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12-Ionic Bonds

# 12-Ionic Bonds - 5.111 Lecture Summary#12 Reading for today...

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5.111 Lecture Summary #12 October 5, 2009 12.1 Reading for today: Sections 2.9-2.11, 2.3, 2.12 Reading for Lecture 13: Sections 3-1,3.2 IONIC BONDS – CLASSICAL MODEL AND MECHANISM HARPOON MECHANISM – gas phase Na atom and Cl atom Herschbach, Polanyi, and Lee, 1986 Na (g) Na + (g) + e - Δ E = IE = 496 kJ/mol Cl (g) + e - Cl - (g) Δ E = -EA = -349 kJ/mol ________________________ __________________ Na (g) + Cl (g) Na + (g) + Cl - (g) Δ E = 147 kJ/mol Δ E = IE(Na) – EA(Cl) PROBLEM: An energy deficit! SOLUTION: The rope! Coulomb attraction Na + (g) + Cl - (g) NaCl (g) Δ E = -592 kJ/mol _________________________ ______________ Na (g) + Cl (g) NaCl (g) Δ E = -445 kJ/mol Simple model predicts: Δ E d = 445 kJ/mol Experiment: Δ E d = 412 kJ/mol Na Cl e - Na + Cl -

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5.111 Lecture Summary #12 October 5, 2009 12.2 Coulomb energy of interaction () 2 0 -e Ur = 4 πε r for two unlike charges At r e = 2.36 A U(r e ) = -592 kJ/mol Harpooning energetically allowed for r < r* At r*, Coulomb energy compensates for energy needed to produce Na + and Cl - 2 0 -e 4 πε r* = - [IE(Na) – EA(Cl)] ( ) ( ) ( ) ⎡⎤ ⎣⎦ 2 -19 23 -1 2 -10 -12 2 -1 -1 5 -1 0 1.602x10 C 6.022x10 mol -e r* = = = 9.45x10 m 4 πε IE(Na)-EA(Cl) 4 π 8.854x10 C J m 1.47x10 Jmol r* = 9.45 A Limitations of harpoon model:
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12-Ionic Bonds - 5.111 Lecture Summary#12 Reading for today...

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