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Unit 3-3 Bond strength and Resonance

Unit 3-3 Bond strength and Resonance. Covalent Bond Strength. The strength of a bond is measured by determining how much energy is required to break the bond, called the bond enthalpy .

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Unit 3-3 Bond strength and Resonance

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  1. Unit 3-3 Bond strength and Resonance

  2. Covalent Bond Strength • The strength of a bond is measured by determining how much energy is required to break the bond, called the bond enthalpy. • The bond enthalpy for a Cl—Cl bond, D(Cl—Cl), is measured to be 242 kJ/mol. What’s the energy of forming Cl-Cl? • Notation: D (X-X)

  3. Average Bond Enthalpies • This table lists the average bond enthalpies for many different types of bonds. • Average bond enthalpies are positive, because bond breaking is an endothermic process.

  4. Average Bond Enthalpies NOTE: These are average bond enthalpies, NOT absolute bond enthalpies; the C—H bonds in methane, CH4, will be a bit different than the C—H bond in chloroform, CHCl3.

  5. Enthalpies of Reaction • Yet another way to estimate H for a reaction is to compare the bond enthalpies of bonds broken to the bond enthalpies of the new bonds formed. Hrxn = (bond enthalpies of bonds in reactants)  (bond enthalpies of bonds in products)

  6. Enthalpies of Reaction CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) In this example, one C—H bond and one Cl—Cl bond are broken; one C—Cl and one H—Cl bond are formed.

  7. Enthalpies of Reaction So, Hrxn = [D(C—H) + D(Cl—Cl)]  [D(C—Cl) + D(H—Cl)] = [(413 kJ) + (242 kJ)]  [(328 kJ) + (431 kJ)] = (655 kJ)  (759 kJ) = 104 kJ

  8. By adding or subtracting Hrxns˚ of intermediate reactions. • Hess Law • Hf˚of reactants and products must be given. • Bond enthalpy (D) difference between reactants and products. • Ds must be given and reaction must be balanced. Hrxn˚Calculation summary

  9. Bond Enthalpy and Bond Length • We can also measure an average bond length for different bond types. • As the number of bonds between two atoms increases, the bond length decreases. Total bond enthalpy?

  10. Bond strength: Page 297 8.53, 8.54, 8.65, 8.69 Homework

  11. Resonance The Lewis structure we would draw for ozone, O3.

  12. Resonance in Ozone, O3 Neither structure is correct. Oxygen bond lengths are identical, and intermediate to single and double bonds

  13. Resonance Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.

  14. Resonance • One Lewis structure cannot accurately depict a molecule such as ozone. • We use multiple structures, resonance structures, to describe the molecule.

  15. Resonance • In truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon. • They are not localized, but rather are delocalized.

  16. Resonance in Polyatomic Ions Resonance in a carbonate ion: Resonance in an acetate ion:

  17. Resonance • Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule. Benzene, C6H6 • The actual structure is an average of all the resonance • structures. • The bond lengths in the ring are identical, and • between those of single and double bonds.

  18. Resonance Bond Length and Bond Energy • Resonance bonds are shorter and stronger than single bonds. • Resonance bonds are longer and weaker than double • bonds. • It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.

  19. Resonance: 8.52(a) and (b), 8.55 HOmework

  20. Exceptions to the Octet Rule • There are three types of ions or molecules that do not follow the octet rule: • Ions or molecules with an odd number of electrons. • Ions or molecules with less than an octet. • Ions or molecules with more than eight valence electrons (an expanded octet).

  21. Structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons. Because F and H can only form one bond. Fewer Than Eight Electrons

  22. More Than Eight Electrons • The only way PCl5 can exist is if phosphorus has 10 electrons around it. • It is allowed to expand the octet of atoms on the 3rd row or below. • Presumably d orbitals in these atoms participate in bonding.

  23. More Than Eight Electrons Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.

  24. More Than Eight Electrons • This eliminates the charge on the phosphorus and the charge on one of the oxygens. • The lesson is: Only the elements on 2nd period obey octet rule strictly (they only have 4 orbitals on outermost shell). For the rest, the bonds they form are dermermined by their oxidation states.

  25. Exceptions to the octet rule: 8.59 (a), (b), (c), (d), 8.62 Homework

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