This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Molecular Polarity Bond Polarity Bond Polarity
HCl is POLAR because it has a positive end and a negative end.
+ •• •• 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 folder in MODELS.) This model, calc’d using CAChe software, shows that the electron density is greater around Cl than around H. Why are water molecules attracted to a balloon that has a static electric charge? H Cl • • Polarity arises because Cl has a greater share in bonding electrons than does H. + H Cl • •
•• •• Bond Polarity
Due to the bond polarity, the H—Cl bond energy is H—Cl GREATER than expected for a “pure” covalent bond. ENERGY ENERGY 339 kJ/mol calc’d 339 kJ/mol calc’d kJ/mol 432 kJ/mol measured 432 kJ/mol measured kJ/mol Electronegativity, Electronegativity,
is a measure of the ability of an atom is in a molecule to attract electrons to itself. Concept proposed by Linus Pauling 1901-1994 Linus Pauling, 1901-1994 Pauling, BOND BOND “pure” bond “pure” bond real bond real bond Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in ELECTRONEGATIVITY, . The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Chemistry areas: bonding, electronegativity, electronegativity, protein structure Page 1 4 3.5 3 2.5 2 1.5 1 0.5 0
1 2 3 4 5 6 7 O N C H F Cl S Si P Electronegativity, Electronegativity ,
See Figure 9.10 See Figure 9.10 Bond Polarity Bond Polarity
Which bond is more polar (or DIPOLAR)? O—H O—F 3.5 - 2.1 3.5 - 4.0 0.5 1.4 Molecular Polarity
Molecules—such as HCl and H2O— can be POLAR (or dipolar). They have a DIPOLE MOMENT. The polar MOMENT. HCl molecule will turn to align with an electric field.
POSITIVE H—Cl NEGATIVE 8 9 10 11 12 13 14 15 16 17 18 • F has maximum . • Atom with lowest is the center atom is in most molecules. • Relative values of determine BOND determine POLARITY (and point of attack on a molecule). OH is more polar than OF O—H -+ O—F +- 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. Molecular Polarity Molecular Polarity
Molecules will be polar if a) bonds are polar AND b) the molecule is NOT “symmetric” Molecular Molecular Polarity, Polarity, H2O H2O
•• O H •• H polar O H + H Symmetric molecules, Fig. 9.12, p. 414 Page 2 Carbon Dioxide
• • Molecular Polarity, BF3 Molecular Polarity, BF3
F B F F
B atom is positive and F atoms are negative. Microwave oven O
•• C O
•• • • • CO2 is NOT polar is even though the CO bonds are polar. • CO2 is symmetrical. is Consequences of H2O Polarity Positive C Positive C atom is reason atom is reason CO22 + H22O CO + H O gives H22CO33 gives H CO B—F bonds in BF3 are polar. 3 are -0.75 +1.5 -0.75 But molecule is symmetrical and NOT polar Molecular Polarity, HBF2 2
H B F F
B atom is po.sitive but po. po.sitive H & F atoms are negative. Polarity of Methane, CH4 4
F F Polarity of CH3F 3
H H C CH H H H H C H H H B—F and B—H bonds in HBF 2 2 are polar. But molecule is NOT symmetrical and is polar. Methane is symmetrical and is NOT polar. 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. Page 3 Substituted Ethylene Substituted Ethylene 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 . POLAR • C—F bonds are MUCH more polar than C—H bonds. • Because both C—F bonds are on opposing ends of molecule, molecule is NOT NOT POLAR . Page 4 ...
View Full Document
- Fall '08