Covalent Chemical BondIn a chemical bond, electrons are shared

between two atoms. The negative electrons act as “glue” holding the two positive nuclei together.

Atoms have different capacities to attract electrons; we call this property electronegativity (EN). Nonmetals have higher EN than metals.

Nonpolar Covalent BondsH : H (also shown as H – H; single bond)

O :: O (O = O; double bond)Cl : Cl (Cl – Cl)

Each of the atoms in these homonuclear diatomics are identical and attract the electrons in the chemical bonds equally.

We say that the electrons are shared equally and that the bond is nonpolar.

Polar Covalent BondWhen the covalent bond forms between

unlike atoms, one of the atoms may attract the electrons in the bond more (due to higher EN).

H - F F attracts the electron pair more than H so the charge separation is δ+H – Fδ- where δ+ and δ- are partial charges. We say that HF has a dipole or that the H – F bond is polar.

δ+C ≡ Oδ-

Intermolecular forces (IMF): Dipole-Dipole Forces

Molecules with dipoles can attract each other even though they are neutral.

I.e. + end of one molecule attracts the – end of another molecule while at the same time, repulsions between like charges are minimized.

Hydrogen or H-bonds are intermolecular or between-molecule bonds

Dipole-dipole forces = @ 1% of the forces in a chemical bond.

Water is a Polar Molecule

Perhaps the most important polar molecule is water.

Because the water molecule is bent, it has a dipole, which in turn determines many of water’s properties.

What if Water were Linear?H ― O ― HDescribe the charge separation situation.Describe the existence of H-bonds.

Hydrogen BondingFortunately water is bent or non-linear and is

capable of extensive H-bonding.H-bonding in water results in water’s

unusually high BP, heat capacity and surface tension.

It results in ice being less dense than liquid water.

H-BondsH-bonding is not restricted to water. Let the

H-bond be represented by XH ···Y where X and/or Y = N, O, F (small and highly EN atoms).

δ+H – Fδ- ··· δ+H – Oδ- – Hδ+ ··· δ-F – Hδ+··· (of course, the water molecule is BENT)

Ionic BondsUnlike covalent bonds in which electrons are shared, in an ionic bond (M—N), electrons are transferred from atom M to atom N due to their unequalENs.

M ends up + and N ends up -.Then the chemical bond forms when these

two oppositely charged ions attract. M+ + N- M+—N-

Ionic BondNote full charges develop, not δ+ and δ-

partial charges.Ionic bonds form when a metal (M) and

nonmetal (N) combine. Metal atoms give up electrons and nonmetal atoms accept them. Note location on the Periodic Table.

Nonmetal atom + nonmetal atom covalent molecule(either nonpolar (F—F) or polar (δ+C ≡ Oδ-)

Chilling Out Chili Peppers (p 83)The “hotness” of a food can be reduced by

diluting the chemical responsible for the heat. This occurs when the “hot” chemical dissolves in a solvent.

So now the question is, what molecular properties promote dissolution? What molecules dissolve the “hot” chemical?

In general, like dissolves like.

Chile PeppersThe active ingredient in chili peppers is

capsaicin (C18H27NO3), an alkaloid which maintains its heat even after cooking.

Polar water dissolves ionic compounds and other polar molecules.

Capsaicin is a nonpolar molecule. So water is not an effective solvent for capsaicin.

CapsaicinNote the long carbon backbone with many

C-H (nonpolar bonds) and essentially no polar bonds to interact with water.

(CH3)2CHCH=CH(CH2)4

CONHCH3C6H3-4-(OH)-3-(OCH3)

What Dissolves Capsaicin?On the other hand, nonpolar molecules like

oils, fats and, to a lesser extent, alcohols, do dissolve capsaicin.

Drink milk or add sour creamThe Scoville Scale assigns heat to capsaicin

and other members of the capsaicinoids. http://www.thinkgeek.com/tshirts/generic/3813/

Why Does Water Bead on a Waxed Car?

Surface TensionSurface tension is the resistance of a liquid to

an increase in its surface area.Recall the strong attractive IMF (H-bonding)

between water molecules. When a water molecule is only partially

surrounded by water, it will be attracted to other water molecules and away from the dissimilar one.

More on Surface TensionThis force toward the interior of water leads

to a spherical shape which resists disruption. E.g. insects can walk on water, a needle can float, water dripping from a faucet forms drops, oil-vinegar separation.

Molecules with large attractive IMF have large surface tensions.

Quantifying Surface TensionSurface tension

values.Erg = energy valuehttp://

www.surface-tension.de/

erg/cm2 at 20oC

Ethanol 22.10

Ethylene glycol

47.70

Glycerol 64.00

Water 72.80

Mercury 425

Spherical Bubbles in Soda (p 84)The gas in soda is carbon dioxide, CO2. It is

nonpolar with these bonds O=C=OThe soda is mostly water, a polar molecule.

So we expect unfavorable water-CO2 interactions.

The interface that minimizes these unfavorable interactions is a sphere because it has the smallest surface area.

Water-Repellent or Water-Proof Fabric (p 89)Based on the discussion on water beading on

a waxed car and spherical CO2 bubbles in soda, what conclusions can be make about the properties of these fabrics?

Waterproofing Surface Coatings

Scotchgard ®Patsy Sherman and Samuel Smith of 3M co-

invented Scotchgardhttp://inventors.about.com/library/inventors/

blscotchgard.htmTeflon ®

http://www.teflon.com/Teflon/teflonissafe/what_is_teflon.htmlWax

Waterproofing Surface CoatingsWax: type of lipidSilicone: polymers with silicon plus C, H, O...

http://www.fluorochemsilanes.co.uk/silicone%20coatings.htm

Requirement 1Fabric coating must adhere well to fabric Adhere = formation of chemical bond

between fabric and coating Example: cellulose fabric (with OH

functional groups; Fig 7.6.1) + organosilicon (trimethylchlorosilane)

cellulose-OH group + [(CH3)2SiCl] (g)

→ cellulose-O-Si(CH3)3 + HCl

Requirement 2Fabric coating should have unfavorable

interactions with water.Replacement of hydrophillic –OH group by

hydrophobic O-Si(CH3)3 group repels water.Note that the Scotchgard chemistry is

similar.

Polydimethylsiloxane (PDMS): Active Component in Silicones

Waterproof Fabrics

Gore-Tex ®http://www.gore-tex.com/remote/Satellite/home

eVent ®http://www.eventfabrics.com

Gore-TexGore-Tex = composite of ePTFE + POPUePTFE = hydrophobic = expanded

polytetrafluoroethylene

POPU = oleophobic = polyalkylene oxide polyurethane-urea

PTFE

Why do we Like Cotton Bath Towels? (p 95)Observation 1: Cotton towels are more

absorbent than polyester towels.Observation 2: Cotton towels take longer to

dry than polyester.Explain

LiquidsWhy do liquids bead up?What is surface tension?How does capillary action work?

Cohesive forces between moleculesAdhesive forces between molecules and

surface of container.What is the basis of viscosity?Give examples of H-bonding.