6.5.2 Role of Hydration in the Dissolving Process

This topic explains how hydration affects the dissolving process. It describes the energy changes involved when a solute dissolves in a solvent and how lattice energy and enthalpy of hydration determine whether the process is exothermic or endothermic.

Definition

Hydration: The process in which solvent molecules surround the solute particles on all sides.

Enthalpy of Hydration (ΔH_hyd): The energy released when solute particles bind to water molecules in solution.

Enthalpy of Solution (ΔH_solution): The net heat change when a substance dissolves.

Main Idea

The energy change of solution formation depends on three types of interactions between solute-solute, solvent-solvent, and solute-solvent particles. When a solution is formed, the interactions between solute particles are broken. At the same time, the solvent molecules also move apart to accommodate the solute particles. Because it takes energy to break the interaction, both processes are endothermic. At the same time, interactions between the solute and solvent particles occur, i.e., hydration occurs. Solvent molecules surround the solute particles on all sides. Energy is released in these interactions, so it is exothermic. Thus, the strength of these two types of interactions determines whether the dissolution process is endothermic or exothermic.

Core Concept

Δ H_solution = Δ H_lattice - Δ H_hyd

• Lattice energy (ΔH_lattice): Energy required to break the solute particles out of the solid lattice (endothermic).
• Enthalpy of hydration (ΔH_hyd): Energy released when solute particles bind to water molecules (exothermic).
• Enthalpy of solution (ΔH_solution): Difference between lattice energy and enthalpy of hydration.

Detailed Explanation

The Dissolving Process

• The attractive force of the solute particles must be overcome to break the solute particles out of the solid lattice.
• A strong force must bind these separate solute particles to the water molecules in the solution.
• Therefore, the heat of solution measures the net flow of energy that occurs when a substance dissolves.

Energy Components

• The energy required to break the solute particles is equal to the lattice energy (ΔH_lattice) of the solute.
• The energy released when solute particles bind to water molecules in solution is equal to the enthalpy of hydration (ΔH_hyd) .
• For other solvents, this energy is called the heat of dissolution.
• The enthalpy of solution includes both the energy required to push the solvent molecules apart and the energy released when they surround the solute particles.

Formula

Δ H_solution = Δ H_lattice - Δ H_hyd

Endothermic vs. Exothermic Dissolution

• When ΔH_solution is very large, the solute is unlikely to be soluble.
• But if ΔH_solution is moderately positive, then solute dissolves.
• Example: When ammonium nitrate dissolves in water, 25.7 kJ mol⁻¹ energy is absorbed, and the flask becomes cold.
• This is because the crystal binds NH₄⁺ and NO₃⁻ ions more tightly than the solution.

NH₄NO_3(s) + xH₂O_(l) → NH₄⁺_(aq) + NO₃^-_(aq)

• On the other hand, if the solvent binds solute particles more tightly than the crystal, energy is released as the solute dissolves, and we have an exothermic solution process.
• Example: When NaOH dissolves in water, 44.5 kJ mol⁻¹ energy is released, and the flask becomes hot.

NaOH_(s) + xH₂O_(l) → Na⁺_(aq) + OH^-_(aq)

Other Examples

CaCl_2(s) + H₂O_(l) → Ca²⁺_(aq) + 2Cl^-_(aq) Δ H_soln = -83.0 kJ mol^-1

MgSO_4(s) + xH₂O_(l) → Mg²⁺_(aq) + SO₄^2-_(aq) Δ H_soln = -91.2 kJ mol^-1

Table 6.3: Enthalpies of Solutions of Some Ionic Solids