Structure and bonding: 2.40 - Intermolecular forces

Syllabus ref: 4.4

In is extremely important to differentiate between the terms 'intermolecular' and 'intramolecular'. Intermolecular means from one molecule to another. When intermolecular forces are overcome the molecules separate from one another. This happens when a liquid boils, for example. The molecules themselves remain integral and unaffected. The only thing that happens is that they cease to be attached to one another.

Intramolecular forces hold the molecule itself together; these are normally called chemical bonds, although there are cases of hydrogen bonding from one part of a molecule to another part of the same molecule.

Nature of science:

Obtain evidence for scientific theories by making and testing predictions based on them. London (dispersion) forces and hydrogen bonding can be used to explain special interactions. For example, molecular covalent compounds can exist in the liquid and solid states. To explain this, there must be attractive forces between their particles which are significantly greater than those that could be attributed to gravity

Understandings

Intermolecular forces include London (dispersion) forces, dipole-dipole forces and hydrogen bonding.

The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds.

Applications and skills

Deduction of the types of intermolecular force present in substances, based on their structure and chemical formula.

Explanation of the physical properties of covalent compounds (volatility, electrical conductivity and solubility) in terms of their structure and intermolecular forces.

In Chapter 2.40