Match each observed physical property or phenomenon of ionic (electrovalent) compounds on … — JAMB UTME

Match each observed physical property or phenomenon of ionic (electrovalent) compounds on the left with its correct underlying thermodynamic or structural explanation on the right.

Magnesium oxide ( $MgO$ ) exhibits an exceptionally high melting point ( $2852^\circ\text{C}$ ) compared to sodium chloride ( $NaCl$ , $801^\circ\text{C}$ ).Lattice energy is directly proportional to the product of ionic charges ( $q_1 q_2$ ), making the electrostatic forces between divalent ions vastly stronger than between monovalent ions.
Anhydrous aluminium iodide ( $AlI_3$ ) exhibits marked covalent character and a low melting point ( $191^\circ\text{C}$ ) despite forming between a metal and a non-metal.A small cation with high charge density ( $Al^{3+}$ ) strongly polarizes the large, highly polarizable electron cloud of the anion ( $I^-$ ), inducing electron cloud distortion.
Sodium hydroxide ( $NaOH$ ) dissolves exothermically in water despite requiring energy to break its crystal lattice.The total enthalpy of ion hydration released upon interaction with polar water molecules is greater in magnitude than the lattice enthalpy of the crystal lattice.
Solid calcium fluoride ( $CaF_2$ ) is an electrical insulator, but conducts electricity readily when melted.Ions are held in fixed positions within a rigid three-dimensional crystal lattice in the solid state, becoming mobile charge carriers only when the lattice breaks down.

Answer

1 matches with the explanation of charge product dependence on lattice energy; 2 matches with the explanation of Fajans' rules of polarization; 3 matches with the explanation of hydration enthalpy exceeding lattice enthalpy; 4 matches with the explanation of ion mobility in molten versus solid states.

Each physical property directly corresponds to its underlying quantum mechanical or thermodynamic principle: lattice energy scales with charge product (

MgO

NaCl

), polarization of anion electron clouds by small high-charge cations creates covalent character (

AlI_3

), exothermic dissolution occurs when hydration energy exceeds lattice energy (

NaOH

), and electrical conduction requires mobile ions that are locked in solids but liberated upon melting (

CaF_2

Step-by-Step Solution

Analyze the high melting point of

MgO

versus

NaCl

Lattice energy is governed by Coulomb's law:

E \propto \frac{|q_1 q_2|}{r}

MgO

consists of

Mg^{2+}

and

O^{2-}

(product = 4), while

NaCl

consists of

Na^+

and

Cl^-

(product = 1). Higher charge product leads to stronger lattice attraction and a higher melting point.

Identify the primary thermodynamic factor controlling lattice strength in ionic crystals.

Analyze the anomalous covalent behavior of

AlI_3

Apply Fajans' rules: Covalency increases with high cation charge density and large anion size.

Al^{3+}

has high charge density and

I^-

is large and easily polarized, leading to electron cloud sharing (covalent character).

Explain deviations from purely electrovalent behavior using polarization principles.

Analyze the thermochemistry of dissolution of

NaOH

Dissolution enthalpy

\Delta H_{soln} = \Delta H_{lat} + \Delta H_{hyd}

. If hydration enthalpy released is greater in magnitude than the lattice enthalpy required to separate ions, the net process is exothermic.

Relate lattice energy and hydration energy to dissolution energetics.

Analyze electrical conductivity in solid versus molten

CaF_2

Solid ionic compounds contain ions held rigidly in a lattice structure. When melted, thermal energy breaks the lattice, producing free-moving ions capable of carrying electrical current.

Distinguish between mobile charge carriers (molten state) and immobile lattice positions (solid state).

Key Concept

Thermodynamic and structural factors governing ionic lattice stability, polarization (Fajans' rules), solution energetics, and state-dependent conductivity.

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