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Lec20_drobny_11 - Lecture 20 Chemical Bonding Reading...

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Lecture 20: Chemical Bonding • Reading: Zumdahl 13.1-13.5 Recommended Problems: 13.11, 13.13, 13.15, 13.17, 13.19, 13.23 • Outline Types of Chemical Bonds – Electronegativity Bond Polarity and Dipole Moments Bond Enthalpies
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Chemical Bonds In broad terms, a chemical bond is a term used to characterize an interaction between two atoms that results in a reduction in Gibbs free energy for the system relative to the isolated atoms. The degree of energy reduction or “stabilization” is given by the energy required to break the bond (known as the “bond energy”)
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Chemical Bonds (cont.) Stabilization
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Chemical Bonds (cont.) Prototypical bonding example: NaCl (Sodium Chloride) When NaCl is heated to the point of melting, one can demonstrate that the resulting solution conducts electricity. This observation demonstrates that the solution (liquid NaCl) contains charged species. Those species are Na + and Cl - .
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Chemical Bonds (cont.) Why the formation of Na + and Cl - ? In short, Na + and Cl - are more energetically stable than atomic Na and Cl. With the transfer of an electron from Na to Cl, two ions of opposite charge are produced. The Coulombic attraction between these ions is largely responsible for the stabilization.
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Properties of Ions (cont.) In this example involving NaCl, we have a metal (Na) bonding to a non-metal (Cl). Metal/non-metal binding generally results in ionic bonding.
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Chemical Bonds (cont.) What is the extent of Coulombic stabilization? Na + and Cl - +1 -1 0.276 nm x N a -504 kJ/mol
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Units: Electrical Charge In the text, charge is expressed in Coulombs. In Ch. 13, this convention seems to change, as shown in the prior example.
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