JEE Main · 2023 · 13 Apr 2023 · Shift 2 · Chemical Thermodynamics

The standard enthalpy of formation of an ionic solid is governed chiefly by its lattice enthalpy, which grows as the ions become smaller and pack more closely. Among the sodium halides the fluoride ion is the smallest, so $\mathrm{NaF}$ has the largest lattice enthalpy and the most exothermic, most negative, formation enthalpy; the value becomes steadily less exothermic through $\mathrm{NaCl}$ and $\mathrm{NaBr}$ to $\mathrm{NaI}$, which has the smallest magnitude. Arranged in order of increasing magnitude of the formation enthalpy, that is from the least to the most exothermic, the sodium halides run $\mathrm{NaI}<\mathrm{NaBr}<\mathrm{NaCl}<\mathrm{NaF}$.