Intermolecular Forces

Intramolecular Forces: The attractive forces between atoms and ions within a molecule e.g Ionic bond, covalent bonds (e.g. polar, nonpolar, double, triple) Relatively strong Intermolecular Forces: The attractive forces between molecules E.g. Van der Waals forces: London dispersion forces, dipole-dipole forces, hydrogen bonds Weak (in comparison to intramolecular forces) I.e. much less energy to melt H2O (inter) than for it to decompose into H2 and O2 (intra)

Intermolecular Forces & Physical Properties The strength of the intermolecular forces determines: State Melting point and boiling point Surface tension Hardness/texture Solubility

Intermolecular Forces If covalent bonds were the only forces at work, most molecular compounds would be gases as there would be no attraction between molecules strong enough to group the molecules as liquids or solids

van der Waals Forces Dipole-dipole London dispersion Hydrogen bonding

Dipole-dipole Forces of attraction between oppositely charged ends of polar molecules. Relatively strong intermolecular force Dipole-dipole force

London Dispersion Attractive forces between all molecules, including nonpolar molecules Result of temporary displacements of the electron cloud around atoms in a molecule (extremely short-lived dipoles) Therefore weaker than dipole-dipole

Hydrogen Bonding Strong dipole-dipole force between the positive hydrogen atom of one molecule and highly electronegative atom of another molecule (O, N, F) H-bond

Discussion/Recap: Rank the intermolecular forces from weakest to strongest. How might intermolecular forces affect physical state? How might intermolecular forces affect melting point?

Surface Tension Molecules in a liquid are attracted by molecules on all sides But, molecules at the surface are only attracted down or sideways. Penny drop, alka seltzer

Practice Problems Prelab: Intermolecular Forces p. 88 #7-12 Variables Hypothesis Observation table p. 88 #7-12