Hydrogen – Hydrides

Introduction

The lightest element, Hydrogen, has the most straightforward electronic configuration, 1s1 

consists of a proton and a single electron. Ordinary elemental Hydrogen exists as a diatomic molecule, called Dihydrogen. The colourless, odourless, and tasteless gas Dihydrogen is not a very active element, reacting with only certain elements. A compound formed by two different elements is called a binary compound. Binary compounds formed by various elements with Dihydrogen are called hydrides. Hydrogen is usually in an oxidation number of -1 in its hydrides. 

 Hydrogen hydrides are of 3 types based on the electronegativity of the element.:

1. Ionic hydrides are formed by alkali and alkaline earth metals
2. Covalent hydrides are formed by elements covalently bonded with Hydrogen
3. Metallic hydrides- formed by transition metals bonding with Hydrogen

Types of Hydrides

Electronegativity of an element, which is the ability of an atom to attract electrons towards itself and hence the type of bond formed, determines the kind of hydride it would form with Hydrogen. 

Hence, Hydrogen forms three types of hydrides: ionic, covalent and interstitial hydrides.

Ionic or Saline or Salt like hydrides

Compounds of Hydrogen with strongly electropositive metals, i.e. alkali and alkaline earth metals and some highly positive members of lanthanide series, except for Be and Mg, are called Ionic hydrides.

Properties:

• Ionic hydrides are colourless or greyish crystalline solids. Alkali metal hydrides have ions and act as salts, hence ionic hydrides. They form non conducting substances and have a rock-salt structure.
• They possess high melting and boiling points.
• They show electric conductivity in a molten (fused) state.
• They undergo an oxidation-reduction reaction with water to produce Hydrogen and a basic solution. 

 LiH(s) + H2O(l) ——> H2(g) + LiOH(aq) 

 Hence, ionic hydrides are used as a source of hydrogen gas where transportation of H2(g) is impractical, as in for inflating weather balloons.

• They have a density higher than those of metals from which they are formed as H- ions occupy holes in the lattice of the metal.
• As the size of the cation increases, the stability of hydrides decreases.

 LiH > NaH > KH > RbH > CsH

 CaH2 > SrH2 > BaH2

• On strong heating they burn in air due to their decomposition into metal and hydrogen.

 MH2——> M + H2

• These hydrides show reducing properties.

 2CO + NaH ——> HCOONa + C

 Fe3O4 + NaH ——> 3Fe + 4NaOH

Uses:

Ionic hydrides are used as reducing agents, as solid fuels and for the ready synthesis of Hydrogen via hydrolysis.

Covalent or Molecular Hydrides 

 When elements are covalently bonded with Hydrogen, they form covalent hydrides. These hydrides are formed by all the true nonmetals (except zero group elements) and the elements like Al, Ga, Sn, Pb, Sb, Bi, Po, etc., which usually are metallic. For example, NH3, H2O, H2O2 and HF.

Based on relative numbers of electrons and bonds in their Lewis structure, covalent hydrides are classified as:-

• Electron deficient molecular hydrides:

These hydrides have too few electrons for writing their conventional Lewis structure.

For example, Diborane (B2H6)

• Electron-precise molecular hydrides:

Elements of group 14 form these hydrides. The molecules formed are tetrahedral in shape. 

For example, Methane (CH4)

• Electron-rich molecular hydrides:

These hydrides have excess electrons, which are present as lone pairs.

For example, NH3, H2O and HF, due to highly electronegative atoms, possess hydrogen bonding.

Properties:

• These hydrides are formed by discrete covalent molecules held together by Van Der Waals’ forces. Hence they are primarily gases, liquids and in some cases, solids of low melting and boiling points.
•  These hydrides are generally volatile.
• They show low electrical conductivity.
• As the size of the central atom increases, the electronegativity decreases and hence the thermal stability for the group also decreases.
•  These hydrides behave as Lewis bases due to one or more lone pairs.

 These hydrides become increasingly acidic as we move left to the right along a given period.

• Some hydrides such as SiH4 act as reducing agents.

Uses:

Covalent hydrides have a wide range of applications, such as,

• Boron hydrides are used as high-energy fuels and propellants.
• Ammonia is used as a source of Hydrogen at any destination.
• Phosphine is used for making Holme’s signals.
• HF used for etching of glass.

Metallic or Interstitial Hydrides

Transition metals bonding with hydrogen form metallic hydrides. These hydrides are called so as they are electrically conducting.

Commonly the formula of these compounds is MHC, where c is often not an integer.

These compounds are known as Interstitial compounds as previously it was perceived that Hydrogen occupies interstices in the metal lattice, producing distortion without any change in the metal itself.

Properties: 

• These hydrides are hard, having a metallic lustre.
• They show magnetic properties.
• They are good conductors of electricity which decreases as temperature increases.
• The hydrides have a density lower than the metals they are formed by as the crystal lattice expands due to the absorption of Hydrogen.
• These hydrides generally undergo reversible decomposition into hydrogen gas and metal.

Uses:

• They can be used as hydrogen storage media.
• They are used in catalytic reduction and hydrogenation to prepare many valuable compounds.
• They are used for storing and transporting Hydrogen as these hydrides release hydrogen when heated and treated with acid.

Conclusion

 Hydrides are the binary compounds of Hydrogen with other elements. An element’s electronegativity determines the kind of hydride it would form with Hydrogen. Hence, Hydrogen forms three types of hydrides: ionic, covalent and interstitial hydrides.