Intermolecular Forces (4.3.1)

Dipole-Dipole Forces Dipole-dipole forces exist between neutral polar molecules. Polar molecules need to be close together. There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble. If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity.

Dipole-Dipole Forces

Dipole-Dipole Forces

London Dispersion (van der Waals) Forces Weakest of all intermolecular forces. It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). For an instant, the electron clouds become distorted. In that instant a dipole is formed (called an instantaneous/temporary dipole).

One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). The forces between instantaneous dipoles are called London dispersion forces.

Polarizability is the ease with which an electron cloud can be deformed. The larger the molecule (the greater the number of electrons) the more polarizable. London dispersion forces increase as molecular weight increases. London dispersion forces exist between all molecules. London dispersion forces depend on the shape of the molecule.

The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces between spherical molecules are lower than between sausage-like molecules.

London Dispersion Forces

London Dispersion Forces

Hydrogen Bonding Special case of dipole-dipole forces. By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high. Intermolecular forces are abnormally strong.

H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). Electrons in the H-X (X = electronegative element) lie much closer to X than H. H has only one electron, so in the H-X bond, the + H presents an almost bare proton to the - X. Therefore, H-bonds are strong.

Hydrogen Bonding

Hydrogen bonds are responsible for: Ice Floating Solids are usually more closely packed than liquids; Therefore, solids are more dense than liquids. Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water. In water the H-O bond length is 1.0 Å. The O…H hydrogen bond length is 1.8 Å. Ice has water molecules arranged in an open, regular hexagon. Each + H points towards a lone pair on O.

Summary of Intermolecular Forces Ionic Compound – contains positive ions (usually metals) and negative ions. Held together by ionic bonds. Covalent Compound – contains 2 or more nonmetals (no ions) Nonpolar – contains only London dispersion forces (LDF) Polar – contains LDF and dipole-dipole forces Polar with H bonded to N, O, or F (with unshared pair) – contains LDF, dipole-dipole forces, and hydrogen bonds. Larger molecule, stronger LDF (all other factors equal) Mixture – ions mixed with polar molecules – contains ion-dipole forces (very strong) – like Na+ and Cl- ions in water.

Examples – determine the types of forces present in each: H2O CCl4 SO2 LiF Ca(NO3)2 aqueous solution HF PCl3

Examples – determine the types of forces present in each: H2O LDF, dipole-dipole, H-bonds CCl4 LDF SO2 LDF and dipole-dipole LiF ionic bonds Ca(NO3)2 aqueous solution ion-dipole forces HF LDF, dipole-dipole, H-bonds PCl3 LDF and dipole-dipole

Relative Strengths of Forces Bonds (covalent, ionic, metallic) Hydrogen bonds Dipole-dipole forces London dispersion forces