CHEMISTRY · Single correct · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
An investigator studies the oxide MX2, where M4+ (ionic radius 62 pm) is surrounded by X2− (ionic radius 184 pm). Two independent substitution strategies are proposed:
• Strategy 1: Replace 5 % of M4+ with M′3+ (ionic radius 67 pm).
• Strategy 2: Replace 5 % of X2− with X′1− (ionic radius 181 pm).
Assuming lattice geometry is otherwise preserved and electrostatic interactions dominate, which strategy is predicted to produce the larger decrease in the magnitude of the lattice energy, and why?
(a) Strategy 1, because the average cation charge decreases while the cation–anion distance remains essentially unchanged.
(b) Strategy 1, because the increase in cation radius lengthens the cation–anion separation, a factor that outweighs any change in ion charge.
(c) Strategy 2, because substituting X2− with lower-charge X1− reduces the average magnitude of the ∣z+z−∣ term far more than the small change in ion separation, leading to the greatest drop in lattice energy.
(d) Strategy 2, because lowering the negative charge on some anions lessens anion–anion repulsion, thereby lowering the lattice energy.