Question

Arrange the following in increasing order of lattice energy:

NaF, NaCl, NaBr, NaI

(A) NaI < NaBr < NaCl < NaF

(B) NaF < NaCl < NaBr < NaI

(C) NaI < NaCl < NaBr < NaF

(D) None of these

Answer 1

Hint: Lattice energy is related to the formation of crystalline solids from its ions. The Born-Haber cycle is a process which is used in the calculation of lattice energy or we can say that lattice enthalpy of a crystalline solid is measured of the energy which is released when the ions combine to make a compound.

Complete answer:

The energy which is used to decompose the ionic solids into its constituent’s cations and anions is known as lattice energy. For example when sodium and chloride ions combine to form sodium chloride. Both sodium and chloride ions are present in a gaseous state, 787.3kj/mol of energy release, this is known as lattice energy. So as we go down the group, ionic radius increases which decreases the lattice energy. When we move from left to right in a period or as the charge on the ions increases the lattice energy increases. Lattice energy is directly proportional to the charge on the ion and it is inversely proportional to the size of the ion.

$\text{Lattice energy}\propto {\displaystyle \frac{\text{Charge on the ion}}{\text{Size of the ion}}}$

In the given options the largest anion will be  $I^{-}$ and the smallest anion will be  $F^{-}$ . So, the correct order will be NaI < NaBr < NaCl < NaF

Hence the correct answer is option (A).

Note: The effects of lattice energies are:

The greater the lattice energy, more will be the stability of the ionic compounds. The lattice energy is greater for small and highly charged ions. The lattice energy affects the solubility of the ionic compounds.