11-7-11 - Chemistry 1A F11 11.7.11 Dr. Shaka Lattice...

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212 Chemistry 1A F11 11.7.11 Dr. Shaka Course Code 40000 Lattice Enthalpy • Recall that we have a formula for the electric potential energy of two point charges separated by a distance r : E = 1 4 "# 0 q 1 q 2 r • In an ionic solid there are an infinite number of positive and negative ions (sometimes multiply charged) and they are arranged in a crystalline array. • The energy (per mole) is the lattice enthalpy.
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213 Chemistry 1A F11 11.7.11 Dr. Shaka Course Code 40000 Lattice Enthalpy • Brilliant theoreticians worked out how to obtain closed formulas for the lattice enthalpy of different arrangements. NaCl (Adapted from Wikipedia) Rock-salt structure
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214 Chemistry 1A F11 11.7.11 Dr. Shaka Course Code 40000 Lattice Enthalpy • Unfortunately, we can’t measure the lattice enthalpy directly, because the reaction is LiF " Li + g ( ) + F # g ( ) • This is easier for the theoreticians to calculate, but in the lab it’s almost impossible to set up a clean measurement. • We could always get, instead, LiF " Li g ( ) + F g ( )
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Chemistry 1A F11 11.7.11 Dr. Shaka Course Code 40000 Born-Haber Cycle • Using conservation of energy, we can set up a number of reactions that we can measure in the lab, and whose sum comes out to the lattice enthalpy. This is the Born-Haber cycle . • We begin with the solid elements, get them into the gas phase, transfer an electron, and then let the ions crash out into a solid again. • All these reactions can be measured in
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11-7-11 - Chemistry 1A F11 11.7.11 Dr. Shaka Lattice...

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