Chapter 9 -Chapter 10 -Lecture 2

Chapter 9 -Chapter 10 -Lecture 2 - General Chemistry I Fall...

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Unformatted text preview: General Chemistry I Fall 2007 Joann S. Monko Chemistry 9th ed. Raymond Chang 2 Phenylalanine, an amino acid H C H C C H C H 1 C C H H 2 H C H C N H O C H 5 3 O 4 H 3 Phenylalanine 4 Clicker Question Consider the molecule CH3SH 1. C = 4 ced, sp3, Td, Td 5. not shown S = 4 ced, sp3, Td, Td C = 6 ced, sp3d2, Oct., Oct. S = 2 ced, sp, Linear, Linear C = 4 ced, sp3, Td, Td S = 4 ced, sp3, Td, Bent C = 3 ced, sp2, T.Planar, T. Planar 2. 3. 4. 5 Chemistry, 8th ed. Chang Bonding in Ethylene 6 Chemistry, 8th ed. Chang Triple Bond Formation 7 Chemistry, 8th ed. Chang Delocalized Molecular Orbitals Benzene, C6H6 8 Chemistry, 8th ed. Chang Formal Atom Charges 9 Atoms in molecules can have a charge (+ or -). The predominant resonance structure: charges are as close to 0 as possible. Formal charge = Group # (# of Bonds) - (# of LP e-'s) Carbon Dioxide CO2 6-2-4=0 O C O Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 4-4-0=0 Thiocyanate Ion, SCN- 10 6 - 1 - 6 = -1 5-3-2=0 S C N 4-4-0=0 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 11 Thiocyanate Ion, SCN S C N S S N C N C Which is the most important resonance form? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 12 Boron Trifluoride, BF3 F B F B +1 -1 F F F F What if we form a B--F double bond to satisfy the B atom octet? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Properties What is the effect of bonding & structure on molecular properties? 13 *Bond Order *Length *Strength *Polarity Buckyball in HIV-protease Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Order # of bonds between a pair of atoms 14 Double bond Single bond Acrylonitrile Triple bond Bond Order Fractional bond orders - molecules with resonance structures. Consider NO2 15 N N O O O O The N--O bond order = 1.5 Total # of e - pairs used for a type of bond Bond order = Total # of bonds of that type 3 e - pairs in N -- O bonds Bond order = 2 N -- O bonds Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 16 Clicker Question What is the bond order for the carbonate ion, CO3-2? a. 1.25 b. 1.33 c. 1.5 d. 1.75 Bond Orde r -related to two important bond properties: (a) bond strength (b) bond length Bond length the distance between the nuclei of two bonded atoms. 110 pm 17 414 kJ 123 pm 745 kJ C-O single bond 143 pm & 358 kJ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Length 18 Depends on bond order H--F H--Cl H--I Depends on size of atoms Bond distances Angstrom units 1 = 10-2 pm. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Strength 19 --measured by the energy req'd to break a bond. BOND H--H C--C C=C CC NN STRENGTH (kJ/mol) 436 346 602 835 945 The GREATER the number of bonds (bond order) the HIGHER the bond strength & the SHORTER the bond. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Using Bond Energies 20 H--H + Cl--Cl ----> 2 H--Cl Net energy = Hrxn = = energy required to break bonds -- energy evolved when bonds are made Hrxn = D (bonds broken) - D (bonds formed) H--H = 436 kJ/mol Cl--Cl = 242 kJ/mol H--Cl = 432 kJ/mol Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 21 Using Bond Energies 2 H--O--O--H O=O + 2 H--O--H Is the reaction exo- or endothermic? Which is larger: A) energy req'd to break bonds B) or energy evolved on making bonds? H-O = 463 kJ/mol O-O = 146 kJ/mol O=O = 498 kJ/mol 2144 kJ/mol 2350 kJ/mol Hrxn = -206 kJ/mol exothermic Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Polarity + - 22 HCl is POLAR -it has a positive end & a negative end. H Cl Cl has a greater share in bonding e-'s than does H. Cl - slight negative charge (- ) H - slight positive charge (+ ) H is + (red) and Cl is - (yellow) Calc'd charge is + or - 0.20 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Electronegativity, - a measure of the ability of an atom in a molecule to attract electrons to itself. 23 TWO unshared Nobel prizes: Peace & Chemistry Chemistry areas: bonding, electronegativity, protein structure Concept proposed by Linus Pauling 1901-1994 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. + - H Cl Bond Polarity 24 The H--Cl bond energy is GREATER than expected for a "pure" covalent bond. ENERGY 339 kJ/mol calc'd 432 kJ/mol measured BOND "pure" bond real bond Difference = 92 kJ. proportional to the difference in ELEC TRONEGATI VI TY, . Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Electronegativity 25 Chemistry 8th ed. Raymond Chang 26 4 3.5 3 O N C H F Cl S Si P Electronegativity 2.5 2 1.5 1 0.5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 F = maximum . Lowest Atom = center atom Relative values of BOND POLARITY (& point of attack on a molecule) Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Polarity Which bond is more polar (or DIPOLAR)? O--H 3.5 - 2.1 1.4 0.5 O--F 3.5 - 4.0 27 OH is more polar than OF - O + H + O - F polarity is "reversed." Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Molecular Polarity Boiling point = 100 C 0 28 Boiling point = -161 C Why do water and methane differ so much in their boiling points? Why do ionic compounds dissolve in water? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Why do CuCl2, acetic acid, and ethanol dissolve in water? 0 29 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Molecular Polarity Molecules (HCl and H2O) can be POLAR. 0 30 They have a DIPOLE MOMENT. The polar HCl molecule will turn to align with an electric field. Figure 9.15 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Dipole Moments, 0 31 The magnitude of the dipole is given in Debye units. Peter Debye (1884 - 1966). Rec'd 1936 Nobel prize: x-ray diffraction & dipole moments. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Molecular Polarity Molecules will be polar if a) bonds are polar AND b) the molecule is NOT "symmetric" 0 32 All above are NOT polar! Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Polar or Nonpolar? 2 2 0 33 Compare CO and H O. Which one is polar? CO is NOT polar even 2 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. -0.75 +1.5 -0.75 though the CO bonds are polar. CO is symmetrical. Polar or Nonpolar? 3 3 2 0 34 Consider AB molecules: BF , Cl CO, and NH . 3 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 35 Clicker Question Consider the molecule SO2 1. Polar bonds, polar molecule, 1.5, S = +1 2. Polar bonds, nonpolar molecule, 1.5, S = -1 3. Polar bonds, nonpolar molecule, 1.5, S = +1 4. Nonpol. bonds, nonpol. molecule, 1.5, S = -1 5. Polar bonds, nonpolar molecule, 2, S = +1 6. Polar bonds, polar molecule, 2, S = -1 7. Polar bonds, polar molecule, 2, S =+1 Molecular Polarity, BF & HBF F B F F 3 0 36 B is + atoms are - 2 F B--F bonds are polar molecule is symmetrical NOT polar B is + & F atoms are but H H B F F B--F & B--H bonds are polar. The molecule is NOT symmetrical & IS polar. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 37 Is Methane, CH , Polar? 0 4 Methane is symmetrical & is NOT polar. Is CH F Polar? 3 C--F bond is very polar. Molecule is not symmetrical & is polar. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Substituted Ethylene 0 38 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. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Substituted Ethylene 0 39 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 POLAR. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 0 40 Consequences of H2O Polarity Microwave oven Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Chemistry, 8th ed. Chang ...
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This note was uploaded on 04/08/2008 for the course CHEM 100 taught by Professor Monko during the Winter '08 term at Kutztown.

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