A) CLASSIFICATION OF BINARY COMPOUNDS:-
The binary compounds formed between hydrogen and other chemical elements in their nature of stability. In combination with metals, hydride is regarded as a , hydrogen compounds with elements of similar electronegativity have low polarity.
The binary compounds of hydrogen element fall into three classes, although there's a range of structural types and some elements from compounds with hydrogen that don't fall strictly into any one category.
1. Molecular hydride exiting as individual and discreet molecules, and they are usually formed with p-block elements of similar or higher electronegativity than Hydrogen. Their E-H bonds are best regarded as covalent.
2. Saline hydrides, also known as a ionic hydride, are formed with the most electropositive elements.
Saline hydrides such as LiH and CaH₂ are the nonconducting crystalline in group 1 and the heavier elements of group 2 should be regarded as hydride 'salts', containing H⁻ ions.
3. Metallic hydrides are always non-stochiometric and electrically conducting soils and with a metallic lustre.
Metallic hydrides are formed with many f - and d - block elements. The hydrogen atoms are often regarded as occupies interstitial sites with the metal structures and occupation rarely occurs without expansion or phase change. Which is reproduces and summarizes classification and the distribution of the different classes through the periodic table. Its also identifying intermediate hydrides that don't fall strictly into any of these categories, and binary compounds, hydrogen is found in complex anions of some p-block elements, examples being the BH⁻₄ ion, tetrahydridoborate also known in older texts is (Boro hydride) in NaBH₄ or the AlH¯₄ ion is ( tetrahydridoaluminate) also known in older texts as ' (aluminiumhydride) in LiAlH₄.
B) THERMODYNAMIC CONSIDERATION :
The s and p block elements strength E-H bonds decrease down each group and in the d block strength of E- H bonds increase down the group. The standard Gibbs energies of formation of the hydrogen compound of s and p block elements reveal a regular variation in stability, with the possible exception of BeH₂ all the s block hydride are exergonic ( ∆fGø< O) and thermodynamically stable with respect to their elements at room temperature. The trend is erratic in group 13, in that only AlH₃ is exergonic at room temperature. The all other groups of p block elements the H2 compounds of the first members of the group (CH₄, NH₃, H₂O, and HF) are exergonic but the analogous compound of their congeners become progressively less stable down the group, a trend illustrated by decreasing E-H bond to the halogens. For example, SnH₄ is highly endergonic (∆f Gɵ< O) whereas HI is barely.
This thermodynamic trends can be traced to the variation in atomic properties. The H-H bond is strongest single homonuclear bond known apart from D-D or T-T bonds) and order for a compound to be e ergonic and stable with respect to it's elements, it needs to have E-H bonds that are even stronger than H-H. For hydrides of the p- block elements, bonding is strongest with the period 2 elements and becomes progressively weaker down each group. The lowest weaker hydrogen bonds formed by the heaviest p - block elements are due to the poor overlaps between the relatively compact H1s orbital and the more diffuse s and p orbitals of their atoms. Although d-block elements don't form binary molecular compounds and many complexs contain one or more hydride ligands. Metal-Hydrogen bonding strengths in the d block increase down a group because the 3d orbitals are too contracted to overlap well with the H1s orbital and better overlap is afforded by 4d and 5d orbitals.
C) REACTION OF BINARY COMPOUNDS:
The reaction of binary compounds of hydrogen falls into three classes depending on the polarity of the E-H bond.
In compounds where E and H have similar electronegativities, and cleavages of the E-H bonds tends to be homolytic, producing, initially H atom and radical each of which can go on to combine with other available radicals.
For X( E) ≈ X(H) : E− H→E.+ H.
Common examples of homonuclear cleavages including the thermolysis and combustion of hydrocarbons.
In compound where E is more electronegative than H, heterolytic cleavage occurs, releasing a proton.
For X( E) >X (H) : E-H → E⁻ + H⁺
The compound behaves as a Brønsted acid and is able to tanfer H⁺ to a base. In such compounds the Hydrogen atom is termed that protonic. Heterolytic bond cleavage is also occurs in compounds where E is less electronegative than Hydrogen, including Saline hydrides.
For X (E) <X (H) : E- H →E ⁺ + H⁻
In this case the Hydrogen atom is hydridic and a H⁻ ion is transferred to a Lewis acid, The reducing agents NaBH₄ and LiAlH₄ used in organic synthesis examples of hydride - transfer reagents.
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