Lecture Twelve

Question 1

What are intermolecular forces?

Answer 1

Whether something is a solid, liquid of gas depends on its intermolecular forces. This dictates the balance between molecular kinetic energies and intermolecular attractive forces.
When the average KE is large enough, molecules remain separated from one another, and the substance is a gas.
Conversely, when intermolecular attractive forces are large enough, molecules remain close to one another, and the substance is a liquid or a solid.

There are three kinds of intermolecular forces:
Hydrogen bonds,
Dipolar forces,
and Dispersion forces.

Question 2

What are dispersion forces?

Answer 2

Exist because the charge distribution in molecules can be distorted - neighbouring molecules have an influence on each other.
As two molecules approach, the nucleus of one molecule attracts the electron cloud of the other .
electrons are highly mobile, so the electron clouds change shape in response to this attraction.
At the same time, the two electron clouds repel each other, which leads to further distortion to minimise electron-electron repulsion.
This distortion creates a charge imbalance.

Question 3

What are hydrogen bonds?

Answer 3

A hydrogen bond occurs when a small, highly electronegative atom with a lone pair of electrons shares its nonbonding electrons with a positively polarised hydrogen atom.
Two requirements for hydrogen bonds:
1) An electron-deficient atom such as in OH, FH and NH bonds.
2) A highly electronegative atom with an electron pair that can interact with the electron deficient hydrogen atom.
O, N and F, and sometimes S.

Question 4

What are dipole-dipole interactions?

Answer 4

Occurs between polar molecules.

The slightly negative region of one molecules is attracted to the slightly positive region of another.