Intermolecular forces

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. (2.2)

• 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.

Guidance:

• The term “London (dispersion) forces” refers to instantaneous induced dipole- induced dipole forces that exist between any atoms or groups of atoms and should be used for non-polar entities. The term “van der Waals” is an inclusive term, which includes dipole–dipole, dipole-induced dipole and London (dispersion) forces.

• The nature of the hydrogen bond is the topic of much discussion and the current definition from the IUPAC gives six criteria which should be used as evidence for the occurrence of hydrogen bonding. How does a specialized vocabulary help and hinder the growth of knowledge?

Utilization:

• Syllabus and cross-curricular links:
• Option A.5—using plasticizers
• Option A.7—controlling biodegradability
• Option B.3—melting points of cis-/trans- fats
• Biology topics 2.2, 2.3, 2.4 and 2.6—understanding of intermolecular forces to work with molecules in the body

Aims:

• Aim 7: Computer simulations could be used to show intermolecular forces interactions.