Lattice Energy of Ionic Compounds, Basic Introduction, Charge vs Ionic Radius

Lattice Energy Overview

  • Definition: Lattice energy is the energy change that occurs when gaseous ions combine to form a solid ionic compound.

  • Example: For sodium chloride formation, the reaction of a sodium cation in gas with a chloride ion to create solid sodium chloride releases energy known as lattice energy.

Factors Affecting Lattice Energy

  • Charge of Ions (q1, q2):

    • Higher charges lead to increased lattice energy.

    • Example:

      • Magnesium Oxide: Mg (+2) and O (-2) gives a product of -4.

      • Sodium Fluoride: Na (+1) and F (-1) gives a product of -1.

    • Result: MgO has a significantly higher lattice energy due to higher charges.

  • Distance Between Ions:

    • Greater distances between ions decrease lattice energy.

    • Smaller ionic sizes contribute to higher lattice energies.

Comparative Analysis of Magnesium Oxide and Sodium Fluoride

  • Charges:

    • Magnesium Oxide: +2 (Mg), -2 (O)

    • Sodium Fluoride: +1 (Na), -1 (F)

    • Result: MgO has a charge product of 4 times that of NaF.

  • Lattice Energies:

    • Magnesium Oxide: -3,800 kJ/mol

    • Sodium Fluoride: -923 kJ/mol

    • Ratio: 3,800/923 = 4.12 (MgO has over 4 times higher lattice energy).

  • Ion Sizes:

    • Mg (65 pm) is smaller than Na (95 pm).

    • O (140 pm) and F (136 pm) are close in size.

    • Conclusion: Smaller Mg and higher charges of MgO lead to its superior lattice energy.

Comparative Analysis of Sodium Fluoride and Potassium Chloride

  • Charges:

    • Sodium Fluoride: +1 (Na), -1 (F)

    • Potassium Chloride: +1 (K), -1 (Cl)

    • Result: Charges are equal, now consider size.

  • Ion Sizes:

    • Na: 95 pm vs K: 133 pm (K is larger).

    • F: 136 pm vs Cl: 181 pm (Cl is larger).

    • Result: NaF (smaller ions) has higher lattice energy than KCl (larger ions).

  • Lattice Energies:

    • Sodium Fluoride: -923 kJ/mol

    • Potassium Chloride: -715 kJ/mol

General Guidelines for Lattice Energy Determination

  • Prioritize Charges: Higher charges yield higher lattice energies.

  • Consider Ionic Radius:

    • Smaller ionic radius leads to increased lattice energy.

  • Final Assessment: Look for ions with the highest charge and smallest radius for the greatest lattice energy.

Ranking Lattice Energy from Least to Most Exothermic

  1. Potassium Iodide (KI): -649 kJ/mol

  2. Lithium Fluoride (LiF): -1036 kJ/mol

  3. Calcium Oxide (CaO): -3400 kJ/mol

  4. Beryllium Oxide (BeO): -4443 kJ/mol

  5. Aluminum Oxide (Al2O3): -16000 kJ/mol

Justification for Ranking

  • Aluminum Oxide (Al2O3): Highest charge (+3 for Al) and small ionic radius.

  • Beryllium Oxide: Higher lattice energy than Calcium Oxide due to smaller Be ionic radius (31 pm vs 99 pm for Ca).

  • Lithium Fluoride: More exothermic than Potassium Iodide due to smaller ionic sizes.