Polar molecules vs. Nonpolar molecules A molecule with separate centers of positive and negative charge is a polar molecule.

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1 CHM 123 Chapter Polar covalent Bonds and Dipole moments Depending on the relative electronegativities of the two atoms sharing electrons, there may be partial transfer of electron density from one atom to the other. When the electronegativities are not equal, electrons are not shared equally and partial ionic charges develop. Atoms with greater electronegativities will attract more of the shared electron density to themselves, causing a polarity to the bond Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons density to itself. Nonpolar (covalent bond): a bond in which the electron density is shared approximately evenly between the two atoms Polar (covalent) bond: A bond between 2 nonmetal atoms that have different electronegativities and therefore have unequal sharing of the bonding electron pair. As result, there is a partial charge separation occur between the two atoms - Dipole moment: is a quantitative measure of the degree of charge separation in a molecule. It is also used to describe a polar bond. - Partial positive charge ( δ+) and partial negative charg (δ-) indicate the direction of the polarity or dipole. Polar molecules vs. Nonpolar molecules A molecule with separate centers of positive and negative charge is a polar molecule Dang 1

2 The percent ionic character of a covalent bond The dipole moment (μ) of a molecule is the product of the magnitude of the charge (Q) and the distance (d) that separates the centers of positive and negative charge. μ = Q x r A unit of dipole moment is the debye (D). Q = magnitude of the charge ( charge of e - = 1.60 x C) r = distance the charge One debye (D) is equal to 3.34 x C m Example: The bond length in HF is 92.0 pm and the measured dipole moment of HF is 1.82 D. Calculate the expected dipole moment if the charges on the atoms were discrete instead of partial. What is the percent ionic character of H F bond? Dang 2

3 Factors Affecting Dipole Moments on Molecular Polarity Lone-pair electrons on oxygen and nitrogen project out into space away from positively charged nuclei giving rise to a considerable charge separation and contributing to the dipole moment Symmetrical structures of molecules cause the individual bond polarities and lone-pair contributions to exactly cancel Even though a molecule has polar bonds, it still maybe nonpolar. Bond dipoles are vector quantities. Bond dipoles can add to one another to cause molecule polarity or cancel another out to give a nonpolar molecule (depending on the shape of the molecule) Example: Are the following molecules polar or nonpolar * Hydrocarbon molecules containing only the elements C and H are considered to be nonpolar, no matter their shape due to the relative small difference in their electronegativies between C-C and C-H bonds. If there is more than one type of atom (element) on the outside of the structure, one must consider the differing electronegativities involved. Depending on the structure of the molecule, bond dipoles may still cancel out to give overall nonpolar molecules. Dang 3

4 Relationship Between Molecular Geometry and Dipole Moment Formula Molecular Geometry Dipole moment (assume X atoms are to be identical) AX Linear Can be nonzero AX2 Linear Zero Bent Can be nonzero AX3 Trigonal planar Zero Trigonal pyramidal T-shaped Can be nonzero Can be nonzero AX4 Tetrahedral Zero Square planar Seesaw Zero Can be nonzero AX5 Trigonal bipyramidal Zero Square pyramidal Can be zero AX6 Octahedral Zero Dang 4

5 8.6 Type of nonbonding interactions Intermolecular Forces: Attractions between molecules that hold them together. These forces are electrical in origin and result from the mutual attraction of unlike charges or the mutual repulsion of like charges. Strongest Weakest A. Ion-ion interaction (Na + Cl - ) B. Ion-dipoles (Na + H2O, Cl - H2O) C. Hydrogen bonding (OH, NH, HF) (polar interaction) D. Dipole-dipole (polar interaction) E. London dispersion (nonpolar molecules) *Ion-ion interaction between ions - electrostatic interaction between two permanent charges Na + Cl - Ion Dipole Forces: The result of electrical interactions between an ion and the partial charges on a polar molecule *Hydrogen bonding strongest known dipole, due to having H on N or O or F Dang 5

6 *Dipole dipole interaction between polar molecules The result of electrical interactions among dipoles on neighboring molecules As the dipole moment increases, the intermolecular forces increase. As the intermolecular forces increase, the boiling point increases. *London Dispersion Forces: The result of the motion of electrons that gives the molecule a short-lived dipole moment. This induces temporary dipoles in neighboring molecules. These are temporary fluctuations of negative electron clouds from one side to another, relative to the less positive nuclear charge. All molecules and atoms will have them As a temporary dipole is established in one molecule, it induces in all the surrounding molecules Greater surface area, greater London dispersion forces? Why? Higher MW, higher B.P (if all polarity is equal) Dang 6

7 Straight chain vs. branches Example: Will those two compounds interact with each other? Why or Why not? Rank the following molecules from highest B.P to lowest B.P then provide your reasons Kryton methylamine water ethyl alcohol dimethyl ether propane Sodium chloride Kr CH3NH2 H2O CH3CH2OH CH3OCH3 CH3CH2CH3 NaCl Viscosity: The measure of a liquid s resistance to flow. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. Weaker intermolecular force, smaller viscosity E.g: If I pour honey into the cup I will find that the cup drains very slowly. That is because honey's viscosity is large compared to other liquids' viscosities. If I fill the same cup with water, for example, the cup will drain much more quickly. Dang 7

8 Surface Tension: The resistance of a liquid to spread out and increase its surface area. - Surface tension is caused by the difference in intermolecular forces experienced by molecules at the surface of a liquid and those experienced by the molecules in the interior. - Molecules at the surface feel attractive forces on only one side and are thus pulled in toward the liquid, while molecules in the interior are surrounded and pulled equally in all directions - Stronger intermolecular forces, higher in surface tension. Dang 8