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BLBCJM-CH8-07 - Chapter 8 Concepts of Chemical Bonding...

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Chemical Bonding Chapter 8 Concepts of Chemical Bonding
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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
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Chemical Bonding Energetics of Ionic Bonding As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium.
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Chemical Bonding Energetics of Ionic Bonding We get 349 kJ/mol back by giving electrons to chlorine.
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Chemical Bonding Energetics of Ionic Bonding But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!
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Chemical Bonding Ionic Bonding Ionic Bonding
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Chemical Bonding Energetics of Ionic Bonding There must be a third piece to the puzzle. What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.
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Chemical Bonding Lattice Energy This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb’s law: E = κ Q Q
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Chemical Bonding Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions.
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Chemical Bonding Energetics of Ionic Bonding 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.
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Chemical Bonding Energetics of Ionic Bonding These phenomena also helps explain the “octet rule.” Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.
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Chemical Bonding Energetics of Ionic Bond Formation The formation of Na + ( g ) and Cl - ( g ) from Na( g ) and Cl( g ) is endothermic. Why is the formation of Na( s ) exothermic? The reaction NaCl( s ) Na + ( g ) + Cl - ( g ) is endothermic ( H = +788 kJ/mol). The formation of a crystal lattice from the ions in the gas phase is exothermic: Na + ( g ) + Cl - ( g ) NaCl( s ) H = - 788 kJ/mol
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Chemical Bonding Energetics of Ionic Bond Formation Lattice energy: the energy required to completely separate an ionic solid into its gaseous ions. Lattice energy depends on the charges on the ions and the sizes of the ions: κ is a constant (8.99 x 10 9 J·m/C 2 ), Q 1 and Q 2 are the charges on the ions, and d is the distance between ions. d Q Q E l 2 1 κ =
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Chemical Bonding Energetics of Ionic Bond Formation Lattice energy increases as The charges on the ions increase The distance between the ions decreases.
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Chemical Bonding
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Chemical Bonding Covalent Bonding In these bonds atoms share electrons.
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