Question

The correct order of lattice energy is:
A. $LiF$>\[Ca{F_2}\] >$MgO$
B. $MgO$>\[Ca{F_2}\]>$LiF$
C. \[Ca{F_2}\]>$MgO$>$LiF$
D. \[Ca{F_2}\]>$LiF$>$MgO$

Answer 1

Hint: Lattice energy of the ionic compound is defined as the amount of energy released when one mole of ionic compound is formed by the interaction of required number of constituent gaseous cations and gaseous anions.

Complete step by step solution: Lattice energy can be positive or negative depending upon the association or dissociation of ions. For association it is negative and for dissociation lattice energy is positive.
Lattice energy is directly proportional to the charge of the ions and is inversely proportional to the size of ions. Larger the size of ions weaker is the force of attraction between the ions and smaller is the lattice energy. In the above given compounds the highest charge on dissociating is on MgO (magnesium oxide) which is (+2 charge) on Mg and (-2 charge) on O. Lithium fluoride has only +1 and -1 charge on Li and F respectively. Calcium fluoride has a +2 and -1 charge on Ca and F respectively.
 Hence the correct option is option B ($MgO$>\[Ca{F_2}\]>$LiF$).

Additional information: Lattice energies cannot be measured directly but experimental values are obtained from thermodynamic data using the Born-Haber cycle. This cycle is the change in enthalpy that occurs when one mole of a solid crystalline substance is formed from its gaseous ions. This cycle is the change of enthalpy in sublimation, dissociation, ionization and lattice energy.

Note: Both the charge and size of the ions affect lattice energy but between charge and size of the ions, charge on the ions always greatly affects the lattice energy.