Name: Dihydrogen as a Fuel
Uploaded: 2019-07-19
Duration: 7 min 24 s
Description: A good fuel should release a huge amount of energy on combustion. Let’s explore one such fuel that has promising potential for use as a non-polluting fuel of...

Question

What are hydrides? Discuss their various types. How are they formed?

Accepted Answer

Dihydrogen combines with a number of elements to form binary compounds called hydrides. Their general formula being ${MH}_{x}$ where M represents the element and x the number of hydrogen atoms. The various elements which form hydrides are,
(i) All the main group elements except those of noble gases and probably indium and thallium.
(ii) All lanthanides and actinides.
(iii) Transition metals - $Sc, Y, La, Ac, Tc, Zr, Hf, Pd$ and to a lesser extent $V, Nb, Ta, Cr, Cu$ and $Zn$ .
Depending upon the physical and chemical properties, the hydrides have been divided into the following three broad categories:
1. Ionic or Salt-like or Saline hydrides.
2. Metallic or Interstitial hydrides.
3. Molecular or Covalent hydrides.
1. Ionic hydrides. These are formed by those metals whose electronegativity values are appreciably lower than that of hydrogen ( $2.1$ ). Thus, all elements of group 1, i.e., alkali metals and group 2, i.e., alkaline earth metals on heating at high temperatures ( $595 - 973 K$ ) form ionic hydrides. However, ${BeH}_{2}$ and ${MgH}_{2}$ have covalent polymeric structures.
Properties: Some important properties of ionic hydrides are listed below:
(1) These are formed by transfer of electrons from the metals to the hydrogen atoms and thus contain hydride ion, $H^{-}$ . For example, lithium hydride, ${Li}^{+} H^{-}$ and calcium hydride, ${Ca}^{2 +} {(H^{-})}_{2}$ .

Uses: lonic hydrides and their complexes ( ${LiAlH}_{4}$ , ${NaBH}_{4}$ , etc.) are used-as reducing agents. Since on heating ionic hydrides decompose to evolve dihydrogen which ignites spontaneously, therefore, they are used as solid fuels.
Metallic or Interstitial hydrides: d—Block elements of groups $3, 4, 5 (Sc, Ti, V, Y, 21', Nb, La, Hf, Ta, Ac, etc .) 10, ll, 12 (Pd, Cu, Zn, etc .)$ and f-block elements ( $Ce, Eu, Yb, Th, U, etc .$ ) on heating with $H_{2}$ under pressure form hydrides. In group 6, $Cr$ alone forms the hydride $CrH$ . The metals of group 7, 8 and 9 do not form hydrides. In fact, the region of the periodic table from groups 7- 9 which does not form hydrides is referred to as the hydride gap. These hydrides generally have properties similar to those of the parent metals and hence are called metallic hydrides. Properties. Some important properties of those hydrides are listed below:
(i) They are hard, have a metallic luster, conduct electricity and have magnetic properties.
(ii) The density of these hydrides is lower than those of metals from which they are formed since the crystal lattice expands due to inclusion of $H_{2}$ .

Uses. Due to interstitial hydride formation, these metals adsorb large volumes of hydrogen on their surface. This property of adsorption of a gas by a metal is known as occlusion.
Molecular or Covalent hydrides: These are mainly formed by p-block elements and some block elements (Be and Mg) primarily due to the reason that the electronegativity difference between these elements and the hydrogen atom is quite small. Their general formula is either ${XH}_{n}$ (for s-block elements) or ${XH}_{8 - n}$ (for p-block elements) where n is the number of electrons in valence shell. These hydrides usually consist of discrete covalent molecules which are held together by weak van der Waals forces of attraction and hence are called covalent or molecular hydrides.

What are hydrides? Discuss their various types. How are they formed?

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