Lattice Energy — Prelims Strategy

Prelims Strategy

To ace NEET questions on lattice energy, a multi-pronged strategy is essential. Firstly, thoroughly understand the definition: energy released when gaseous ions form a solid lattice. Secondly, master the two primary factors: ionic charge (most dominant, $U \propto |z^+z^-|$) and ionic size ($U \propto 1/r_0$).

Practice comparing compounds like $NaCl$ vs $MgO$ (charge effect) and $LiF$ vs $CsF$ (size effect). Remember that higher charges and smaller ions lead to higher lattice energy. Thirdly, become proficient with the Born-Haber cycle.

This involves memorizing the steps (sublimation, ionization, dissociation, electron affinity, lattice formation) and their corresponding enthalpy changes. Practice numerical problems where you calculate lattice energy or any other unknown enthalpy term.

Pay close attention to stoichiometry (e.g., $2 \times EA_1$ for $MgCl_2$) and the correct signs for exothermic (negative) and endothermic (positive) processes. Finally, understand the implications of lattice energy on physical properties: higher lattice energy means higher melting point, greater hardness, and generally lower solubility (unless hydration energy is exceptionally high).

Watch out for trap options that confuse lattice energy with enthalpy of formation or solution, or those that reverse the trends based on size or charge.