Ch9_Polarity

Ch9_Polarity - Bond Polarity Bond Polarity Molecular...

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Unformatted text preview: Bond Polarity Bond Polarity Molecular Polarity HCl is POLAR because it POLAR has a positive end and a negative end. + Why are water molecules attracted to a balloon that has a static electric charge? + H •• Bond Polarity •• Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond. Cl • • BOND BOND “pure” bond “pure” bond real bond real bond ENERGY ENERGY 339 kJ/mol calc’d 339 kJ/mol calc’d 432 kJ/mol measured 432 kJ/mol measured Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in H •• Cl • • •• Polarity arises because Cl has a greater share has in bonding electrons than does H. Electronegativity , Bond Polarity Bond Polarity This model, calc’d using CAChe software, shows that H is + (red) and Cl is (yellow). Calc’d charge is + or - 0.20. (See PARTCHRG or (See folder in MODELS.) This model, calc’d using CAChe software, shows that the electron density is greater around Cl than around H. Linus Pauling, 1901-1994 is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling 1901-1994 ELECTRONEGATIVITY, . ELECTRONEGATIVITY, Page 1 The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Chemistry areas: bonding, electronegativity , protein structure 4 3.5 N 3 2.5 F O Cl C S H Si 2 P See Figure 9.10 See Figure 9.10 1.5 1 0 Which bond is more polar (or DIPOLAR)? O—H O—F O—H 3.5 - 2.1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 • F has maximum . has Molecules—such as HCl and H2O— can be POLAR (or dipolar). be POLAR 3.5 - 4.0 1.4 0.5 Molecular Polarity Bond Polarity Bond Polarity Electronegativity , Electronegativity Electronegativity, 0.5 They have a DIPOLE MOMENT . The polar They DIPOLE HCl molecule will turn to align with an electric field. OH is more polar than OF • Atom with lowest is the center atom Atom in most molecules. O—H -+ • Relative values of determine BOND Relative BOND POLARITY (and point of attack on a molecule). and polarity is “reversed.” Molecular Polarity The magnitude of the dipole is given in Debye units. Named for Peter Debye (1884 1966). Rec’d 1936 Nobel prize for work on x-ray diffraction and dipole moments. O—F +- P O SI T I V E H—Cl NEG A TIV E Molecular Polarity Molecular Polarity Molecular Molecular Polarity, Polarity, H2O H2O Molecules will be polar if a) bonds are polar AND AND b) the molecule is NOT “symmetric” •• O H •• Symmetric molecules, Fig. 9.12, p. 414 Page 2 H polar O H H + Carbon Dioxide Microwave oven Consequences of H2O Polarity Molecular Polarity, HBF2 2 H B F F • • O C •• •• Positive C Positive C atom is reason atom is reason CO22 + H22O CO + H O gives H22CO33 gives H CO Molecular Polarity, BF3 Molecular Polarity, BF3 • CO2 is NOT polar even though the CO bonds are polar. • CO2 is symmetrical. F B atom is po.sitive but po.sitive but H & F atoms are negative. F F B—F bonds in BF3 are polar. 3 -0.75 +1.5 -0.75 But molecule is symmetrical and NOT polar and NOT polar Polarity of CH3F 3 F F H H H C B atom is positive and F atoms are negative. B Polarity of Methane, CH4 4 H B—F and B—H bonds in HBF 2 2 are polar. But molecule is NOT symmetrical and is polar. O • • C CH H H H H H Methane is symmetrical and is NOT polar. Page 3 C—F bond is very polar. C—F bond is very polar. Molecule is not symmetrical Molecule is not symmetrical and so is polar. and so is polar. Substituted Ethylene Substituted Ethylene • C—F bonds are MUCH more polar than C—H bonds. • Because both C—F bonds are on same side of molecule, molecule is POLAR . side POLAR Substituted Ethylene Substituted Ethylene • C—F bonds are MUCH more polar than C—F C—H bonds. C—H • Because both C—F bonds are on opposing ends of molecule, molecule is NOT ends NOT POLAR . Page 4 ...
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