CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Two ionic solids, CaO and NH₄Cl, are heated from 298 K to 600 K. Experimental X-ray data show that the average cation–anion distance increases by 3.0 % for each solid over this temperature range. The charges and room-temperature ionic radii are provided below.
• Ca²⁺ 100 pm, O²⁻ 140 pm
• N⁴⁺ (in NH₄⁺) treated as a monatomic cation of effective charge +1 and radius 148 pm, Cl⁻ 181 pm
Assume that the magnitude of the lattice energy |U| varies inversely with the cation–anion separation and directly with the product of the ionic charges. Ignore all other contributions.
(a) Calculate the percentage change in the magnitude of the lattice energy for CaO as the temperature rises from 298 K to 600 K.
(b) Repeat the calculation in part (a) for NH₄Cl.
(c) Which solid experiences the larger percentage decrease in lattice energy upon heating? Provide a quantitative comparison and explain why this result occurs, citing the relevant charge and size factors.