Chemistry · Thermodynamics · NEET
It is the heat change when 1 mole of a solid ionic compound splits into its ions as gases. Example: NaCl(s) → Na+(g) + Cl-(g), which needs +788 kJ/mol. It is positive because you must PULL apart ions that are strongly attracted. A bigger lattice enthalpy means a stronger, more stable ionic solid. NEET loves asking you to compare lattice strength (higher charge and smaller ions = larger lattice enthalpy).
You cannot take solid NaCl and simply rip it into free-floating gas ions in a lab and measure the heat. There is no clean single experiment for it. So NCERT uses an INDIRECT method: the Born-Haber cycle. We measure all the OTHER steps (which we CAN measure) and then use Hess's law to solve for the one unknown step, the lattice enthalpy.
Write two paths from the elements to solid NaCl that must give the SAME total (Hess's law). Path 1 is direct: enthalpy of formation ΔfH. Path 2 goes through gas ions: sublime Na, ionize Na, dissociate ½Cl2, add electron to Cl, then form the lattice. Set Path 1 = Path 2 and solve. For NaCl the answer comes out to +788 kJ/mol.
Sublimation of Na = +108.4 (needs energy). Ionization of Na = +496 (needs energy). ½ bond dissociation of Cl2 = +121 (needs energy). Electron gain by Cl = −348.6 (releases energy). Formation of NaCl = −411 (releases energy). Lattice enthalpy (dissociation into ions) = +788 (needs energy). Endothermic steps are +, exothermic steps are −. Getting a sign wrong is the #1 mistake.
It depends on the DIRECTION written. NCERT defines lattice enthalpy as the solid breaking APART into gas ions, so it is POSITIVE (endothermic), e.g. +788 kJ/mol for NaCl. If a question defines 'lattice energy' as ions COMING TOGETHER to form the solid, that is the negative of it (−788). Always read which direction the arrow points before choosing the sign.
They describe the same bonding but are close, not identical. Lattice ENERGY is the internal-energy change (ΔU); lattice ENTHALPY is the heat at constant pressure (ΔH = ΔU + ΔngRT). For NEET they are treated as almost equal and the numbers are used interchangeably in the Born-Haber cycle. Just match the sign to the direction the reaction is written.
The bond dissociation energies of X2, Y2 and XY are in the ratio 1 : 0.5 : 1. ΔH for the formation of XY is −200 kJ mol⁻¹. The bond dissociation energy of X2 will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) Sublimation of Na(s)→Na(g): +108.4. 2) Ionization Na(g)→Na⁺(g): +496. 3) Dissociation ½Cl2→Cl(g): +121. 4) Electron gain Cl(g)→Cl⁻(g): −348.6. 5) Lattice formation Na⁺(g)+Cl⁻(g)→NaCl(s): −788. The direct route is ΔfH(NaCl) = −411 kJ/mol.
Hess's law of constant heat summation. Energy from the elements to the final solid is the same no matter which path you take, so the direct formation enthalpy equals the sum of all the step enthalpies.
Smaller ions and higher charges sit closer and attract more strongly, giving a LARGER lattice enthalpy. So MgO (2+ and 2−) has a much bigger lattice enthalpy than NaCl (1+ and 1−). NEET often asks you to rank ionic solids by lattice strength.
It links thermodynamics with ionic bonding and solubility. A high lattice enthalpy means a stable, hard, high-melting solid that is harder to dissolve. Questions test the sign of each step, Hess's-law calculation, and comparing solids.
