Structure & Bonding – Intermolecular Forces

Condensation and freezing for elements etc

All molecular elements and compounds and monatomic elements condense and freeze at

sufficiently low temperatures. For this to occur, some attractive forces must exist between the

molecules or discrete atoms.

What are the intermolecular forces

Intermolecular forces acting between molecules are known as van der Waals forces. There are

several different types of these, such as London dispersion forces and permanent dipole-

permanent dipole interactions that include hydrogen bonding.

What is London dispersion force

London dispersion forces are forces of attraction that can operate between all atoms and

molecules. These forces are much weaker than all other types of bonding. They are formed as a

result of electrostatic attraction between temporary dipoles and induced dipoles caused by

movement of electrons in atoms and molecules.

What is the LDF strength related to?

The strength of London dispersion forces is related to the number of electrons within an atom or

molecule.

Polar molecule

A molecule is described as polar if it has a permanent dipole.

What can spatial arrangement do to polar covalent bonds

The spatial arrangement of polar covalent bonds can result in a molecule being polar.

Permanent dipole-permanent dipole interactions

Permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction

between polar molecules.

Why are permanent dipole stronger than LDF

Permanent dipole-permanent dipole interactions are stronger than London dispersion forces in

molecules with similar numbers of electrons.

Elements with H bonding

Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element

such as fluorine, oxygen or nitrogen are highly polar. Hydrogen bonds are electrostatic forces of

attraction between molecules that contain these highly polar bonds.

H bond strength

A hydrogen bond is stronger than other forms of permanent dipole-permanent dipole interaction

but weaker than a covalent bond.

How is the MP BP ands viscosity determined?

Melting points, boiling points and viscosity depend on the type and strength of forces between molecules. By looking at a molecule’s polarity and how many electrons it has, you can predict how strong these forces will be.

MP and BP do polar substances compared with non polar

The melting and boiling points of polar substances are higher than the melting and boiling points of

non-polar substances with similar numbers of electrons.

H bonding effect what properties of a substance?

Boiling points, melting points, viscosity and solubility/miscibility in water are properties of

substances that are affected by hydrogen bonding.

H bonding effect on BP

The anomalous boiling points of ammonia, water and hydrogen fluoride are a result of hydrogen

bonding.

H bonding in ice

Hydrogen bonding between molecules in ice results in an expanded structure that causes the

density of ice to be less than that of water at low temperatures.

Ionic compounds solubility compared with polarity

Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as

water, and insoluble in non-polar solvents. Non-polar molecular substances tend to be soluble in

non-polar solvents and insoluble in polar solvents.

To predict the solubility of a compound, key features to be considered are the:

presence in molecules of O-H or N-H bonds, which implies hydrogen bonding

spatial arrangement of polar covalent bonds, which could result in a molecule possessing a

permanent dipole