Ch9_Polarity - Bond Polarity Bond Polarity Molecular...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 ...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online