2 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 bondingGiven the structural formula of a compound, indicate all the intermolecular forces present
3 Intermolecular Forces Melting point, boiling point, evaporation, surface tension, and solubility are related to the strength of attractive forces between moleculesMolecules are often attracted to each other by intermolecular forcesThese forces are much weaker than ionic or a covalent bond
4 1. Dispersion Forces Holds two nonpolar molecules together The diatomic halogen molecules (ex. F2,Cl2, Br2, I2) are held together by dispersion forcesThe weakest intermolecular forceStrength generally increases as the number of electrons in a molecule increaseWhen forces are strongest, the compound will be a solidWhen the forces are weakest, the compound will be a gasWhich halogens would you expect to be solids? Liquids? Gases? Explain.
5 2. Dipole-Dipole Interactions Occur between oppositely charged ends of polar moleculesThe slightly positive region of a polar molecules is weakly attracted to the slightly negative region of another polar moleculeThe interactions between two molecules of hydrochloric acid is an example of this type of force
7 3. Hydrogen Bonds The strongest intermolecular force Hydrogen bonds are not truly bonds, they are forcesHydrogen bonding always involves hydrogenHydrogen atoms involved in hydrogen bonds must be also be attached to a F, O, or N
8 Water vs. AcetoneWater molecules can attract each other through hydrogen bonds, but acetone can'tWHY?This is because it has no H's that are bonded to F, O, or N