Liquid State

 

We have seen that the molecules in a gas in constant random motion. The spaces between them are large and the intermolecular attractions negligible.

However, in a liquid the molecules are in  contact with each other. The forces of attraction between the molecules are strong enough to hold them together. All the same, the molecules are able to move past one another through available intermolecular space.

The molecules in a liquid move in a random fashion. At any other instant the molecules may from clusters, leaving vacant space or 'hole' here and there.

A liquid may be difined as:  A collection of a molecules held close to each other and executing random motion through intervening spaces.

Most of the physical properties of a liquid are actually controlled by the strength of intermolecular attractive forces.

Therefore, before discussing the properties of liquid, we must have  look at nature of intermolecular forces.

Intermolecular Forces in Liquids

Intermolecular forces in liquids are collectively called van der Waals forces a. Therefore forces are essentially electrical in nature and result from the attraction of changes of opposite sign.

Molecules model of a liquid with holes indicated.

The principal kinds of intermolecular attractions are :

1. Dipole - dipole attractions
2. London Dispersion Forces
3. Hydrogen bonding

Dipole - Dipole Attractions

Dipole - dipole attractions exist between molecules that are polar. This requires the presence of polar bond and an unsymmetrical molecules. These molecules have a permanant separation of positive and negative charge. In the illustration the H of HCl is a permanent slightly positive charge. The Cl end of HCl has a permanent slight negative charge. The H atom in one molecule is attracted to the Cl in a neighbor. The intermolecular force is weak compared to a covalent bond, but this dipole- dipole interaction is one of the stronger intermolecular attractions.

London Dispersion Forces

London dispersion forces exist in non polar molecules. These forces result from temporary charge imbalances. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. The nucleus of one atom attracts electrons in the neighbouring atom.At the same time, the electrons in one particle repel the electrons in the neihbor and  create a short listed lived charge imbalance .

Hydrogen Bonding

Hydrogen bonding is a unique type of intermolecular attraction. There are two requirements:

1. Covalent bond between a H atom and either F, O, or N. These are the three most electronegative elements.
2. Interaction of the H atom ⚛️ in this kind of polar bond with a lone pair of electrons on a near by atom luke F, O or N.