Which of the following substances has highest value of standard molar enthalpy of formation at \(298…
Which of the following substances has highest value of standard molar enthalpy of formation at \(298 \mathrm{~K}\) ?
\(\mathrm{NaF}(\mathrm{s})\)
\(\mathrm{NaCl}(\mathrm{s})\)
\(\mathrm{NaBr}(\mathrm{s})\)
\(\mathrm{Nal}(\mathrm{s})\)
Solution
Higher the stability of a substance, higher will be its standard molar enthalpy of formation value, \(\left|\Delta_f H^{\circ}\right|\)
Among the halides, \(\mathrm{F}^{-}\)is smallest and \(\mathrm{I}^{-}\)is largest in size. So, according to Fajan's rule, \(\mathrm{NaF}\) is mostly ionic in nature.
\(\mathrm{NaI} < \mathrm{NaBr} < \mathrm{NaCl} < \mathrm{NaF}\)
\(\longrightarrow\) Ionic nature
\(\longrightarrow\) Thermal stability
So, \(\mathrm{NaF}\) has highest value of \(\left|\Delta_f H^{\circ}\right|\).