Alkali Metals Halides MCQ - Practice Questions with Answers

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

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

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:

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

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:

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