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Jun 26, 2016

Hydrides

Let us first understand what are hydrides?

In a nutshell, a binary compound of hydrogen with a metals and non-metals are called hydrides.

Now let us get into a little more depth to understand and analyse about these hydrides.

Dihydrogen will combine with almost all elements, except Noble gases under certain reaction conditions forming binary compounds which are popularly called hydrides and these hydrides can be expressed as EHx (e.g., CaH2) or EmHn (e.g.,B2H6) where ‘E’ is the symbol of the element.

In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, it is a compound in which one or more hydrogen centres possess nucleophilic, reducing, or basic properties. Compounds containing hydrogen bonded to metals, non-metals or metalloid may also be referred to as hydrides, even though in this case the hydrogen atoms can have a protic character.

The hydrides are classified into three types, namely,

1.Ionic or Saline or Salt-like hydrides

2. Covalent hydrides

3.Metallic or non-stoichiometric hydrides

Now let us take a closer look at these 3 different types of hydrides.

Dihydrogen when combines with s-block elements forms Ionic or Saline hydrides. These are highly electropositive in nature though significant covalent character is found in the lighter metal hydrides like Lithium hydride (LiH), Beryllium hydride (BeH2) & Magnesium hydride (MgH2). BeH2 & MgH2 are polymeric in nature.

Characteristic

These ionic hydrides are:

i.Crystalline

ii.Non-volatile

iii. Non-conducting in solid state, but are conducting in their molten state.

Note: Upon electrolysis of such hydrides they liberate Dihydrogen gas at anode which in fact confirms the existence of hydrogen as hydride ion (H-).

Hydride ion melt upon electrolysis liberating hydrogen gas at anode confirming the existence of hydride ion

Chemical property:

Saline hydrides react violently with water producing Dihydrogen gas.

Note: Since LiH is unreactive at moderate temperatures with Oxygen or Chlorine it is used in the synthesis of other useful hydrides.

Example:

Dihydrogen can form molecular compounds with many p-block elements.

Ex., CH4, NH3, H2O, HF etc,.

Note: For convenience compounds of hydrogen with non-metals have also been considered as hydrides. They are covalent compounds in addition to being highly volatile.

Note: 1.Electron rich hydrides have excess of electrons being present as lone pairs. Group 15 has 1 lone pair (NH3 where N: atom has one lone pair of electrons).Group 16 has 2 lone pairs(H2O where :O: atom has two lone pairs of electrons), & Group 17 has 3 lone pairs(HF where atom has two lone pairs of electrons)

Fluorine atom having 3 lone pair of electrons

2. Due to presence of such number of lone pair of electrons they will behave as Lewis bases. i.e., they are electron donors. In hydrides, as the lone pair of electrons are present on highly electronegative atoms like Nitrogen, Oxygen & Fluorine results in hydrogen bond formation between the molecules which causes association of molecules.

Metallic or Non-stoichiometirc hydrides are also called Interstitial hydrides.

Metallic hydrides are usually non-stoichiometric compounds, having relatively low heats of formation and the mobility of hydrogen. They are described, chemically, in terms of any of the following three models in which hydrogen is considered

i) a small interstitial atom,

ii) a proton, or iii) a hydride anion.

Non-stoichiometric hydrides are Chemical compound which are solid inorganic compounds, having elemental composition whose proportions cannot be represented by integers very often, in such materials, some small percentage of atoms are missing or may be too many atoms are packed into an otherwise perfect metal lattice.

**crystallographic defects** Shown is a two-dimensional slice through a primitive cubic crystal system showing the regular square array of atoms on one face (open circles, o), and with these, places where atoms are missing from a regular site to create *vacancies*, displaced to an adjacent acceptable space to create a Frenkel pair, or substituted by a smaller or larger atom not usually seen (closed circles, • ), *in each case resulting in a material that is moved toward being measurably non--stoichiometric.*

These Non-stoichiometric hydrides are formed by many d-block (except Groups-7,8 & 9) and f-block elements. These hydrides conduct heat & electricity though do not as efficiently as their parent metals do. They are called non-stoichiometric hydrides as they contain deficient hydrogen.

Example: Lanthanum hydride(LaH2.87), Ytterbium hydride (YbH2.55), Titanium hydride(TiH1.5–1.80), Zirconium hydride (ZrH1.3–175), Vanadium hydride (VH0.56), Nickel hydride (NiH0.6–0.7) Palladium hydride (PdH0.6–0.8) etc,.

It was earlier believed that in these metallic hydrides-hydrogen occupied the interstitial spaces in the metal lattice producing distortions without any change in the metal lattice hence they are called interstitial hydrides. However, modern understanding of non-stoichiometric compounds view them as homogeneous, and not mixtures of stoichiometric chemical compounds. According to recent studies except for hydrides of Nickel, Palladium Cerium and Actinium other hydrides of this class possess a different lattice from that of the parent metal.

Since the non-stoichiometirc compound solids are overall electrically neutral, the defect is compensated by a change in the charge of other atoms in the solid, either by changing their oxidation state, or by replacing them with atoms of different elements with a different charge. Non-stoichiometric compounds exhibit special electrical or chemical properties because of the defects; for example, when atoms are missing, the other atoms can move through the solid more rapidly.

1.The property of absorption of hydrogen on transition metals s widely used in catalytic reduction/hydrogenation reactions for the preparation of large number of compounds.

2. Non-stoichiometric compounds have applications in Ceramic and superconductive material and in electrochemical (i.e., battery ) system designs.

3. Some metals like Palladium, Platinum can accommodate a very large volume of hydrogen and they are used hydrogen storage media and in turn as a source of energy.

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