16.3: Attractions Between Molecules

Objectives Objectives: Compare the relative strengths of forces between molecules based on the melting point and boiling point of the substances. Compare the strength of the forces of attraction between molecules of different elements. Identify the elements necessary for hydrogen bonding Given the structural formula of a compound, indicate all the intermolecular forces present

Intermolecular Forces Melting point, boiling point, evaporation, surface tension, and solubility are related to the strength of attractive forces between molecules Molecules are often attracted to each other by intermolecular forces These forces are much weaker than ionic or a covalent bond

1. Dispersion Forces Holds two nonpolar molecules together The diatomic halogen molecules (ex. F2,Cl2, Br2, I2) are held together by dispersion forces The weakest intermolecular force Strength generally increases as the number of electrons in a molecule increase When forces are strongest, the compound will be a solid When the forces are weakest, the compound will be a gas Which halogens would you expect to be solids? Liquids? Gases? Explain.

2. Dipole-Dipole Interactions Occur between oppositely charged ends of polar molecules The slightly positive region of a polar molecules is weakly attracted to the slightly negative region of another polar molecule The interactions between two molecules of hydrochloric acid is an example of this type of force

3. Hydrogen Bonds The strongest intermolecular force Hydrogen bonds are not truly bonds, they are forces Hydrogen bonding always involves hydrogen Hydrogen atoms involved in hydrogen bonds must be also be attached to a F, O, or N

Water vs. Acetone Water molecules can attract each other through hydrogen bonds, but acetone can't WHY? This is because it has no H's that are bonded to F, O, or N

Hydrogen bonds have about 5% the strength of an average covalent bond This force is extremely important in determining the properties of water such as surface tension