8.6 - 8.8 covalent bonds

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8.6 Partial Covalent Character of ionc Bonds

  • No individual bonds are completely ionic
  • All “ionic” bonds have some degree of covalent character
  • Ionic bonds with a polyatomic ion are a great example (polyatomic ion is held together by a covalent bond)
  • Any compound that conducts an electric current when melted will be classified as ionic
  • Salt = ionic compound
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8.7 The covalent chemical bond: a model

  • Chemical bonds can be viewed as forces that cause a group of atoms to behave as a unit
  • Bonds result from the tendency of a system to seek its lowest possible energy (most stable)
  • Bonding is a model proposed to explain molecular stability. It is a human invention
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8.8 Covalent bond energies and chemical reactions

  • Single bond: bond in which one pair of electrons is shared
  • Double bond: bond in which two pairs of electrons are shared
  • Triple bond: bond in which three pairs of electrons are shared
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8.8 cont.

  • ∆H (enthalpy) : sum of the energy required to break old bonds (positive signs) plus the sum of the energies released in the formation of new bonds (negative signs)
  • ∆H = ∑ D (bonds broken) - ∑D(bonds formed)
  • Where ∑ represents the sum in terms, and D represents the bond energy per mole of bonds (D always has a positive sign)
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Ex.

  • The formation of HF
  • H2 + F2 --> 2HF
  • H – H 432 kJ/mol
  • F – F 154 kJ/mol
  • H – F 565 kJ/mol
  • Reactants --> Atoms --> Products
  • First step needs energy second step releases energy
  • ∆H = DH-H +DF-F – 2DH-F
  •               = (1 mol * 432 kJ/mol) + (1 mol * 154 kJ/mol) – (2 mol * 565 kJ/mol)
  •        = -544 kJ released
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Ex

  • Using bond energies listed, calculate ∆H for the reaction of methane with chlorine and fluorine to give Freon – 12 (CF2Cl2)
  • CH4 + 2 Cl2 + 2 F2 --> CF2Cl2 + 2 HF + 2 HCl
  • C – H 413 kJ/mol
  • Cl – Cl 239 kJ/mol
  • C – F 485 kJ/mol
  • H – Cl 427 kJ/mol
  • C – Cl 339 kJ/mol
  • F – F 154 kJ/mol
  • H – F 565 kJ/mol
  • Reactant bonds broken
  • CH4 : 4 mol C – H                            4* 413 = 1652
  • 2 Cl2 : 2 mol Cl – Cl                         2* 239 = 478
  • 2 F2 : 2 mol F – F                            2* 154 = 308
  •                                                                     = 2438
  • Product bonds formed
  • CF2Cl2 : 2 mol C – F                       2* 485 = 970
  •                2 mol C – Cl                      2* 339 = 678
  • HF : 2 mol H – F                               2* 565 = 1130
  • HCl : 2 mol H – Cl                             2* 427 = 854
  •                                                                     = 3632
  • ∆H = 2438 – 3632 = -1194 kJ
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