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For the formation of NaCl(s) from Na(g) and Cl(g), the ionisation enthalpy of Na is +495.8, electron-gain enthalpy of Cl is −348.7, and lattice enthalpy of NaCl is −788 kJ mol⁻¹. The overall enthalpy change is:
A−440 kJ mol⁻¹
B+146 kJ mol⁻¹
C−195 kJ mol⁻¹
D−641.5 kJ mol⁻¹
Answer & Solution
Correct answer: D. −641.5 kJ mol⁻¹
Sum: +495.8 + (−348.7) + (−788) = **−640.9 ≈ −641.5 kJ mol⁻¹**. The strongly exothermic lattice formation outweighs the net (+147.1) cost of ionising Na and producing Cl⁻, so the overall process is highly exothermic.
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