Lecture 17sf

Lecture 17sf - Energetics of Ionic Compound Formation...

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Energetics of Ionic Compound Formation Consider the formation of NaCl from its elements: Na(s) + ½Cl 2 (g) → NaCl(s) H f = -411 kJ We can think of this process as proceeding by the following steps: (1) Change solid sodium into gaseous sodium (2) Change Cl 2 (g) into Cl(g) atoms (3) Transfer the valence electron from Na to Cl (4) Na + (g) and Cl - (g) attract, forming the solid NaCl lattice. These steps are shown below: (1) Na(s) → Na(g) H 1 = 107 kJ (2) ½Cl 2 (g) → Cl(g) H 2 = 122 kJ (3a) Na(g) → Na + (g) + e - H 3a = 496 kJ (3b) Cl(g) + e - → Cl - (g) H 3b = -349 kJ (4) Na + (g) + Cl - (g) → NaCl(s) H 4 = x H 1 is the enthalpy of sublimation of Na. H 2 is ½ the bond energy of Cl 2 H 3a is the ionization energy of sodium H 3b is the electron affinity of chlorine (with sign changed, since electron affinity in our text is defined as the energy released in the process of adding an electron to a gaseous atom) The reverse of H 4 is called the lattice energy of NaCl, and is defined in our text as the H of the reaction breaking down the NaCl lattice into its gaseous ions. We can calculate H 4 using Hess’ Law: H 1 + H 2 + H 3 + H 4 = H f of NaCl(s) 107 + 122 + 147 + H 4 = -411 kJ H 4 = -787 Lattice energy is defined as the reverse of reaction 4. Reaction 4 forms the NaCl lattice from its gaseous ions. The reverse process breaks down the NaCl lattice into its gaseous ions. Na + (g) + Cl - (g) → NaCl(s) H 4 = -787
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NaCl(s) → Na + (g) + Cl - (g) Lattice energy = 787 This entire cycle of steps is known as the Born-Haber cycle. The cycle may be diagrammed as shown below: Na(s) + ½Cl 2 (g) NaCl(s) H f = -411 kJ Na(g) + Cl(g) Na + (g) + Cl - (g) H 3 Note that every step in this cycle is endothermic except for the lattice energy. It is the attraction of the positive and negative gaseous ions to form a solid lattice that makes the entire reaction exothermic. Especially note that step 3—the transfer of an electron from gaseous Na to gaseous Cl—is also endothermic, being the sum of the IE of Na and the EA of Cl (496- 349 = 147). It is a common misconception that NaCl forms because Na “wants” to get a complete shell by giving off an electron and Cl “wants” to get a complete shell by gaining an electron. In actuality, the transfer of the electron requires energy. The transfer of the electron may be thought of as the “set-up” step for what really drives the reaction: the attraction of the ions to form a solid lattice. Born-Haber Cycle for MgCl
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This note was uploaded on 02/20/2012 for the course 160 161 taught by Professor Kim during the Fall '08 term at Rutgers.

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Lecture 17sf - Energetics of Ionic Compound Formation...

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