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Halides of Alkali Metals - (Concept)

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

2 \mathrm{M}+\mathrm{X}_{2}\: \rightarrow \: \mathrm{MX}+\mathrm{Heat}

The reactivity order of alkali metals towards halogens is:

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

Ionic nature, solubility in H2O increases while lattice energy decreases.
For example:

\mathrm{LiF}<\mathrm{NaF}<\mathrm{KF}<\mathrm{RbF}<\mathrm{CsF}

\mathrm{KF}<\mathrm{KCl}<\mathrm{KBr}<\mathrm{KI}

These halides are colourless, however, on heating, they become coloured due to nonstoichiometry and crystal defects.

NOTE: 

  • LiF is ionic but insoluble in H2O due to very high lattice energy while LiCl, LiBr, LiI 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 K, Rb, Cs can also form polyhalides. For example:
    \mathrm{KI}+\mathrm{I}_{2} \longrightarrow \mathrm{KI}_{3}
Exam Chapter
MHT-CET S - Block Elements (Alkali and Alkaline Earth Metals)
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