Four binary ionic compounds that all crystallise in rock-salt–type lattices are…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Four binary ionic compounds that all crystallise in rock-salt–type lattices are listed below. Relevant ionic radii (pm) for the gaseous ions are provided.
Compound
Cation charge
Anion charge
r₍cation₎ / pm
r₍anion₎ / pm
LiF
+1
–1
76
133
Na₂O
+1
–2
102
140
MgS
+2
–2
72
184
AlN
+3
–3
53
146
Assume the closest cation–anion separation equals the sum of the tabulated radii.
(a) Arrange the four compounds in order of increasing lattice energy and provide a clear justification that references both ionic charge and inter-ionic distance.
(b) Using your ranking, predict which compound will have the highest melting point and which will have the lowest. Explain your reasoning.
(c) AlN is virtually insoluble in liquid water, whereas LiF—despite a relatively large lattice energy—shows modest solubility. Propose an explanation that reconciles these observations by considering the interplay between lattice energy and hydration enthalpy.