Types of Hydrides - Classification of Hydrides - Definition of Hydrides

Types of Hydrides:

§       Binary compounds of the elements with hydrogen are called hydrides. The hydrides can be divided into  in three classes:

a)    Ionic or salt-like hydrides (formed between H and electropositive elements- group 1 and 2)

b)    Covalent or molecular hydrides (formed between H and group 3,4,5,6 ,7 elements)

c)     Metallic or interstitial hydrides (formed between hydrogen and transition elements)

 

§        The Properties of Hydrides:

1. Stability of Hydrides:

·        The unstable hydrides form from elements whose electronegativities are close to the electronegativity of hydrogen.

·        The H₂ molecule is single-bonded so that, unlike O₂, no extra electronegativity difference is required to overcome its bond energy.

·        The fact that some hydrides have positive H_f values must be due to the stability of these elements in their standard states.

·        The unstable hydrides do not have enough extra stability to their bonds to overcome the extra stability of the element in its standard state relative to a single-bonded diatomic molecule.

 

·        For Example:

While N₂ has a triple bond with extra stability, the three N-H bonds are sufficiently strong to make NH₃ a thermodynamically stable molecule due to different electronegativities. For PH₃, AsH₃, and SbH₃, the stability of their hydride bonds is not sufficient to overcome the extra stability of phosphorus, arsenic, and antimony in their standard states.

·        An electronegativity difference of about 0.5 unit is sufficient to stabilize most hydrides relative to their elements in their standard states.

 

2. Acidic and Basic Character of Hydrides:

·        The ionic hydrides are strong bases, when dissolve in water they form bases.

                                                        

·        Along any given row of the periodic table, the covalent hydrides become increasingly acidic as the atomic numbers increase. Thus CH₄, has negligible acidic properties, but NH₃ donates a proton to very strong bases to form NH₂^-, H₂O loses a proton more easily, and HF is a moderately strong acid.

 

3. Reducing power of hydrides:

     ·        The saline hydrides (ionic hydride) are good reducing agents.

·        In addition, the hydride ion reacts rapidly and completely to reduce water or any other proton donor to hydrogen, as in

                   

      ·        Thus we can regard the hydride ion as a powerful reductant and a very strong base.