Lattice Energy or Lattice Enthalpy
Lattice energy measure the strength of bonds in an ionic solid and is important for understanding the stability, solubility, and hardness of ionic compounds. Greater the lattice energy, higher the stability, less the solubility and more harder the ionic solid.
Lattice Energy is a type of potential energy that may be defined in two ways.
Lattice Dissociation Enthalpy
Lattice energy is the energy required to break an ionic solid to its component atoms into gaseous ions. In this process, lattice energy is always positive because, this is an endothermic process. For NaCl, the lattice dissociation enthalpy is +787 kJ mol-1.
Lattice Association Enthalpy
Lattice energy is the energy released when an ionic solid is formed from its gaseous ions. In this process, lattice energy is always negative because, this is an exothermic process. For NaCl, the lattice formation or association enthalpy is -787 kJ mol-1.
Lattice energy is usually expressed in kilojoules per mole (kJ/mol).
Lattice energy can't be measured directly, but it can be estimated using the Born-Haber cycle. The magnitude of lattice energy is associated with other physical characteristics, such as solubility, hardness, and volatility.
Factors Affecting Lattice Energy
There are several factors that affects lattice energy. Some of them are discussed below-
Ionic Charge
Lattice energy is directly proportional to the ionic charge that means lattice energy increases with increase in ionic charge, as the charge on the ions increases, the ionic bond gets stronger.
Ionic Radius
Ionic radius is inversely proportional to the lattice energy that means, lattice energy decreases with increasing ionic radius, as the ionic radius decreases, the ionic bond becomes stronger.
Effective Nuclear Charge
Effective nuclear charge is directly proportional to lattice energy. Higher effective nuclear charge leads to a smaller ionic radius and vice-versa.
Polarizability of Ions
Lattice energy decreases with increasing the polarizability of the ions because, polarizable ions can more easily distort their electron clouds in response to the electrostatic forces of the other ions, which reduces the strength of the forces holding them together. Less polarizable ions lead to higher lattice energy because they maintain stronger ionic character.
Ionization Energy
Lattice energy generally increases with the ionization energy of the ions because, higher ionization energy means more energy required to remove an electron from an atom, which means that the ion will be more stable and have a higher lattice energy.
Electron Affinity
Lattice energy increases with the electron affinity of the ions because, higher electron affinity means more energy required to add an electron to an atom, which means that the ion will be more stable and have higher lattice energy.
Crystal Structure
The arrangement of ions in the crystal lattice affects the Madelung constant (M). Different structures have different values of Madelung constant which affects the overall lattice energy.
Simple cubic: Lower lattice energy.
Face-centered cubic (FCC): Higher lattice energy due to more efficient packing.
Temperature
Higher temperatures can weaken ionic bonds and decrease lattice energy better to say lattice enthalpy.
Pressure
Lattice energy generally increases on increasing the pressure as ions come closer on increased pressure.
University Questions
Why is lattice energy very high for ionic crystal?
How does lattice energy depend on internuclear distance and charges on the ions?
How to calculate lattice energy?
Calculate the lattice energy of NaCl using Born-Haber cycle.
Calculate the lattice energy of NaCl using the Born-Lande equation.
Explain why the lattice energy of CaO is more exothermic than the lattice energy of LiF.
The lattice energy of CaO is more exothermic than that of LiF due to the higher charges on the ions and the smaller ionic radii in CaO. These factors lead to a much stronger electrostatic attraction between the ions in CaO, resulting in a more exothermic lattice energy compared to LiF.
