Intermolecular Forces and Polarity Intra versus Inter Remember

Intermolecular Forces and Polarity

Intra – versus Inter – • Remember that INTRAmolecular forces are the covalent bonds which hold a molecule together, and INTERmolecular forces are the attractive forces that hold different molecules together.

Intermolecular Forces (IMF) • The attractive force a molecule can exert is important in understanding what that molecule can mix with. • As well, knowing the strength of the IMFs acting on a molecule helps us understand melting/boiling points

Polarity • Polar Molecules = molecules created due to an unequal sharing of electrons • This unequal sharing creates a DIPOLE, a molecule with partial positive and a partial negative end • Since positives and negatives are naturally attracted to one another, dipoles exert an attractive force on each other.

Dipole-Dipole • All polar molecules are dipoles • The partial positive end of one dipole is attracted to the partial negative end of another

Polarity and Lewis Structures • From a Lewis Structure it is quite easy to identify a polar molecule, it will either have – Lone pairs on the central atom – One of the atoms bonded to the central atom will be different than the others – Both POLAR

Types of Intermolecular Forces • All molecules are first divided into Polar and Nonpolar • Nonpolar molecules only exhibit LONDON DISPERSION forces (van der Waals) • Polar molecules can either exhibit normal DIPOLE-DIPOLE interactions or in some cases the stronger force of HYDROGEN-BONDING

London Dispersion • This is the weakest IMF • It is only present in nonpolar molecules • It’s created by the momentary grouping of electrons on one side of the electron cloud

Hydrogen Bonding • This IMF occurs in the special case when HYDROGEN is bonded to NITROGEN, OXYGEN, or FLUORINE

Polarity and Mixing • The ability for one compound to mix with another is based on whether the two compounds can physically connect • Polar molecules can only mix with other polar (or ionic) molecules. • Polar molecules CANNOT mix with nonpolar molecules

Polarity and Phase Changes • Recall that states of matter (solid, liquid, and gas) are described by the arrangement of particles • Changing from one state to another involves changing the particle arrangement • The ease at which this occurs is based upon the force of attraction between the individual particles

Polarity and Phase Changes • Polar molecules require more energy to change state than nonpolar molecules • But remember, ionic compounds will always melt/boil at higher temperatures than molecules