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  Inorganic Chemistry

Hydrides and Halides of Alkali Metals :

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Hydrides

(i) These metals combine with H to give white crystalline ionic hydrides of the general of the formula MH.

(ii) The tendency to form their hydrides, basic character and stability decreases from Li to Cs since the electropositive character decreases from Cs to Li.

2M+ H2 → 2MH ; Reactivity towards H2 is Cs < Rb < K < Na < Li.

(iii) The metal hydrides react with water to give MOH and H2 ; MH + H2 O → MOH + H2

(iv) The ionic nature of hydrides increases from Li to Cs because of the fact that hydrogen is present in the these hydrides as H– and the smaller cation will produce more polarisation of anion (according to Fajans rule) and will develop more covalent character.

(v) The electrolysis of fused hydrides give H2 at anode. NaHfused contains Na+ and H– i.e.,

At cathode: Na+ + e– → Na; At anode: H– → 1/2 H2 + e–

(vi) Alkali metals also form hydrides like NaBH4, LiAlH4 which are good reducing agent.

Halides

(i) Alkali metals combine directly with halogens to form ionic halide M+X– .

(ii) The ease with which the alkali metals form halides increases from Li to Cs due to increasing electropositive character from Li to Cs.

(iii) Lithium halides however have more covalent nature. Smaller is the cation, more is deformation of anion and thus more is covalent nature in compound. Also among lithium halides, lithium iodide has maximum covalent nature because of larger anion which is easily deformed by a cation. Thus covalent character in lithium halides is, LiI > LiBr > LiCl > LiF

(iv) These are readily soluble in water. However, lithium fluoride is sparingly soluble. The low solubility of LiF is due to higher forces of attractions among smaller Li+ and smaller F– ions (high lattice energy).

(v) Halides having ionic nature have high m.pt. and good conductor of current. The melting points of halides shows the order, NaF > NaCl > NaBr > Nal

(vi) Halides of potassium, rubidium and caesium have a property of combining with extra halogen atoms forming polyhalides.

KI + I2 → KI3 ; In KI3(aq) the ions K+and I–3 are present

 

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