Intermolecular Forces and Properties

3.1 Intermolecular Forces

Essential Knowledge:

*These are taken directly from College Board

• London dispersion forces are a result of the Coulombic interactions between temporary, fluctuating dipoles. London dispersion forces are often the strongest net intermolecular force between large molecules. 

  • • Dispersion forces increase with increasing contact area between molecules and with increasing polarizability of the molecules. 

    • The polarizability of a molecule increases with an increasing number of electrons in the molecule; and the size of the electron cloud. It is enhanced by the presence of pi bonding. 

    • The term “London dispersion forces” should not be used synonymously with the term “van der Waals forces.” (SAP-5.A.1)  

• The dipole moment of a polar molecule leads to additional interactions with other chemical species. 

  • • Dipole-induced dipole interactions are present between a polar and nonpolar molecule. These forces are always attractive. The strength of these forces increases with the magnitude of the dipole of the polar molecule and with the polarizability of the nonpolar molecule. 

    • Dipole-dipole interactions are present between polar molecules. The interaction strength depends on the magnitudes of the dipoles and their relative orientation. Interactions between polar molecules are typically greater than those between nonpolar molecules of comparable size because these interactions act in addition to London dispersion forces. 

    • Ion-dipole forces of attraction are present between ions and polar molecules. These tend to be stronger than dipole-dipole forces. (SAP-5.A.2 )  

• The relative strength and orientation dependence of dipole-dipole and ion-dipole forces can be understood qualitatively by considering the sign of the partial charges responsible for the molecular dipole moment, and how these partial charges interact with an ion or with an adjacent dipole. (SAP-5.A.3) 

• Hydrogen bonding is a strong type of intermolecular interaction that exists when hydrogen atoms covalently bonded to the highly electronegative atoms (N, O, and F) are attracted to the negative end of a dipole formed by the electronegative atom (N, O, and F) in a different molecule, or a different part of the same molecule. (SAP-5.A.4 )

• In large biomolecules, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule. (SAP-5.A.5)