Hydrogen, Oxygen, and Water
Interactive Student Tutorial
18.5 Binary Hydrides
Compounds containing hydrogen and just one other element are binary hydrides. Table 18.2 summarizes the formulas and melting points of the simplest main group hydrides.
The ionic hydrides are saltlike, high-melting, white crystalline compounds formed by the alkali metals and the heavier alkaline earth metals Ca, Sr, and Ba. Ionic hydrides can be prepared by direct reaction of the elements at about 400°C.
The alkali metal hydrides contain alkali metal cations and H anions in a face-centered cubic crystal structure. Molten alkali metal hydrides are ionic and conduct electricity. The H anion is a good proton acceptor, and ionic hydrides therefore react with water to give H2 gas and OH ions.
This reaction of an ionic hydride with water is a redox reaction. Ionic hydrides are good reducing agents. Potassium hydride catches fire in air because of a rapid redox reaction with oxygen.
Compounds in which hydrogen is attached to another element via a covalent bond are called covalent hydrides. These include nonmetallic hydrides such as B2H6, CH4, NH3, and H2O, and hydrogen halides such as HX. Covalent hydrides consist of small, discrete molecules that have relatively weak intermolecular forces and are gases or volatile liquids at room temperature.
The reaction of hydrogen with lanthanide, actinide, or d-block transition metals results in metallic hydrides with the general formula MHx, where x represents the number of H atoms in the simplest formula. Also called interstitial hydrides, these compounds are thought to consist of a crystal lattice of metal atoms with the smaller hydrogen atoms occupying holes, or interstices, between the larger metal atoms.
Bonding in metallic hydrides is not well understood,
and the state of hydrogen (H atoms, H+ cations, H
anions) is unknown. The hydrogen atoms can fill different numbers of interstices,
so these may be nonstoichiometric
compounds in which atomic composition cannot be expressed as a
ratio of small whole numbers.
Stoichiometric metallic hydride
Nonstoichometric metallic hydride
The properties of metallic hydrides depend on their
composition, which is a function of the partial pressure of H2
gas in the surroundings. Interstitial hydrides are of current interest
as potential hydrogen-storage devices.