HYDRIDES

HYDRIDES

A hydride is any class of chemical compound in which hydrogen is combined with another element. Three basic types of hydrides—saline (ionic), metallic, and covalent—may be distinguished on the basis of type of chemical bond involved. A fourth type of hydride, dimeric(polymeric) hydride, may also be identified on the basis of structure. Aluminum and, possibly, copper and beryllium hydrides are non-conductors that exist in solid, liquid, or gaseous forms. All are thermally unstable, and some explode on contact with air or moisture.

SALINE (IONIC) HYDRIDES

Saline, or ionic, hydrides are defined by the presence of hydrogen as a negatively charged ion, H−. The saline hydrides are generally considered those of the alkali metals and the alkaline-earth metals (with the possible exception of beryllium hydride, BeH2, and magnesium hydride, MgH2).

These metals enter into a direct reaction with hydrogen at elevated temperatures (300–700 °C [570–1,300 °F]) to produce hydrides of the general formulas MH and MH2. 

The hydride ion in the saline hydrides is a strong base, and these hydrides react instantly and quantitatively with the hydrogen ion (H+) from water to produce hydrogen gas and the hydroxide ion in solution.

H− + H2O → H2 + OH−

Because saline hydrides react vigorously with water, giving off large volumes of gaseous hydrogen, this property renders them useful as light, portable sources of hydrogen.

METALLIC HYDRIDES

The transition metals and inner transition metals form a large variety of compounds with hydrogen.

Metallic hydrides are formed by heating hydrogen gas with the metals or their alloys.

Metallic alloy like hydrides possess some of the characteristics of metals, such as lustre and strong electrical conductivity. They tend to have variable physical properties, with some being more brittle and others being harder than the metals from which they are made. 

Metallic hydrides essentially consist of protons (positive hydrogen ions, H+) and metal atoms in an electron sea.

COVALENT HYDRIDES

Covalent hydrides are primarily compounds of hydrogen and non-metals, in which the bonds are evidently electron pairs shared by atoms of comparable electronegativities.

For example, most non-metal hydrides are volatile compounds, held together in the condensed state by relatively weak van der Waals intermolecular interactions.

Covalent hydrides are liquids or gases that have low melting and boiling points, except in those cases (such as water) where their properties are modified by hydrogen bonding.

For example, NH3, H2O, and HF are held together in the liquid state primarily by hydrogen bonding.

The hydrogen compounds formed with the elements that follow oxygen—H2S, H2Se—are all volatile, toxic gases with repulsive odours. They are easily prepared by adding dilute acid to the corresponding metal sulfide, selenide.

Each of the halogens forms a binary compound with hydrogen, HX. At ambient temperature and pressure, these compounds are gases, with hydrogen fluoride having the highest boiling point owing to intermolecular hydrogen bonding. 

APPLICATIONS Hydrides such as sodium borohydride, lithium aluminium hydride,  are

commonly used as reducing agents in chemical synthesis.

Hydrides such as sodium hydride and potassium hydride are used as strong bases in organic synthesis. The hydride reacts with the weak acid releasing H2.

Hydrides such as calcium hydride are used as drying agents, to remove trace water from organic solvents. The hydride reacts with water forming hydrogen and hydroxide salt.

Hydrides are important in storage battery technologies such as nickel-metal hydride battery.

Various metal hydrides have been examined for use as a means of hydrogen storage for fuel cell-powered electric cars and other purposed aspects of a hydrogen economy.

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