INTERMOLECULAR FORCES - UCLA Chemistry （分子间作用力,加州大学洛杉矶分校化学）.pdf

___________________________INTERMOLECULAR FORCES___________________________ Intermolecular forces- forces of attraction and repulsion between molecules that hold molecules, ions, and atoms together. Intramolecular - forces of chemical bonds within a molecule Boiling Point and Electronegativity Boiling Point - the temperature at which the liquid form of a compound vaporizes into a gas. - the attractive forces in the liquid must be overcome to become a gas. - boiling point of 0 = no attractive force Trends: Boiling Point example from Bruice 5.2 Explain trend in boiling points of H Halides: HCl, - 85`C; HBr, -67`C; HI, -15`C Answer: Stronger intermolecular forces result in higher boiling points. The dipole moments increase with the polarity of the H-X bond (the greater the difference in EN between the H and halogen atoms, the stronger the dipole-dipole attractions.) The strength of London forces increases with the number of electrons Cl is more EN than I so HCl should have a greater boiling point but this data suggests that London forces dominate dipole-dipole interactions for these molecules since Iodine is larger than Cl. Relative Boiling Points amide carboxylic acid nitrile Ester ~acyl chloride ~aldehyde~ keytone Amides have highest boiling points because they have strong dipole-dipole interactions because of resonance. Electronegativity - the attractive force that an atom exerts on an electron (e-) - closer e- to nucleus = more electronegative - more protons, more positive, more attractive to e- Electronegative difference:  Ionic Bonds = 2.0  Polar Covalent Bonds = 0.5 - 2.0  Nonpolar Covalent Bonds (equal sharing of e-) = 0.5 - Electrostatic Attraction (common in every noncovalent molecular force)= when opposite charges with permanent dipoles attract bonding IONIC_____