intermolecular forces: liquids and solids

gen chem 2 exam 1

Van der Waals forces

weak intermolecular forces that arise from temporary dipoles in molecules; include London dispersion forces, dipole-dipole interactions, dipole-induced dipole forces, and hydrogen bonding forces

instantaneous dipole

caused by fluctuations in electron density, leading to temporary positive and negative regions within a molecule; ex: H2 (symmetrical molecule) does not have charge distribution, but at any one instant the 2 e- might be located towards one end of the molecule, forming an instantaneous dipole

induced dipole

when a molecule with a temporary dipole is approached by an unpolarized molecule, causing a temporary charge separation in the latter

dispersion forces

interactions between instantaneous dipole and induced dipole; charge separation, polarity, e- cloud distortion; result of the fluctuation of electron density; impact physical properties such as boiling and melting point

polarizability

the tendency of the electron cloud to distort, describes the ease with which a dipole can be induced

factors affecting polarizability and dispersion forces

1. number of electrons: polarizability/dispersion forces increase as number of electrons/molar mass increases

2. shape of molecule: electrons in elongated molecules are more easily displaced/polarized than those in small, compact, symmetrical molecules with similar masses

3. surface area: increase in SA increased attractive forces

to be considered when determining a permanent dipole:

1. electronegativity: elements with different electronegativities bonded to each other form polar bonds

2. overall shape of molecule: symmetrical = nonpolar

dipole moment

a measure of the extent to which a separation of charge exists in a molecule

dipole vectors

which direction is the dipole moment? (arrow points towards more electronegative atom)

dipole-dipole interactions

molecules line up based on partial charges; opposite poles attracted to each other; phase changes occur when thermal motion upsets the ordered array of dipoles

hydrogen bonding

special dipole-dipole interactions when a hydrogen atom is bonded to N, O, or F (highly electronegative atoms) AND a nearby small electronegative atom in another molecule or chemical group

hydrogen bonding in water

responsible for the tetrahedral arrangement of water molecules, 4 hydrogen bonding sites; in solid, crystalline water, creates voids that lead to a density that is lower than liquid water and a higher volume

why do smaller molecules have stronger dipole-dipole interactions?

there is more charge density and closer proximity of dipoles, leading to stronger attractions between molecules.

ion-dipole interactions

permanent dipoles and ions interact; contributes to the solubility of ionic salts in polar solvents

ion interactions

stronger than dipole interactions, involve full charges instead of partial charges; leads to high boiling/melting points of ionic compounds; depend on size/charge of ion, dipole moment of molecule, and size of molecule