NEET Chemistry Notes Hydrogen – Compounds of Hydrogen

NEET Chemistry Notes Hydrogen – Compounds of Hydrogen

Compounds of Hydrogen

Compounds of Hydrogen

The detailed description of the compounds of hydrogen e.g. hydrides, water, hard and soft water etc., are given below.

Hydrides

Dihydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary compounds, called hydrides. Based upon their physical and chemical properties, hydrides are of the following three types:

•  Ionic or Saline or Salt Like Hydrides

Ionic hydrides are stoichiometric compounds of dihydrogen formed with most of the s-block elements, which are highly electropositive in character, e.g. LiH, BeH₂ andMgH₂. In fact, BeH₂ andMgH₂ are polymeric in structure.
Due to their high reactivity with water ionic hydrides are used to remove traces of water from organic solvents.

•  Covalent or Molecular Hydrides

Dihydrogen forms molecular compounds with most of the p-block elements. Most familiar examples are CH₄,NH₃, H₂0 and HF etc. Molecular hydrides are further classified, according to the relative number of electrons and bonds.

in their Lewis structure, into following types:

1. Electron deficient hydrides have incomplete octet, so behave as Lewis acids, i.e. are electron acceptors, e.g. B₂H₆. These hydrides are formed by the elements of group 13.
2.  Electron precise hydrides are obtained from elements of group 14 (e.g. CH₄) which are tetrahedral in geometry.
3.  Electron rich hydrides have excess electrons, which are present as lone pairs. Elements of group 15-17 form
   such type of hydrides, (two lone pairs) and

•  Interstitial Hydrides or Metallic or Non-stoichiometric

These are formed by many d-block and f-block elements, however, the metals of group 7, 8 and 9 do not form hydride (hydride gap). These hydrides are mainly formed by

•  transition metals of group 3, 4, 5 of d-block
•  Cr metal of group 6
• f-block elements, e.g.LaH_{2 .87}, YbH₂.₅₅  etc.
• Metallic hydrides are non-stoichiometric. They have metallic lattice and hydrogen is present at the interstitial sites. These conduct heat and electricity just like metals except hydrides of Eu and Yb.  EuH₂ and YbH₂ are ionic and stoichiometric)

Water (H₂0)

The water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. The physical and chemical properties of water are given below:

Physical Properties

Water (H₂0) is polar in nature. It exists in liquid state at room temperature due to intermolecular hydrogen bonding. HOH bond angle is 104.5° and O—H bond length is 95.7 pm. H₂0 (ice) has four hydrogen bonds per molecule and hence, has a highly ordered three dimensional cage like structure. Ice has low density than H₂0 (liquid) but H₂0 has maximum density at 3.98°C.

Chemical Properties

Water is amphoteric in nature.

Water reacts with metals and non-metals both.

In hydrated salts, water may remain in five types such as coordinated water, hydrogen bonded water, lattice water, clathrate water and zeolite water.
Zeolites are the open structures of Si0₄ and Al0₄ tetrahedron that contains cavities or certain channels in which H₂0 molecules gets trapped.
Clathrate contains host molecules that crystallises with an open structure containing water molecules,  A number of compounds such as calcium hydride, calcium phosphide etc., undergo hydrolysis with water. The hydrolysis of hydrides with H₂0 is highly exothermic and may be explosive as H₂ catches fire. CO₂ is reduced by hot metal hydride, so it cannot use to extinguish such fire.

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