Intermolecular Forces Relative Magnitudes of Forces The types

Intermolecular Forces

Relative Magnitudes of Forces The types of bonding forces vary in their strength as measured by average bond energy. Strongest Covalent bonds (400 kcal/mol) Hydrogen bonding (12 -16 kcal/mol ) Dipole-dipole interactions (2 -0. 5 kcal/mol) Weakest London forces (less than 1 kcal/mol)

Hydrogen Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen Hydrogen bonding between ammonia and water

Hydrogen Bonding in DNA Thymine hydrogen bonds to Adenine T A

Hydrogen Bonding in DNA Cytosine hydrogen bonds to Guanine C G

Dipole Related Forces Between Ion - Dipole Ion charge and dipole charge H - bonding Dipole - Dipole Ion – induced dipole Dipole – induced dipole Decreasing Strength Type Polar bonded hydrogen and dipole charge Dipole charge and dipole charge Ion charge and polarizable e- cloud Dipole charge and polarizable e- cloud Stronger than dipole-related forces: Ionic, metallic, and covalent bonding Weaker than dipole-related forces: London dispersion forces

London Dispersion Forces The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors. Fritz London 1900 -1954 London forces increase with the size of the molecules. Synonyms: “London forces”, “dispersion forces”, and “dispersion-interaction forces”

London Dispersion Forces

London Forces in Hydrocarbons

Boiling point as a measure of intermolecular attractive forces