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8 Concepts of Chemical Bonding

8 Concepts of Chemical Bonding - Chapter 8 Concepts of...

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1 Chapter 8 Concepts of Chemical Bonding Chemical Bonds Three basic types of bonds: Ionic: Electrostatic attraction between ions Covalent: Sharing of electrons Metallic: Metal atoms bonded to several other atoms 2
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2 Ionic Bonding Energetics of Ionic Bonding As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium. We get 349 kJ/mol back by giving electrons to chlorine. But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic! Energetics of Ionic Bonding (cont) What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion. 4
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3 Lattice Energy This third piece of the puzzle is the lattice energy : The energy associated with electrostatic interactions is governed by Coulomb’s law: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. E el = κ Q 1 Q 2 d Charges of the particules Distance between their centers Proportionality constant (8.99x10 9 jm/C 2 ) Lattice Energy (cont) Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions. Table 8.2 6
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4 By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process. Fig 8.4 The Born-Haber cycle Energetics of Ionic Bonding Energetics of Ionic Bonding (cont) H 0 f (NaCl) s = - 411 Kj/mol H = - H Lattice H 0 f (NaCl) s = H 0 f (Na) g H 0 f (Cl) g + + I 1 (Na) + E(Cl) - H Lattice H Lattice = 788 Kj/mol Fig 8.4 Na s + 1/2Cl 2(g) NaCl s R o u t e A 8 Na g + Cl - g NaCl s R o u t e B +
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5 Octet rule : Elements lose or share electrons to achieve a more stable electron configuration ( isoelectronic with the closest noble gas) . Energetics of Ionic Bonding (cont) These phenomena also helps explain the “octet rule.” Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration.
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