Consider the alkali halide crystal KCl exhibiting ionic bonds. t crystallizes in
ID: 575666 • Letter: C
Question
Consider the alkali halide crystal KCl exhibiting ionic bonds. t crystallizes in the rock salt structure schematically depicted on the right with positively charged potassium ions (blue) and negatively charged chloride ions (green). a) Name the crystal structure of the potassium sublattice. b) Calculate the equilibrium bonding distance Teq using the strategy provided in Lecture 7 using the correct value for the Madelung constant of this ionic crystal structure as well as the g parameters: Z· = 2.05 . 10-8 erg, r 0.326A. Compare your result to the KCl lattice parameter of 6.292 Å. Calculate the lattice energy and compare it to the experimental value of 165.8 kcal/mol. From your result in c) give the lattice energy per bond in eV and determine the cohesive energy per bond. Compare it to the cohesive energy of the van der Waals interaction of the noble gas element with the electronic configuration closest to K and Cl c) d)Explanation / Answer
Cohesive energy for Argon is 0.080 ev/atom i.e. 7.74 kJ/mol.
Above values have been taken from :
p. 595 in Arthur M. Halpern. "From Dimer to Crystal: Calculating the Cohesive Energy of Rare Gas Solids." Journal of Chemical Education, volume 89, number 5, 2012, pp. 592–597. doi:10.1021/ed200348j
p. 50 in Charles Kittel. Introduction to Solid State Physics, 8th edition. Hoboken, NJ: John Wiley & Sons, Inc, 2005.
Compare this value with the cohesive energy obtained for the KCl crystal.