Intermolecular Forces of Attraction

Key Questions

  • Polar molecules dissolve in polar solvents (ex. water) and non-polar molecules will dissolve in non-polar solvents (ex. hexane).

    The type of forces that exist between neighboring molecules will be determined by the properties of those molecules.

    Polar molecules will be attracted to each other by either hydrogen bonding or dipole-dipole interactions. These intermolecular forces are made possible by a large difference in electronegativity values for two atoms bonded to each other.

    In water, the electronegativity difference between oxygen (3.5) and hydrogen(2.1) is 1.4 (3.5-2.1=1.4). This, and waters bent shape, make water a polar molecule. Another polar molecules is ammonia (#NH_3#), whose trigonal pyrimidal shape and electronegativity different in N-H bonds of 0.9 make this substance soluble in water.

    Non-polar molecules are attracted to each other by London forces (dispersion) and either do not have dipoles (ex. #CH_4#), or they have multiple dipoles which cancel each other out due to their geometry (#CO_2# is nonpolar because its linear shape makes the molecule nonpolar due to the two dipoles O=C=O cancelling each other out).

  • They are not really different because they are all electrical in nature.

    Forces between ions tend to be a lot stronger than forces between molecules, because ions are formally charged, while molecules are electrically neutral.

    Na+   Cl-   (strong attraction between formal charges)

    The following is an example of an intermolecular force. Below, each H-Cl molecule is electrically neutral (each Cl has 3 lone pairs of electrons not drawn in) Since Cl is more electronegative than H, the electrons shared between H and Cl are more likely near the Cl atom. This makes the Cl end of the molecule partially negative, and the H end partially positive.

    The intermolecular force below is represented by the dotted line. It is an attraction between the "kind of negative" Cl of one HCl molecule and the kind of positive H on a different HCl.

                           H-Cl  - -  - - -  H-Cl

    It is electrical just like the ionic attraction, but since neither the H or Cl are formally charged, this dipole-dipole attraction is weaker.

  • Intermolecular forces occur whenever two particles come close to each other.

    Intermolecular forces between molecules include hydrogen bonds and dipole-dipole attractions. Dipole-induced dipole forces and London dispersion forces are weaker forces.

    If ions are present, we can have ion-ion, ion-dipole, and ion-induced dipole attractions.

    Some of these forces are stronger than others. Some of them decrease more rapidly with increasing distance.

    Even so, the forces that can occur will always occur when the particles are close enough.