Lecture Notes: Determining Lattice Energy of Ionic Compounds

Key Concepts

• Lattice Energy:

  • Defined as the energy change when gaseous ions form a solid ionic compound.
  • Example: Formation of sodium chloride from sodium cation and chloride ion.
  • Depends on two factors: charge of ions ( (q_1, q_2) ) and distance between ions.

• Factors Influencing Lattice Energy:

  • Charge of Ions:
    • Lattice energy increases with increasing charge of the ions.
  • Distance Between Ions:
    • Lattice energy decreases with increasing distance between ions.

Important Principles

• Ions with larger charge values have greater lattice energy.
• Ionic compounds with smaller ions tend to have higher lattice energy.

Examples and Analysis

Example 1: Magnesium Oxide vs. Sodium Fluoride

• Charges:
  • Magnesium (Mg): (+2), Oxygen (O): (-2)
  • Sodium (Na): (+1), Fluoride (F): (-1)
• Analysis:
  • Product of charges for MgO ( = -4 ) and NaF ( = -1 ).
  • MgO has higher lattice energy due to higher charge product.
  • Lattice Energy Values:
    • MgO: (-3800) kJ/mol
    • NaF: (-923) kJ/mol
  • MgO's lattice energy is over four times greater than NaF.

Example 2: Sodium Fluoride vs. Potassium Chloride

• Charges:
  • Both have same charge: Na ((+1)), K ((+1)); F ((-1)), Cl ((-1))
• Sizes of Ions:
  • Sodium is smaller than potassium; fluoride is smaller than chloride.
• Analysis:
  • Smaller ionic size in NaF results in higher lattice energy than in KCl.
  • Lattice Energy Values:
    • NaF: (-923) kJ/mol
    • KCl: (-715) kJ/mol

Ranking Ionic Compounds by Lattice Energy

• Criteria:
  • First evaluate ionic charges.
  • If charges are the same, evaluate ionic radii.
  • Higher lattice energy corresponds to higher charge and smaller ionic radius.

Ranking Example

• Compounds: Aluminum Oxide ((Al_2O_3)), Beryllium Oxide ((BeO)), Calcium Oxide ((CaO)), Lithium Fluoride ((LiF)), Potassium Iodide ((KI))
• Charges:
  • Aluminum ((+3)), Beryllium ((+2)), Calcium ((+2)), Lithium, Potassium ((+1))
  • Oxygen ((-2)), Fluoride, Iodide ((-1))
• Order based on Lattice Energy:
  • Highest: Aluminum Oxide (due to (+3) charge)
  • Followed by Beryllium Oxide, Calcium Oxide, Lithium Fluoride
  • Lowest: Potassium Iodide
• Lattice Energy Values:
  • (Al_2O_3): ~16,000 kJ/mol
  • (BeO): 4443 kJ/mol
  • (CaO): 3400 kJ/mol
  • (LiF): 1036 kJ/mol
  • (KI): 649 kJ/mol

Conclusion

• To determine the greatest lattice energy, focus on compounds with:
  • Highest Ionic Charges
  • Smallest Ionic Radii
• This allows accurate prediction of lattice energy associated with different ionic compounds.