Page 3 - Microsoft Word - Lesson note-HYDROGEN
P. 3

Hydrogen forms ionic hydrides with s- block elements which are highly
                    electropositive. It forms Covalent hydrides with elements of higher
                    electronegativity such as p-block elements.


                    All the type of hydrides can be classified into following three categories:

                         ionic or saline hydrides

                         covalent or molecular hydrides

                         metallic or non-stoichiometric hydrides

                    Alkali metal hydrides are good reagents for preparing other hydride
                    compounds. Molecular hydrides (e.g., BH3, CH4, NH3, H2O) are of great
                    importance in day-to-day life. Metallic hydrides are useful for ultra-purification
                    of dihydrogen and as dihydrogen storage media.
                    Among the other chemical reactions of dihydrogen, reducing reactions leading
                    to the formation hydrogen halides, water, ammonia, methanol, vanaspati
                    ghee, etc. are of great importance. In metallurgical process, dihydrogen is used
                    to reduce metal oxides. In space programmes, it is used as a rocket fuel. In fact,
                    it has promising potential for use as a non-polluting fuel of the near future
                    (Hydrogen Economy).


                    Water
                    Water is the most common and abundantly available substance. It is of a great
                    chemical and biological significance. The ease with which water is transformed
                    from liquid to solid and to gaseous state allows it to play a vital role in the
                    biosphere. The water molecule is highly polar in nature due to its bent
                    structure. This property leads to hydrogen bonding which is maximum in ice
                    and least in water vapour.

                    The polar nature of water makes it:

                    (a) a very good solvent for ionic and partially ionic compounds


                    (b) to act as an amphoteric (acid as well as base) substance; and

                    (c) to form hydrates of different types. Its property to dissolve many salts,
                    particularly in large quantity, makes it hard and hazardous for industrial use.


                    Self-ionization of water: One water molecule acts as an acid by donating a
                    proton to another water molecule which acts as a base. This is also known as
                    autoprotolyis of water.

                    Water free from soluble salts of calcium and magnesium is called soft water.
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