CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A series of sodium halides—NaCl, NaBr, and NaI—are isostructural. The tabulated gaseous ionic radii (pm) are:
Ion
Radius / pm
Na⁺
102
Cl⁻
181
Br⁻
196
I⁻
220
Experimental data show that the lattice parameter (edge length) of the cubic unit cell, the lattice energy, and the enthalpy of fusion all vary systematically across the series.
(a) Predict the qualitative trend (increase or decrease) in unit-cell edge length from NaCl to NaI. Support your answer with appropriate reasoning.
(b) Explain how the observed trend in lattice energy across the series arises from changes in both ionic charge (if any) and inter-ionic distance.
(c) Using your explanation from part (b), account for the corresponding trend in enthalpy of fusion for the three salts.