Intermolecular Forces Ch. 16: Liquids and Solids Intermolecular Forces

Intra- vs. Inter- intramolecular forces intermolecular forces inside molecules hold atoms together into molecule intermolecular forces between molecules get weaker as phase changes from S – L – G When a substance changes state, molecule stays together but intermolecular forces weaken

Dipole-Dipole Attraction when molecules with dipole moments line up to minimize repulsion and maximize attraction very weak compared to covalent and ionic bonds

Hydrogen Bonding happens between H and N, O, or F very strong type of dipole-dipole attraction because bond is so polar because atoms are so small

London Dispersion Forces in every molecular compound only important for nonpolar molecules and noble gas atoms weak, short-lived caused by formation of temporary dipole moments

Practice which has highest boiling pt? HF, HCl, or HBr? Identify the most important intermolecular forces : BaSO4 H2S Xe C2H6 P4 H2O CsI ionic dipole-dipole H-bonding London Dispersion

Which has stronger IMF’s? CO2 or OCS CO2: nonpolar so only LD OCS: polar so dipole-dipole PF3 or PF5 PF3: polar so dipole-dipole PF5: nonpolar so only LD SF2 or SF6 SF2: polar so dipole-dipole SF6: nonpolar so only LD SO3 or SO2 SO3: nonpolar so LD only SO2: polar so dipol-dipole

Liquids and Solids

Properties of Liquids low compressibility no rigidity high density compared to gases beads up as droplets molecules on surface are subject to attractions from side and below surface tension resistance to an increase in surface area greater tension means stronger IMF’s

Properties of Liquids capillary action viscosity spontaneous rising of a liquid up a tube from cohesive forces (IMF’s) and adhesive forces (between liquid and container) viscosity measure of a liquid’s resistance to flow strong IMF’s  highly viscous large, complex molecules  highly viscous

Practice Quiz