}$$

where q₁ and q₂ are the net charge differences centered at two points in space, and r is the distance between them.

It is important to keep clearly in mind that we are talking about interactions between molecules here, not the chemical bonds that hold atoms together within the molecule. In both cases, however, the source of the force is the same: electrical charges create the fields that provide the attractive (or repulsive) forces between units.

In these interactions, the forces are primarily electrostatic, that is, due to differences in electrical charges situated in the particles themselves. Gravitational forces between such small masses are negligible at the average distances between molecules--the particles would have to be much closer (and overcome the repulsive forces of their positive nuclei) before mass attractions would affect their behavior. But even where molecules are not held together by covalent bonds, they can still associate because of temporary or permanent polarities which result in electrical attractions.

Permanent charges: ions

An ion is a permanently charged particle (either mono or polyatomic), because it has an unequal number of protons and neutrons. Such a particle has a permanent non-zero charge, and a very strong attraction for an oppositely charged ion or repulsion for a like-charged molecule. The charge difference between ions with at least one extra electron or proton is so great that the forces create a non-molecular structure called a lattice. Technically, ion-ion interactions are not intermolecular forces, because the resulting solid is not composed of discrete molecules. The only way to pull it apart is to dissociate the individual ions.

Permanent Dipoles: polar covalent bonds

Dipoles occur when the charge in a single molecule is unevenly distributed. One side of the molecule will have more negative charge, the other side more positive charge. This actually happens fairly frequently as electrons move around the electrons in a molecule and are attracted first to one or the other nucleus, and repelled by electrons around one or more atoms.

A permanent dipole in the molecule occurs when the atoms of the elements involved have different electronegativities — that is, they pull on the electrons of the other atoms with different amounts of force. Elements on the right side of the periodic table are strongly electronegative while elements on the left tend to be weakly electronegative. So in a CO molecule, for example, the O pulls the electrons in the molecule more than the C does, and the C winds up with a slight positive charge while the O winds up with a slight negative charge. Obviously, if we get two molecules with dipoles together, they will orient themselves so that their + and - charges line up +- +- (like two N-S magnets will orient themselves so they are N-S N-S) with the opposite forces attracting one another.

In the case of hydrogen, we have special considerations. Hydrogen units with carbon and most other elements in a primarily covalent bond, but when it units with nitrogen, oxygen, or fluorine, the strong electronegativity of these elements forces hydrogen's single electron to stay between the two atoms. This leaves the single positively-charged proton in the nucleus unshielded, since hydrogen has no other electrons. The positive side of the hydrogen atom is free to attract passing negative dipoles and ions almost as though it were an independent, charged ion. Such attractions form H-bonds.

Dipole-ion interactions are the strongest intermolecular forces after ion-ion interactions. Next in strength are dipole-dipole interactions, which include H-bonds.

Practice with the Concepts

Hydrogen Bonding

Which substance is most likely to have hydrogen bonds: CH₄, CH₃OH, H₃COCH₃, or H₂C=CH₂

Discussion Questions

• Are molecules with permanent dipoles always more tightly bonded than molecules with one induced dipole?
• What factors affect the strength of dipole forces?

Optional Readings

Check out the Intermolecular Forces discussion at the free chemistry textbook site for more diagrams and examples.

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