Intermolecular Forces

Molecular Substances “intra”= within the molecule. vs. “inter”= between molecules. For molecular compounds, the intramolecular forces are STRONG covalent bonds. The intermolecular forces are WEAK; they are not bonds.

Types of Intermolecular Forces 1. London dispersion forces 2. Dipole-dipole forces 3. Hydrogen bonding forces

London Dispersion Forces As the electrons move about the molecule, they are usually spread out in a balanced manner.

London Dispersion Forces This balanced arrangement of the electrons causes the molecule to have no permanent positive or negative end.

London Dispersion Forces How do molecules without permanent positive and negative ends attract each other? ?

London Dispersion Forces In their random movements, the electrons sometimes are NOT distributed in a balanced manner around the molecule! When this happens, a temporary polar molecule is formed! δ- δ+

London Dispersion Forces London dispersion forces are weak intermittent forces between nonpolar molecules. A temporary polar molecule will cause neighbor-ing nonpolar molecules to become temporary polar molecules. δ+ δ- δ+ δ-

London (dispersion) Forces Caused by instantaneous dipoles. This is caused by the constant motion of electrons. At any instant, there will be an uneven distribution of the electrons in a molecule. ALL molecular compounds and all noble gas elements exhibit dispersion forces. The strength of dispersion forces depends on 2 factors. # of electrons. How spread out a molecule is. See board. Molecules that are more spread out or have more electrons are “more polarizable”. Molecules that are “more polarizable” have stronger dispersion forces.

Example -- Explain why I2 has a higher melting point than F2. Both I2 and F2 are made up of nonpolar molecules that only have London’s forces between them. Since I2 has more electrons than F2, I2 is more polarizable. This results in I2 having stronger London’s forces and a higher melting point.

Dipole-dipole forces Dipole: one side of a molecule is positive and the other side is negative. Dipole-dipole forces are caused by a permanent dipole in a molecule. Ex: H-Cl Chlorine has a stronger attraction for the electrons that are “shared”.

Dipole-dipole forces Dipole-dipole forces only occur in polar molecules. The dipole in one molecule is attracted to the dipole in a nearby molecule.

Example – Explain why H2Se has a higher boiling point than GeH4. Both H2Se and GeH4 have London’s forces between molecules. Because they have the same number of electrons, the London’s forces will be similar in strength. Because H2Se is a polar molecule, there are also dipole-dipole forces between its molecules. Because H2Se has more forces holding its molecules together, it requires a higher temperature to boil.

Hydrogen Bonding Forces A unique force that ONLY occurs between Hydrogen in one molecule and Nitrogen, Oxygen or Fluorine in a neighboring molecule. Not as strong as a bond, but is significantly stronger than the London’s and dipole-dipole forces.

Strength of Attraction Bonds: Covalent, Ionic, Metallic. ----- Hydrogen Bond Forces Van der waal’s (dipole-dipole & dispersion) Increasing Strength

Example – Which will have the higher boiling point? Kr or Ne Both Kr and Ne are noble gases that only have London’s dispersion forces attracting the atoms. Kr is much larger with more electrons making it more polarizable. Therefore, Kr has the higher boiling point due to its stronger dispersion forces.

Example – Which will have the higher boiling point? H2O or H2S The attractive forces between molecules of H2O (dispersion, dipole-dipole, and hydrogen bonding) are stronger than the attractive forces between molecules of H2S (dispersion and dipole-dipole). Stronger forces require higher temperatures to break, so H2O will have the higher boiling point.