BLBCJM-CH8-07

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

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Unformatted text preview: Chemical Bonding Chapter 8 Concepts of Chemical Bonding 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 Chemical Bonding Energetics of Ionic Bonding As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium. Chemical Bonding Energetics of Ionic Bonding We get 349 kJ/mol back by giving electrons to chlorine. Chemical Bonding Energetics of Ionic Bonding But these numbers dont explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic! Chemical Bonding Ionic Bonding Ionic Bonding 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. 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 Coulombs law: Chemical Bonding Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions. 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. 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. 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 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: is a constant (8.99 x 10 9 Jm/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 = Chemical Bonding Energetics of Ionic Bond Formation Lattice energy increases as The charges on the ions increase The distance between the ions decreases. Chemical Bonding Chemical Bonding Covalent Bonding In these bonds atoms share electrons....
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This note was uploaded on 08/17/2008 for the course CHE 101 taught by Professor Churchhill during the Spring '08 term at SUNY Buffalo.

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

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