QJEEM21Chd9ab: JEE Main 2021 Chemistry - Class 11 JEEM21Chd9ab Solution: The correct set from the following in which both pairs are in…

Correct option is i.e. LiF > LiCl; MgO > NaCl. Melting point is directly proportional to lattice energy. Lattice energy is the energy required to separate a mole of an ionic solid into gaseous ions. It depends upon charge of ions and size of ions.

$$M.P.\propto L.E.\propto \frac{Charge}{Size}$$

$$\begin{array}{cc}Li\to +1& Li\to +1\\ F=-1& Cl\to -1\end{array}$$

Both LiF and LiCl having same charge, so melting point will depend on size.

Larger the size of anion, lesser the lattice energy and hence, melting point order is LiF > LiCl.

Similarly, $$\begin{array}{cc}MgO& NaCl\\ Mg\to 2+& Na\to 1+\\ O\to 2-& Cl\to 1-\end{array}$$

MgO having + 2 charge which is greater than NaCl (+ 1) charge. So, greater the charge on the ions greater will be lattice energy and hence, melting point order is MgO > NaCl.