Chemistry · Chemical Bonding · NEET
It is the energy needed to break one mole of a solid ionic compound completely apart into its free gaseous ions. Imagine a NaCl crystal where Na+ and Cl- ions are tightly packed and attracting each other. Lattice enthalpy is the energy you must supply to pull all those ions apart until they are gaseous and infinitely far from each other. NCERT defines it as 'the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions.'
NCERT gives the lattice enthalpy of NaCl as 788 kJ/mol. This means 788 kJ of energy is required to separate one mole of solid NaCl into one mole of Na+ (g) and one mole of Cl- (g), taken to an infinite distance. This is the exact number and example NEET expects, so memorise 788 kJ/mol for NaCl.
When defined the NCERT way (breaking the solid into gaseous ions), lattice enthalpy is POSITIVE because you must put energy IN to separate ions that attract each other. The reverse process, where gaseous ions come together to form the solid (M+ (g) + X- (g) to MX (s)), releases energy and is negative. NEET usually uses the 'separation' definition, so treat lattice enthalpy as a large positive number unless the question clearly says 'lattice formation'.
NCERT clearly states that the real measure of the stability of an ionic compound is its enthalpy of lattice formation, not just achieving an octet. A high lattice enthalpy means the ions are held together very strongly, so a lot of energy would be needed to break the crystal. That strong binding is exactly what makes the ionic solid stable. So do not judge stability only by octet completion; judge it by lattice enthalpy.
Because an ionic crystal is three-dimensional. The process involves both attractive forces (between opposite charges) and repulsive forces (between like charges) among many ions at once. NCERT says it is not possible to calculate lattice enthalpy directly from simple attraction and repulsion; you must also include factors linked to the crystal geometry. In practice, lattice enthalpy is found indirectly using the Born-Haber cycle.
Two main factors: (1) higher ionic charge and (2) smaller ionic size. Larger charges attract each other more strongly, and smaller ions can sit closer together, so both raise the lattice enthalpy. For example, MgO (charges +2 and -2, small ions) has a much larger lattice enthalpy than NaCl (charges +1 and -1). This is why high-charge, small-ion compounds form very stable, high-melting ionic solids.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The lattice enthalpy of an ionic solid is the energy required to completely separate one mole of the solid ionic compound into its gaseous constituent ions. This is the exact NCERT definition.
788 kJ/mol. That much energy is needed to break one mole of solid NaCl into one mole of Na+ (g) and one mole of Cl- (g) at infinite separation.
It is found indirectly using the Born-Haber cycle, which links lattice enthalpy to ionization enthalpy, electron gain enthalpy, sublimation, bond dissociation, and the formation enthalpy of the compound.
NCERT says lattice enthalpy is the true measure of the stability of an ionic compound. A large lattice enthalpy means strong ion binding, which is why the ionic solid forms and stays stable, even more than just completing octets.
Yes. Higher ionic charge and smaller ionic size both increase lattice enthalpy, because the electrostatic attraction between ions becomes stronger.
