Question

$\Delta_{r} H$ of which reaction correctly represents the lattice enthalpy of $NaCl ( s ) ?$

• A. $NaCl ( s ) \rightarrow Na ( g )+ Cl ( g )$
• B. $NaCl ( s ) \rightarrow Na ^{+}( g )+ Cl ^{-}( g )$
• C. $Na ( g )+ Cl ( g ) \rightarrow NaCl ( s )$
• D. $Na ^{+}( g )+ Cl ^{-}( g ) \rightarrow NaCl ( s )$

Answer 1

Answer: (B)

Lattice enthalpy of an ionic solid like $NaCl (s)$ is the energy required to completely separate one mole of solid ionic compound into gaseous constituent ions. For $NaCl (s)$
$\underset{1 \text{mole}}{NaCl (s)} \xrightarrow {\text{Lattice enthalpy}} Na ^+(g) + Cl ^+ (g)$
So here, $\Delta_{u} H=\Delta_{r} H=$ enthalpy of reaction.