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Bond Order, Length, and Energy

Bond Order, Length, and Energy. Bond Order. The number of bonding electron pairs shared by two atoms in a molecule. Can be whole numbers as well as fractional numbers. Draw H2, NH3, and CH4. Each have a bond order of 1 Draw CO2 and C2H4

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Bond Order, Length, and Energy

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  1. Bond Order, Length, and Energy

  2. Bond Order • The number of bonding electron pairs shared by two atoms in a molecule. • Can be whole numbers as well as fractional numbers. • Draw H2, NH3, and CH4. • Each have a bond order of 1 • Draw CO2 and C2H4 • The C=O bond in CO2 has a bond order of 2, as does the C=C bond in C2H4. • Draw N2 and CO • Bond orders of 3

  3. Bond Order • Draw O3 • It has 2 resonance structures, so how do I calculate the bond order? • Bond Order = _[Number of shared pairs in all X-Y bonds]_ [Number of X-Y links in the molecule or ion] • Bond Order = _[Number of bonds]_ [Number of structures] • Each O-O bond has a total of 3 shared pairs/2 structures • Bond Order = 1.5

  4. Bond Order • Draw C2O4-2 … • How many resonance structures are there? • What’s the bond order between Carbon and Oxygen? • Draw OCN- … • How many resonance structures are there? • What’s the bond order between C and O? C and N?

  5. Bond Length • Is the distance between the nuclei of two bonded atoms. • Related to 2 things: • Size of the atom • The larger the atom, the longer the bond • H-F < H-Cl < H-Br < H-I • Bond Order • When comparing bonds between the same two atoms, the larger the bond order, the shorter the bond • C-O > C= O > C= O • Single bonds are longer than double bonds, which are longer than triple bonds

  6. Bond Length • Compare the bond lengths between carbon monoxide, carbon dioxide, and carbonate ion. Arrange them in order of increasing bond length. • Carbon Monoxide < Carbon Dioxide < Carbonate Ion Bond order: 3 2 1.3

  7. Bond Enthalpy • Bond Dissociation Enthalpy • the enthalpy change for breaking a bond in a molecule with the reactant and products in the gas phase • Enthalpy (ΔH) • The process of breaking bonds in a molecule is always endothermic • ΔH is positive. Energy must be put in to break a bond. • The process of forming bonds in a molecules is always exothermic • ΔH is negative. Energy is released when a bond is formed

  8. Bond Enthalpy ΔHrxn = [∑(Bond enthalpies bonds broken)] – [∑(Bond enthalpies bonds formed)] or ΔHrxn = [∑ ΔHbonds broken ] – [ ∑ ΔHbonds formed]

  9. Practice Problems • Compare the nitrogen-nitrogen bond in hydrazine, H2NNH2, with that in “laughing gas”, N2O. In which molecule is the nitrogen-nitrogen bond shorter? In which is the bond stronger?

  10. Practice Problems • The compound oxygen difluoride is quite reactive, giving oxygen and HF when treated with water. OF2(g) + H2O(g)  O2(g) + 2HF(g) ΔHrxn = -318kJ/mol●rxn Using bond dissociation enthalpies, calculate the bond dissociation energy of the O-F bond OF2. H-O = 463 kJ/mol O=O = 498 kJ/mol H-F = 565 kJ/mol

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