Chemistry: Intermolecular Forces

polarity

the presence and strength of intermolecular forces

electronegativity

a measure of the relative attraction that the element has for bonding electrons.

• molecules are not charged but some behave as they are

polar bonds

dipole-dipole forces

occur between polar molecules where negative pole on one molecule is attracted to positive pole on another molecule

• form strong attractive forces

• greater dipole, stronger attraction between molecule and stronger intermolecular force, hence higher melting point.

hydrogen bonding

three small highly electronegative atoms (N, F, O) are bonded to a hydrogen atom, the largest possible dipole is set up within molecule

• must have at least one lone pair of electrons - attracted to positive dipole on adjacent hydrogen atom

• strongest of intermolecular forces

hydrogen bonding explains these spec properties of water

• relatively, high melting/boiling points

• expansion on freezing

• good solvent properties

dispersion forces

weakest type of intermolecular force, present in all covalent molecules

how dispersion forces are formed

• operate between all covalent molecules

• arise due to consisten movement of electrons within atoms (electrostatic in nature)

• movement results in instantaneous dipole

• attract each other, b/c not permanent attraction is weak

strength of dispersion forces

• number of electrons in molecule: electrons increases strength increases, stronger instantaneous dipole

• shape of molecule

why substances can dissolve

polar solutes will dissolve in polar solvents

non-polar solutes dissolve in non-polar solvents

can dissolve in water

polar molecules
ionic substances
some polar molecules that form ions

stationary phase

a solid or liquid coating a solid, onto which the components of a sample adsorb

• adsorption is the attraction or sticking of one substance to the surface of another

mobile phase

a liquid component which carries the components of a mixture along the stationary phase

desorption

unsticking and dissolving back into mobile phase

rate components move depend on

• attraction between component and stationary phase: greater attraction, slower movement

• solubility of component in mobile phase: greater solubility, faster movement

polarity depends on

• number/arrangement of electronegative atoms

• proportion of molecules that is hydrocarbon