Role of Forces in the Liquid Phase

What are three main intermolecular forces that exist among liquids?

1. Dipole interactions: A dipole is formed when an unequal distribution of electrons occurs in a molecule due to the different electronegativities of their individual atoms. When a dipole exists in a liquid, the positive ends of the molecules will align themselves with the negative ends of other molecules.
   • Acetone is a good example of a polar molecule because the very electronegative oxygen `pulls' the electrons more towards its side, creating a dipole allowing for dipole-dipole interactions.

     Figure 34.5: $e^{-}$ charge distribution in acetone.

     

2. Hydrogen bonding: The strongest of the listed intermolecular forces; which exist between hydrogen and lone pairs of electrons found on N, O, or F (Remember: ``E-N.O.F. bonds exist,'' pronounced ''Enough bonds exist.''
   • Boiling points (b.p.) are directly related by the number of hydrogen bonds that a molecule can form in its liquid state. For example, water can form up to four bonds, whereas hydrogen fluoride (HF) can only form one. Therefore, water has a higher b.p.^35.1
   • Hydrogen bonds are the reason why ice floats: The crystalline lattice of ice is comprised of a regular array of hydrogen bonds which space the water molecules farther apart than they are in liquid water. This in turn causes the density of water to decrease upon freezing.

     Figure 34.6: Hydrogen bonding in water.

     

3. Van der Waals forces (aka London dispersion forces): The weakest force among the ones listed. These forces occur when the electron distribution of a molecule momentarily changes when the molecule interacts with other molecules, producing an imbalance of electron distribution around a nucleus. The part where electrons concentrate is more negative while the other part is more positive. Noble gases exhibt this property and the electron cloud is distributed in such a fashion that appears oval. Nevertheless, in theory, all molecules are able to possess Van der Waals forces and they are the weakest force among the ones listed above.
   • Van der Waals forces are transient
   • This is what allows geckoÕs to stick on glass, leaves, etc.!