Lattice Energy (Crystal Energy)

The lattice energy is the amount of energy evolved when gaseous ions of opposite charges are brought together to form an ionic crystal composed of 1 mole of formula units of a compound.

OR

Lattice energy is defined as the amount of energ’y required to convert one mole of an ionic solid to its gaseous ions.

The concept of Lattice energ’y was originally developed for rock-salt structured and Sphalerite structure.The sphalerite structure is made up of zinc and sulfur.The compounds like Na-Cl and Zns ,where the ions occupy high symmetry crystal lattice sites.

 In the case of NaCl, Lattice energy is the energ’y released by the reaction
Na+ + Cl−  → Na Cl
which would be amount to -786 kJ/mol
In this case Na and cl in gas form and NaCl in solid phase.
lattice energy is depend upon

The lattice energ’y is depend upon the strength of the interaction  between  the particles  in the solid and in the geometry of the crystal structure.It is a measure of the stability of an ionic solid. The greater the lattice energ’y,the stronger the bond.

Lattice energy maybe determined in two different ways:

  • By theoretical  calculation
  • By experimental measurement

The theoretical approach involves the calculation of interaction energies among all the particles in a crystal.The calculation gives a result which is simply represented as follows;

lattice energy formula
Where A is a constant whose value depends upon the kind of crystal lattice.And q+ and q- are the charges on the ions , r+ and r- are the ionic radii.Where B is an small constant of repulsion.
The important quantities which determine the magnitude of the lattice energ’y are, therefore ,ionic charges q, and the ionic radii.

Equation description:

Equation shows , that the crystal lattice energ’y increases in magnitude as the charges on the ions increase. Thus most ionic solids  containing multiply charged ions have higher crystal lattice energ’y. Than those containing only mono valent ions.The equation also shows that the crystal lattice energy gets larger as the separation between cation and anion decrease, that is as r+ and r- decrease.
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