1. Chemical Reaction Equations
When calcium chloride (CaCl₂) dissolves in water, it undergoes dissolution and hydration, primarily in two scenarios:
(1) Dissolution and Hydrolysis of Anhydrous Calcium Chloride (CaCl₂)CaCl2(s)+excess H2O(l)→Ca2+(aq)+2Cl−(aq)+heatCaCl2(s)+excess H2O(l)→Ca2+(aq)+2Cl−(aq)+heat
- This process is an exothermic reaction (ΔH < 0), releasing significant heat.
(2) Formation of Hydrates (e.g., Dihydrate CaCl₂·2H₂O)CaCl2(s)+2H2O(l)→CaCl2⋅2H2O(s)+heatCaCl2(s)+2H2O(l)→CaCl2⋅2H2O(s)+heat
- This reaction is also exothermic, but the heat released is less than that of anhydrous CaCl₂ dissolving directly.
2. Explanation of the Exothermic Phenomenon
The heat release during the dissolution of calcium chloride is determined by two energy changes:
(1) Lattice Energy Breakdown (Endothermic)
- The ionic bonds (electrostatic interactions between Ca²⁺ and Cl⁻) in solid CaCl₂ are broken, requiring energy input.
(2) Hydration Energy Release (Exothermic)
- When Ca²⁺ and Cl⁻ enter the water, they form hydrated ions:
- Ca²⁺ attracts water molecules to form [Ca(H2O)6]2+[Ca(H2O)6]2+ (hexaaquacalcium ion).
- Cl⁻ binds to water molecules via hydrogen bonds.
- The energy released during hydration > energy absorbed during lattice breakdown, resulting in a net exothermic effect.
(3) Comparison of Heat Release
- Anhydrous CaCl₂ releases more heat (~82.8 kJ/mol) due to more intense hydration.
- Hydrated CaCl₂ (e.g., dihydrate) releases less heat because some water molecules are already bound.
3. Practical Applications of the Exothermic Effect
- De-icing Agent: The heat released during CaCl₂ dissolution accelerates snow and ice melting, making it more efficient than NaCl.
- Self-Heating Materials: Used in emergency heat packs (utilizing the exothermic reaction of CaCl₂ + water).
- Laboratory Safety: Preparing concentrated CaCl₂ solutions can cause rapid temperature rise or even boiling; thus, water should be added slowly with stirring.
