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Covalent Chemical Bond In a chemical bond, electrons are shared between two atoms. The negative electrons act as “glue” holding the two positive nuclei.

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Presentation on theme: "Covalent Chemical Bond In a chemical bond, electrons are shared between two atoms. The negative electrons act as “glue” holding the two positive nuclei."— Presentation transcript:


2 Covalent Chemical Bond In 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.

3 Nonpolar Covalent Bonds H : 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.

4 Polar Covalent Bond When the covalent bond forms between unlike atoms, one of the atoms may attract the electrons in the bond more (due to higher EN). H - FF attracts the electron pair more than H so the charge separation is δ+ H – F δ- where δ+ and δ- are partial charges. We say that HF has a dipole or that the H – F bond is polar. δ+ C ≡ O δ-

5 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.

6 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.

7 Figure 10.3 a-b (a) Polar H 2 O Molecules (b) H-Bonding Among H 2 O Molecules

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

9 Hydrogen Bonding Fortunately 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.

10 H-Bonds H-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 – F δ- ··· δ+ H – O δ- – H δ+ ··· δ- F – H δ+ ··· (of course, the water molecule is BENT)

11 Ionic Bonds Unlike 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 -

12 Ionic Bond Note 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 ≡ O δ- )

13 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.

14 Chile Peppers The active ingredient in chili peppers is capsaicin (C 18 H 27 NO 3 ), 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.

15 Capsaicin Note the long carbon backbone with many C-H (nonpolar bonds) and essentially no polar bonds to interact with water. (CH 3 ) 2 CHCH=CH(CH 2 ) 4 CONHCH 3 C 6 H 3 -4-(OH)-3-(OCH 3 )

16 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 cream The Scoville Scale assigns heat to capsaicin and other members of the capsaicinoids.

17 Why Does Water Bead on a Waxed Car?

18 Surface Tension Surface 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.

19 Figure 10.6 A Molecule in the Interior of a Liquid is Attracted by the Molecules Surrounding It

20 More on Surface Tension This 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.

21 Quantifying Surface Tension Surface tension values. Erg = energy value http://www.surface- http://www.surface- erg/cm 2 at 20 o C Ethanol22.10 Ethylene glycol 47.70 Glycerol64.00 Water72.80 Mercury425

22 Spherical Bubbles in Soda (p 84) The gas in soda is carbon dioxide, CO 2. It is nonpolar with these bonds O=C=O The soda is mostly water, a polar molecule. So we expect unfavorable water-CO 2 interactions. The interface that minimizes these unfavorable interactions is a sphere because it has the smallest surface area.

23 Water-Repellent or Water-Proof Fabric (p 89) Based on the discussion on water beading on a waxed car and spherical CO 2 bubbles in soda, what conclusions can be make about the properties of these fabrics?

24 Waterproofing Surface Coatings Scotchgard ® Patsy Sherman and Samuel Smith of 3M co-invented Scotchgard htm htm Teflon ® htmlWax htmlWax

25 Waterproofing Surface Coatings Wax: type of lipid Silicone: polymers with silicon plus C, H, O... tm tm

26 Requirement 1 Fabric 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 + [(CH 3 ) 2 SiCl] (g) → cellulose-O-Si(CH 3 ) 3 + HCl

27 Requirement 2 Fabric coating should have unfavorable interactions with water. Replacement of hydrophillic –OH group by hydrophobic O-Si(CH 3 ) 3 group repels water. Note that the Scotchgard chemistry is similar.

28 Polydimethylsiloxane (PDMS): Active Component in Silicones

29 Waterproof Fabrics Gore-Tex ® eVent ®

30 Gore-Tex Gore-Tex = composite of ePTFE + POPU ePTFE = hydrophobic = expanded polytetrafluoroethylene POPU = oleophobic = polyalkylene oxide polyurethane-urea


32 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

33 Liquids Why do liquids bead up? What is surface tension? How does capillary action work? Cohesive forces between molecules Adhesive forces between molecules and surface of container. What is the basis of viscosity? Give examples of H-bonding.

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