Intermolecular Attractions and the Properties of Liquids and Solids Chapter 12 Intermolecular Attractions and the Properties of Liquids and Solids

Gases, Liquids and Solids

Intermolecular Attractions Intermolecular forces depend on distance Gases have very small attractive forces Solids/liquids have larger attractive forces since molecules are closer to each other Intermolecular forces - attractions between two molecules Intramolecular forces - chemical bonds that hold molecule together Intermolecular forces weaker than Intramolecular forces

van der Waals’ Forces HCl molecules H and Cl atoms held tightly by covalent bond Strength of chemical bond keeps molecule intact Attractions between HCl molecules are weaker (4% as strong) Attraction between molecules determine physical properties Notice disorientation!

Dipole-dipole attractions HCl(g) - polar molecule with partial charges Polar molecules tend to line up so partial negative and near partial positive Still net attraction!! (Dipole-dipole!) Why weak? Charges associated are only partial charges Ordinary temperatures (Thermal energy) causes the dipoles to be somewhat misaligned reducing effectiveness of attractions

Hydrogen Bonds Important Dipole-dipole attraction when hydrogen bonds to very small, highly electronegative atom Think FON (HF, OH, and NH) Why Hydrogen bonding? Ends of bond contain substantial positive and negative charges Charges highly concentrated due to small size Positive ends can get very close to negative of another molecule due to small size

Hydrogen Bonds in Water In Liquid water - molecules experience hydrogen bonds that continually break and re-form As water freezes, molecules become locked and participate in 4 hydrogen bond Resulting structure has larger volume than liquid water Ice cubes float in more dense liquid

London Dispersion forces Nonpolar molecules still have attraction (although weak) to hold substance together 1930 - Fritz London, German Scientist Nonpolar substances can still have attraction Atoms constantly moving Motion in one particle affects neighboring particles Electrons repel and push away At any given moment, the electron density of molecule can be unsymmetrical At particular instant, instantaneous dipole!