Alkali Metals Halides MCQ - Practice Questions with Answers

Alkali metals react with halogens to form metal halides, which are ionic in nature.

$$
2 \mathrm{M}+\mathrm{X}_2 \rightarrow \mathrm{MX}+\text { Heat }
$$

The reactivity order of alkali metals towards halogens is:

$$
\mathrm{Cs}>\mathrm{Rb}>\mathrm{K}>\mathrm{Na}>\mathrm{Li}
$$

Ionic nature, solubility in $\mathrm{H}_2 \mathrm{O}$ increases while lattice energy decreases.
For example:

$$
\begin{aligned}
& \mathrm{LiF}<\mathrm{NaF}<\mathrm{KF}<\mathrm{RbF}<\mathrm{CsF} \\
& \mathrm{KF}<\mathrm{KCl}<\mathrm{KBr}<\mathrm{KI}
\end{aligned}
$$

These halides are colourless, however, on heating, they become coloured due to nonstoichiometry and crystal defects.
NOTE:
- LiF is ionic but insoluble in $\mathrm{H}_2 \mathrm{O}$ due to very high lattice energy while $\mathrm{LiCl}, \mathrm{LiBr}$, Lil are covalent less soluble or insoluble as their hydration energy is less than lattice energy.
- Other MX are crystalline solids with high melting and boiling points.
- Halides of $\mathrm{K}, \mathrm{Rb}, \mathrm{Cs}$ can also form polyhalides. For example:

$$
\mathrm{KI}+\mathrm{I}_2 \longrightarrow \mathrm{KI}_3
$$