Lattice Energy | Ionic Bonding and Properties of Ionic Compounds

Clues:

1. Ionic Charges: Lattice energy is directly proportional to the product of the charges of the ions. Higher charges generally result in higher lattice energies.
2. Ionic Radii: Lattice energy is inversely proportional to the sum of the ionic radii. Smaller ions result in higher lattice energies.
3. Use Coulomb's Law: Recall Coulomb's law for lattice energy.
4. Comparing Compounds:
   Compare the charges of the cations and anions for each compound.
   Compare the sizes of the ions involved. Smaller ions will result in a higher lattice energy for compounds with similar charges.

Answer:

The compounds ordered according to their expected lattice energies from highest to lowest are: MgO > CaS > NaCl > KBr

MgO (Magnesium Oxide):

• Ionic Charges: Mg²⁺ and O^2-, both have charges of ±2.

• Ionic Radii: Magnesium and oxide ions are relatively small.

• Lattice Energy: Highest due to the high charges (±2) and relatively small ionic sizes.

CaS (Calcium Sulfide):

• Ionic Charges: Ca²⁺ and S^2-, both have charges of ±2.
• Ionic Radii: Calcium and sulfide ions are larger than magnesium and oxide ions but still have a significant lattice energy.
• Lattice Energy: Second highest due to the charges of ±2, though the ions are larger than in MgO.

NaCl (Sodium Chloride):

• Ionic Charges: Na⁺ and Cl^-, both have charges of ±1.

• Ionic Radii: Sodium and chloride ions are relatively small but larger than Mg²⁺ and O^2-.

• Lattice Energy: Lower than MgO and CaS because the charges are lower (±1) despite the small ionic sizes.

KBr (Potassium Bromide):

• Ionic Charges: K⁺ and Br^-, both have charges of ±1.

• Ionic Radii: Potassium and bromide ions are larger than sodium and chloride ions.

• Lattice Energy: Lowest among the four due to the larger ionic sizes and lower charges (±1).

By considering both the charges and the sizes of the ions involved, we can rank the compounds based on their expected lattice energies accurately.