Hydrides And Their Classification

Hydrides and Their Classification

Binary Compounds of hydrogen with other elements are called hydrides.The type of hydride which an element forms depends on the electronegativity of the element,and hence on the type of the bond formed.The electronegativity of hydrogen is 2.1,thus more electropositive element formed ionic hydride.p-block elements formed covalent hydrides because the electronegativity difference between H and p-block element is small.Many d-block elements and some lanthanides and actinides elementsform metallic hydrides.Hence the nature of hydride depends upon the nature of element combining with hydrogen.On the basis of type of bonding in hydrides,these are classified into the following three classes.

1.Ionic or salt like hydrides

2.Covalent or molecular hydrides

3.Metallic or interstitial hydrides

Ionic or salt like hydrides

At high temperature the metals of group IA(alkalii metals) and the heavier group IIA metals(alkaline earth metals)Ca,Sr and Ba form ionic hydrides.Examples are LiH,NaH,CaH₂,etc.These type of hydrides are only formed by the elements with an electronegativity value very lower than the value for hydrogen,thus allowing the hydrogen to attract electron from the metal forming metal cation and hydride ion(H^-).These compounds are solids with high melting points and are classified as ionic hydrides.The evidences for the existence of ionic hydrides are:

i)Molten LiH conducts electricity, and H₂ is liberated at the anode,showing the presence of the hydride ion(H^-).

ii)The other ionic hydrides decompose before melting,but they may be dissolved in melts of alkali halides and when the melt is electrolysed then H₂ is evolved at the anode.

iii)The crystal structures of these hydrides are known,and they show no evidence of directional bonding.

The density of the ionic hydrides is greater than that of the metal from which they were formed because of strong polar bonds existing in the ionic lattices.This is due to H^- ions occupying holes in the lattice of the metal,without distorting the metal lattice.Ionic hydrides have high heats of formation, and are always stoichiometric.

Ionic hydrides react with water to liberate hydrogen.

$$LiH+H_2O \rightarrow LiOH+H_2$$

$$CaH_2+2H_2O \rightarrow Ca(OH)_2+2H_2$$

Ionic hydrides show reducing properties at high temperatures which is probably due to the formation of atomic hydrogen.But their reactivity towards water limits their usefulness.

$$2CO+NaH \rightarrow HCOONa+C$$

$$SiCl_4+4NaH \rightarrow SiH_4+4NaCl$$

$$PbSO_4+2CaH_2 \rightarrow PbS+2Ca(OH)_2$$

Lih and NaH are used as strong reducing agents in synthetic chemistry.LiH and NaH are used to produce other important hydrides,particularly lithium aluminium hydride Li[AlH₄] and sodium borohydride Na[BH₄],which have important uses as reducing agents in both organic and inorganic synthesis.CaH₂ is used for military purposes as a source of H₂ for balloons and hence the name of hydrolith.

$$4LiH+AlCl_3 \rightarrow Li[AlH_4]+3LiCl$$

$$4NaH+B(OCH_3)_3 \rightarrow Na[BH_4]+3NaOCH_3$$

Covalent or molecular hydrides

These hydrides are given by most of the p-block elements.There is a small electronegativity difference between p-block elements and hydrogen.Because of lesser difference in the electronegativity between the combining elements,these hydrides are covalent in nature and non conducting.The compounds usually consist of discrete covalent molecules,with only weak vander waals forces holding the molecules together,and so they are usually volatile and have low melting and boiling points.

Some elements (example:Sn,Pb,Sb,Bi,Te,Po and halogens) give only mononuclear hydrides while the elements like B,Al,Ga,In,C,Si,N,O,S give mononuclear as well as polynuclear(or polymeric) hydrides as given below:

$$B \rightarrow B_2H_6,B_4H_10,B_5H_9 etc$$.The compounds usually consist of discrete covalent molecules,with only weak vander waals forces holding the molecules together,and so they are usually volatile and have low melting and boiling points.

$$Al \rightarrow AlH_3,(AlH_3)_n$$

$$Ga \rightarrow Ga_2H_6,Ga_3H_8 etc$$

$$C \rightarrow CH_4 and other higher hydrocarbons$$

$$Si \rightarrow SiH_4,Si_2H_6,Si_3H_6 etc$$

$$N \rightarrow NH_3,N_2H_4$$

$$S \rightarrow H_2S,H_2S_2,H_2S_3,H_2S_5,etc$$

The formula of the simplest hydrides of these elements is XH_n or XH_(8-n) where n is the group in the periodic table to which X belongs.

Preparation

These hydrides are prepared by a variety of synthetic methods.

i)By the direct union of the free element with H₂ at elevated temperature.

$$N_2+3H_2 \xrightarrow[200atm]{450°C,Fe/Mo}2NH_3$$

$$2N_2+O_2 \xrightarrow[]{Spark}2H_2O$$

$$H_2+X_2 \rightarrow 2HX$$

ii)Reaction of a halide with Li[AlH₄] in a dry solvent such as ether.

$$4BCL_3+3Li[AlH_4] \rightarrow 2B_2H_6+3AlCl_3+3LiCl$$

$$SiCl_4+Li[AlH_4] \rightarrow SiH_4+AlCl_3+LiCl$$

iii)Treating the appropriate binary compounds with acid.

$$Al_4Cl_3+12HCl \rightarrow 3CH_4+4AlCl_3$$

$$FeS+H_2SO_4 \rightarrow H_2S+FeSO_4$$

$$Ca_3P_2+3H_2SO_4 \rightarrow 2PH_3+3CaSO_4$$

iv)Converting one hydride into another by prolysis

$$B_4H_10 \rightarrow B_2H_3+other products$$

Metallic or interstitial or alloy type hydrides

Many d-block elements(example Ti,Pd,Zr,V,Nb,etc.),lanthanides and actinides at elevated temperature absorb hydrogen in to the holes or interstices existing between the atoms comprising the metallic lattice without changing the original crystal of the metal and thus give hydrides which are called metallic hydrides.Examples are ZrH_1.9,TiH_1.8,NbH_0.7,VH_0.6,LaH_2.87,YbH_2.55,etc.

However,the elements in the middle of the d-block elements do not form hydrides.The absence of hydrides in this part of the periodic table is sometimes called the hydrogen gap.

Metallic hydrides are usually prepared by heating the metal with the hydrogen.If heated to higher temperatures the hydrides decompoes and this may be used as convenient method of making very pure hydrogen.

Since,the hydrides possesses properties similar to those of the parent metals such as hardness,lustre,magnetic properties and electrical conductivity,these are also called metallic hydrides.The densities of these hydrides are usually lower than those of the parent metal because the crystal lattice has expanded through the inclusion of hydrogen.This distortion of the crystal lattice may make the hydride brittle.Thus when hydride is formed a solid piece of metal turns into finely powdered hydride.If the finely powdered hydride are heated they decompose giving hydrogen and very finely divided metal.These finely divided metals may be used as catalysts.

The chemical composition of these hydrides id variable,since varying temperature and pressure may vary the proportion of hydrogen absorbed by the metal.Hence,many interstitial hydrides are non-stiochiometric in nature.

In view of the variable composition of these hydrides,it has been suggested that these are not true chemical compounds but are regarded as the metals containing absorbed hydrogen.These compounds are also regarded as solid solutions.Metals which can dissolve varying amounts of hydrogen.The absorbed hydrogen atom can be expelled from the interstitial sites simply by heating the hydride and by increasing the gas pressure.Thus these hydrides are good reducing agents and act as effective catalyst in hydrogenation reaction due to their ability to provide H-atoms or H⁺ with delocalized electrons rather than H₂ molecules.

Intermediate Hydrides

There are a number of hydrides which do not fit easily in the above classification.Examples of such hydrides are BeH₂,MgH₂,CuH,ZnH₂,CdH₂,HgH₂,etc(BeH₂)_n is polymeric and is thought to be chain polymer with hydrogen bridges.MgH₂ has properties in between those of ionic and covalent hydrides.while CuH,ZnH₂,CdH₂ and HgH₂ have properties which are intermediate between those of covalent and metallic hydrides.In non of these hydrides the type of bonding has been established.

References

Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.