Question

The lattice energy of NaCl is $788 \text{ kJ mol}^{-1}$. This means that 788 kJ of energy is required}

• A. to separate one mole of solid NaCl into one mole of Na(g) and one mole of Cl(g) to infinite distance
• B. to separate one mole of solid NaCl into one mole of Na$^+$(g) and one mole of Cl$^-$ (g) to infinite distance
• C. to convert one mole of solid NaCl into one mole of gaseous NaCl
• D. to convert one mole of gaseous NaCl into one mole of solid NaCl
• E. to separate one mole of gaseous NaCl into one mole of Na$^+$(g) and one mole of Cl$^-$ (g) to infinite distance

Hint:

Lattice energy always involves two "keywords": Solid (starting point) and Gaseous Ions (end point). Any option missing either of these is incorrect.

Answer 1

The Correct Option is B

Concept: Lattice energy is a measure of the strength of the ionic bonds in an ionic compound. It is defined based on the formation or dissociation of the ionic crystal.
• Technical Definition: The energy required to completely separate one mole of a solid ionic compound into its constituent gaseous ions.
• State of Matter: The starting material must be a solid lattice, and the final products must be gaseous ions.
• Sign Convention: It is an endothermic process (energy required) when breaking the lattice.

Step 1: Analyze the chemical equation representing lattice energy. For Sodium Chloride (NaCl), the process is: \[ NaCl(s) \rightarrow Na^+(g) + Cl^-(g) \quad \Delta H_{lattice} = +788 \text{ kJ/mol} \]

Step 2: Evaluate the options.
• Option (A) is wrong because it mentions neutral atoms ($Na, Cl$), not ions.
• Option (C) and (E) involve gaseous NaCl, which doesn't represent the breakdown of a solid crystal lattice.
• Option (B) correctly identifies the separation of the solid into gaseous ions.