Trends in chemical Reactivity with Hydrogen

All the alkali and alkaline earth metals are highly reactive.They react with air, water,hydrogen,halogens  and a number of other non metals. The alkali metals have one electron in their valence shell and possess large atomic sizes due to which they are highly reactive.

The ionic or saline or salt like hydrides  are the compounds of hydrogen with strong electropositive alkali and alkaline earth metals except beryllium and magnesium.

Reactivity of alkali metals with hydrogen

All the alkali metals react with hydrogen directly at a temperature of 300 to 6000C and form the corresponding hydrides.

The general reaction of alkali metals with hydrogen can be represented as follows:

               2M+H2→2MH

2Li+H2→2LiH Lithium hydride

2Na+H2→2NaH  Sodium hydride

2K+H2→2KH   Potassium hydride

2Rb+H2→2RbH  Rubedium hydride

2Cs+H2→2CsH  Caesium hydride

All these hydrides are ionic in nature.In these compounds hydrogen acts as the hydride ion- H-

All the alkali metal hydrides are white coloured solids with high melting points.

 Ionic hydrides are usually binary compounds as they contain only hydrogen and the most electropositive metal.These compounds are insoluble in solutions.

As the metallic nature of the alkali metals increases from Lithium to caesium, the ionic nature of the corresponding hydrides also increases from LiH to CsH.

LiH < NaH < KH < RbH < CsH

The thermal stability of the alkali metal hydrides decreases from lithium hydride to Caesium hydride as the metallic nature increases from Lithium to caesium.

LiH > NaH > KH > RbH > CsH

 All the alkali metal hydrides  react with water to produce hydrogen gas and the corresponding  hydroxide salt

MH(s)+H2O(l)⟶MOH(aq)+H2(g)

For example, sodium hydride reacts with water and forms sodium hydroxide along with hydrogen gas.

NaH(s)+H2O(l)⟶NaOH(aq)+H2(g)

Fused lithium hydride on electrolysis yields hydrogen at anode.The other alkali metal hydrides decompose before melting.

All the alkali metal hydrides are powerful reducing agents at high temperatures.

NaH+SiCl4⟶NaCl+SiH4

All the alkali metal hydrides react with proton donors such as water, alcohol and ammonia  and liberate hydrogen gas.

NaH+ROH⟶RONa+H2

NaH+NH3⟶NaNH2+H2

Reactivity of alkaline earth metals with hydrogen

Except beryllium and magnesium, all the alkaline earth metals react with hydrogen directly  on heating to form the corresponding hydrides with the general formula, MH2

The general reaction of alkaline earth metals with hydrogen can be represented as follows:

               M+H2→MH2

Ca+H2→MH2    Calcium hydride

Sr+H2→SrH2     Strontium hydride

Ba+H2→BaH2    Barium hydride

Beryllium hydride and magnesium hydride are prepared indirectly by reducing their chlorides with lithium aluminium hydride-LiAlH4

Beryllium hydride is prepared by reducing beryllium chloride with lithium aluminium hydride.

2BeCl2+LiAlH4→2BeH2 + LiCl +AlCl3 

As the metallic nature of the alkaline earth  metals increase from beryllium  to barium, the ionic nature of the corresponding hydrides also increases from Beryllium  hydride to barium hydride

BeH2 < MgH2 < CaH2 < SrH2 < BaH2

Beryllium hydride is covalent and polymeric.It possess hydrogen bridges.It is a colourless solid. Three centered bonds are present in which a banana shaped molecular orbital covers three atoms

Magnesium hydride is partially ionic and polymeric.It is a white crystalline solid.The reaction of magnesium hydride with water is highly violent.It gets ignited on contact with air.It is used as a hydrogen storage material  as it has high amount of hydrogen.

Calcium hydride,strontium hydride and barium hydrides are ionic in nature.

Calcium hydride reacts very quickly with water and liberates hydrogen gas.It is having a salt like structure,so it is called a saline hydride. Calcium hydride is used as a drying agent. 

Calcium, Strontium and Barium hydrides liberate hydrogen at anode on electrolysis in the fused

state. CaH2 is technically called ‘hydrolith’ and used for large scale production of

hydrogen.

CaH2+2H2O→Ca(OH)2 + H2 

The thermal stability of the alkali metal hydrides decreases from beryllium hydride to barium hydride as the metallic nature increases from beryllium to barium.

BeH2 > MgH2 > CaH2 > SrH2 > BaH2

Ionic hydrides

By heating the respective metal with hydrogen under pressure and at moderate temperatures these hydrides are prepared.

All the alkali metal hydrides have rock-salt structure.

They are either colourless or grayish coloured crystalline solids.These solids as such are non-conducting substances.

These hydrides consist of ions and thus they behave as salts.

Ionic hydrides can undergo oxidation reduction reactions with water to produce hydrogen and the corresponding basic solution.

  LiH+H2O→LiOH+H2

Alkaline earth metal hydrides and LiH combine with nitrogen and form the corresponding nitrides with the liberation of hydrogen gas.