Intermolecular Forces

The Kinetic-Molecular Description of Liquids and Solids

The physical properties of a substance depends upon its physical state.

Water vapor, liquid water and ice all have the same chemical properties, but their physical properties are considerably different.

Covalent bonds determine

• molecular shape
• bond energies
• chemical properties

Intermolecular forces (non-covalent bonds) influence

• physical properties of liquids and solids

Gases

• A collection of widely separated molecules
• The kinetic energy of the molecules is greater than any attractive forces between the molecules
• The lack of any significant attractive force between molecules allows a gas to expand to fill its container
• If attractive forces become large enough, then the gases exhibit non-ideal behavior

Liquids

• The intermolecular attractive forces are strong enough to hold molecules close together
• Liquids are more dense and less compressible than gasses
• Liquids have a definite volume, independent of the size and shape of their container
• The attractive forces are not strong enough, however, to keep neighboring molecules in a fixed position and molecules are free to move past or slide over one another

Solids

• The intermolecular forces between neighboring molecules are strong enough to keep them locked in position
• Solids (like liquids) are not very compressible due to the lack of space between molecules
• If the molecules in a solid adopt a highly ordered packing arrangement, the structures are said to be crystalline

The state of a substance depends on the balance between the kinetic energy of the individual particles (molecules or atoms) and the intermolecular forces

• Kinetic energy keeps the molecules apart and moving around, and is a function of the temperature of the substance
• Intermolecular forces try to draw the particles together

• Solids and liquids have particles that are fairly close to one another, and are thus called "condensed phases" to distinguish them from gases

Temperature

• Heating and cooling can change the kinetic energy of the particles in a substance, and so, we can change the physical state of a substance by heating or cooling it.
• Cooling a gas may change the state to a liquid
• Cooling a liquid may change the state to a solid

Pressure

• Increasing the pressure on a substance forces the molecules closer together, which increases the strength of intermolecular forces
• Increasing the pressure on a gas may change the state to a liquid
• Increasing the pressure on a liquid may change the state to a solid

1996 Michael Blaber