CHEMISTRY · Multi-part · ISC · Chemistry · Chemical Bonding and Molecular Structure · Chemical Bonding and Molecular Structure
Problem
The alkali metal fluorides LiF, NaF, KF, and CsF are studied at two different external pressures.
At 1atm each crystal exhibits its ambient lattice constant a0. When subjected to 5.0GPa of pressure, the lattice constant for every fluoride decreases by the same fractional amount, a=0.92a0.
Ionic radii (pm): Li+ 76, Na+ 102, K+ 138, Cs+ 174, F− 133.
Assume that the fluoride anion radius remains effectively constant with pressure and that the magnitude of the lattice energy ∣U∣ is governed solely by the product of the ionic charges and the nearest-neighbor cation–anion separation.
(1) Part 1: (a) Rank the four fluorides by the magnitude of their lattice energies at 1atm.
(2) Part 2: (b) Using the information about lattice compression, predict whether any changes occur in the ranking when the pressure is increased to 5.0GPa. State the new order, or indicate that the order is unchanged.
(3) Part 3: (c) Justify your answer in part (b) with explicit reference to how pressure-induced changes in inter-ionic distance interact with the original size differences among the cations.
(4) Part 4: (d) Discuss whether charge magnitude or ionic size is the dominant factor in determining the absolute lattice energies for this series under high pressure. Support your argument with qualitative or quantitative evidence from the data.