Question

The Correct set from the following in which both pairs are in correct order of melting point is:

• A. $\mathrm{LiF}>\mathrm{LiCl};\mathrm{NaCl}>\mathrm{MgO}$
• B. $\mathrm{LiF}>\mathrm{LiCl};\mathrm{MgO}>\mathrm{NaCl}$
• C. $\mathrm{LiCl}>\mathrm{LiF};\mathrm{NaCl}>\mathrm{MgO}$
• D. $\mathrm{LiCl}>\mathrm{LiF};\mathrm{MgO}>\mathrm{NaCl}$

Answer 1

The correct option is B

$\mathrm{LiF}>\mathrm{LiCl};\mathrm{MgO}>\mathrm{NaCl}$

The correct option is (B):

Explanation for the correct answer:

Melting point:

1. It is basically a temperature at which the solid will start converting into the liquid form.
2. Actually at this temperature the solid and the liquid form will exist in the form of equilibrium.
3. This melting point is just directly proportional to the lattice energy which is actually an energy that is required to separate the mole of the ionic solid into the gaseous ion and so it will depend upon the charge as well as the size of the ion.
4. $$\begin{gathered} \mathrm{M}.\mathrm{P}\propto \mathrm{L}.\mathrm{P}\propto \frac{\mathrm{Charge}}{\mathrm{Size}} \\ \mathrm{Li}=+1\mathrm{and}\mathrm{F}=-1 \\ \mathrm{Li}=+1\mathrm{and}\mathrm{Cl}=-1 \end{gathered}$$
5. So, as both $\mathrm{LiF}$ and $\mathrm{LiCl}$ are having the same charge so the melting point would depend upon the size.
6. Large the size of anion, lesser would be the lattice energy so the melting point of $\mathrm{LiF}$ is greater than that of $\mathrm{LiCl}$.
7. Similarly, $$\begin{gathered} \mathrm{In}\mathrm{MgO}:\mathrm{Mg}=+2\mathrm{and}\mathrm{O}=-2 \\ \mathrm{In}\mathrm{NaCl}:\mathrm{Na}=+1\mathrm{and}\mathrm{Cl}=-1 \end{gathered}$$
8. $\mathrm{MgO}$ is having $+2$ and $\mathrm{NaCl}$ is having $+1$ charge, so the melting point $\mathrm{MgO}$ is greater than that of $\mathrm{NaCl}$.

Therefore, the correct answer is (B): $\mathrm{LiF}>\mathrm{LiCl};\mathrm{MgO}>\mathrm{NaCl}$