4.3 Chapter summary

4.3 Chapter summary (ESBMV)

Presentation: 23RD

• Intermolecular forces are the forces that act between molecules.

• The type of intermolecular force in a substance, will depend on the nature of the molecules.

• Polar molecules have an unequal distribution of charge, meaning that one part of the molecule is slightly positive and the other part is slightly negative. The molecule is said to be a dipole.

• Non-polar molecules have an equal distribution of charge.

• There are five types of intermolecular forces: ion-dipole forces, ion-induced-dipole forces, dipole-dipole forces, dipole-induced dipole forces and induced dipole forces.

• Ion-dipole forces exist between ions and polar (dipole) molecules. The ion is attracted to the part of the molecule that has an opposite charge to its own.

• Ion-induced dipole forces exist between ions and non-polar molecules. An ion induces a dipole in the non-polar molecule.

• Dipole-dipole forces exist between two polar (dipole) molecules.

• Dipole-induced dipole forces exist between a polar molecule and a non-polar molecule.

• Induced dipole forces exist between two non-polar molecules.

• Dipole-dipole forces, dipole-induced dipole forces and induced dipole forces are collectively called van der Waals' forces.

• Hydrogen bonds are a type of dipole-dipole force that occurs when a hydrogen atom is attached to a highly electronegative atom (oxygen, fluorine, nitrogen). A hydrogen atom on one molecule is attracted to the electronegative atom on a second molecule.

• Intermolecular forces affect the properties of substances.

• Substances with larger molecules have stronger intermolecular forces than substances with smaller molecules.

• Viscosity is the resistance to flow of a liquid. Substances that are very viscous have larger molecules and stronger intermolecular forces than substances with smaller molecules.

• Density is a measure of the mass in a unit volume. Solids have strong intermolecular forces and so have more molecules in one unit volume.

• Substances with weak intermolecular forces will have low melting and boiling points while those with strong intermolecular forces will have high melting and boiling points.

• Thermal expansion is the expansion of a liquid on heating.

• Thermal conductivity is a measure of how much a material conducts heat.

• Water has strong hydrogen bonds which hold the molecules together. It is these intermolecular forces that give water its unique properties.

• Water has the following properties: a high specific heat, absorption of infrared radiation, a large range in which it exists as a liquid, a high heat of vaporisation and has a less dense solid phase.

• Specific heat is the amount of heat energy that is needed to increase the temperature of a unit mass of a substance by one degree.

• Heat of vaporisation is the energy that is needed to change a given quantity of a substance into a gas.