Polarity and Intermolecular Forces

Review We know how to draw Lewis structures for simple molecules and polyatomic ions. We also know how to predict the 3-D geometry of these molecules and ions, if we apply the VSEPR Theory. Electronegativity (EN) is an atom's tendency to attract electrons in chemical bonds. EN increases to the right and up on the periodic table, excluding the noble gases.

Bond Polarity When two nonmetal atoms bond, they share electrons. They may or may not share electrons evenly. Consider the following molecules: F 2 HF

Bond Polarity H vs. F The EN of H is 2.2. The EN of F is 4.0. In F 2, both atoms pull with equal strength on the bonding e-. The e- are shared evenly between them. In HF, the F atom pulls harder than the H atom. The e- are drawn more toward the F atom. H EN = 2.2 F EN = 4.0 FF FH

Bond Polarity EN is same for both atoms. e - density is spread evenly around molecule. Highest e - density occurs between atoms. Bond is nonpolar. EN is much higher for F than for H. e - density is drawn toward F side. F atom acquires partial negative charge. H atom acquires partial positive charge. Bond is polar. FF F H

Bond Polarity In general, a covalent bond is: polar if it occurs between two different atoms. nonpolar if it occurs between two identical atoms.

Dipole Moments Dipole Moment - a measure of the polarity of a bond. Is often represented by a special arrow. FH Arrow points toward more EN atom.

Polarity of Diatomic Molecules Diatomic Molecules - molecules made of only two atoms. If atoms are the same, molecule is nonpolar. If atoms are diff., molecule is polar. NOTE: Polar does not mean charged. Is Cl 2 polar or nonpolar? Is CO polar or nonpolar?

Molecules With 3 or More Atoms A molecule with 3 or more atoms is: Polar if its central atom has lone pairs OR If the outer atoms are not all the same. Nonpolar if its central atom has no lone pairs AND All the outer atoms are identical.

CO 2 vs. H 2 O Consider the Lewis structure of CO 2 : This molecule is nonpolar.

CO 2 vs. H 2 O Consider the Lewis structure of H 2 O: This molecule is polar.

CH 4 vs. CH 3 Cl Neither CH 4 nor CH 3 Cl has any lone pairs on the central carbon atom. Is CH 4 polar or nonpolar? Is CH 3 Cl polar or nonpolar? CCHH H H ClH H H

“Like Dissolves Like” Polar molecules mix with each other. Nonpolar molecules mix with each other. Polar and nonpolar molecules do not easily mix.

Amphipathic Molecules Amphipathic - has a hydrophobic region and a hydrophilic region. Hydrophobic - “water-fearing” Nonpolar. Hydrophilic - “water-loving” Polar or charged. Dish detergents contain amphipathic molecules. Why?

Grease and Water Don't Mix!

Amphipathic Molecules Hydrophilic head Hydrophobic tails

Amphipathic Molecules

Intermolecular Forces Intermolecular force - a force between two molecules that does not result from chemical bonding. Dipole-dipole interaction. Hydrogen bonding. London force.

Dipole-Dipole Interactions Dipole - polar molecule. Like magnets, except poles are  + and  - instead of N and S. Polar molecules generally have higher melting and boiling points than similar nonpolar molecules. EXAMPLE: O 2 (nonpolar) boils at -183ºC. EXAMPLE: NO (polar) boils at -152ºC. NO has a higher boiling point due to its polarity. Still far below the boiling point of any ionic cmpd.

Dipole-Dipole Interactions

Hydrogen Bonding Hydrogen bond - a stronger form of dipole-dipole interaction. Occurs in molecules that have H atoms bonded to O, N, or F atoms. The small size of the H atom allows these molecules to get closer together. Closer together = stronger forces. EXAMPLE: H 2 O has a boiling point of 100ºC. EXAMPLE: H 2 S has a boiling point of -60ºC. The b.p. of H 2 O is higher b/c of hydrogen bonding.

Hydrogen Bonding

London Force London force - attraction between temporary dipoles. e - move randomly around molecules. Nonpolar molecules become temporarily polar. Allows for very weak attractions between nonpolar molecules. Named for Fritz London.

London Forces  

London Forces The more e - a molecule has, the greater its London forces are. Large molecules tend to have higher melting/boiling points than small molecules. London forces apply to all molecules.

London Forces