In this oxidation, a molecule of hydrogen gas is ionized to two electrons and two protons. Writing a half-reaction for oxygen reduction is more complicated, since oxygen can be reduced by either one, two or four electrons, as shown by the square redox scheme to the right, below. In most chemical reactions, molecular oxygen is reduced along the red and blue pathways highlighted in this redox scheme.
The selective reduction of oxygen to water in such biological systems is crucial, not only in order to maximize the energy produced for cellular metabolism but also because hydrogen peroxide is a powerful oxidant and cytotoxin, which harms living cells. Given the energetics presented above, there is a strong thermochemical bias for the production of water over hydrogen peroxide when H 2 and O 2 are reacted together.
For instance, when hydrogen gas is burned in the presence of oxygen, a large amount of energy is released and water is produced as the major product. In cases where the reaction is more controlled, however, such as the consumption of hydrogen and oxygen in a fuel cell, the mechanism and kinetics of the O 2 reduction process can complicate issues greatly.
For instance, the delivery of the protons and electrons derived from the ionization of hydrogen see redox half-reaction above to a molecule of oxygen has to be precisely controlled via a process know as proton-coupled electron transfer in order to ensure that the complete four-electron reduction of O 2 dominates. Metallic hydrides form when hydrogen reacts with transition metals, therefore they will not be introduced in this module. Ionic hydrides form when hydrogen reacts with s-block metals, not including Be and Mg.
These s-block elements are found in Group 1 and Group 2 of the periodic table and are the most active metals. Both Group 1 and Group 2 metals have low electronegativity values less than 1. Hydrogen Reactions Hydrides are binary compounds of hydrogen. Ammonia is a colorless gas with a sharp, pungent odor. Smelling salts utilize this powerful odor. Due to intermolecular hydrogen bonding, the enthalpy of vaporization of liquid ammonia is higher than that of any other liquid except water, so ammonia is useful as a refrigerant.
In the presence of a large excess of ammonia at low temperature, the chloramine reacts further to produce hydrazine, N 2 H 4 :. Anhydrous hydrazine is relatively stable in spite of its positive free energy of formation:. It burns rapidly and completely in air with substantial evolution of heat:. Some rockets use hydrazine as a fuel.
The most important hydride of phosphorus is phosphine, PH 3 , a gaseous analog of ammonia in terms of both formula and structure. Unlike ammonia, it is not possible to form phosphine by direct union of the elements. There are two methods for the preparation of phosphine.
One method is by the action of an acid on an ionic phosphide. The other method is the disproportionation of white phosphorus with hot concentrated base to produce phosphine and the hydrogen phosphite ion:.
Phosphine is a colorless, very poisonous gas, which has an odor like that of decaying fish. The major uses of phosphine are as a fumigant for grains and in semiconductor processing. Like ammonia, gaseous phosphine unites with gaseous hydrogen halides, forming phosphonium compounds like PH 4 Cl and PH 4 I.
Phosphine is a much weaker base than ammonia; therefore, these compounds decompose in water, and the insoluble PH 3 escapes from solution. Hydrogen sulfide, H 2 S, is a colorless gas that is responsible for the offensive odor of rotten eggs and of many hot springs. Hydrogen sulfide is as toxic as hydrogen cyanide; therefore, it is necessary to exercise great care in handling it. Hydrogen sulfide is particularly deceptive because it paralyzes the olfactory nerves; after a short exposure, one does not smell it.
A more effective preparation method is the reaction of a metal sulfide with a dilute acid. For example:. It is easy to oxidize the sulfur in metal sulfides and in hydrogen sulfide, making metal sulfides and H 2 S good reducing agents. The sulfur in H 2 S usually oxidizes to elemental sulfur, unless a large excess of the oxidizing agent is present.
This oxidation process leads to the removal of the hydrogen sulfide found in many sources of natural gas. The deposits of sulfur in volcanic regions may be the result of the oxidation of H 2 S present in volcanic gases.
Hydrogen sulfide is a weak diprotic acid that dissolves in water to form hydrosulfuric acid. Since hydrogen sulfide is a weak acid, aqueous solutions of soluble sulfides and hydrogen sulfides are basic:. Binary compounds containing only hydrogen and a halogen are hydrogen halides.
In general, it is possible to prepare the halides by the general techniques used to prepare other acids. Fluorine, chlorine, and bromine react directly with hydrogen to form the respective hydrogen halide.
This is a commercially important reaction for preparing hydrogen chloride and hydrogen bromide. The acid-base reaction between a nonvolatile strong acid and a metal halide will yield a hydrogen halide. At standard temperature and pressure, hydrogen gas consists of 75 percent orthohydrogen and 25 percent parahydrogen.
Hydrogen is available in different forms, such as compressed gaseous hydrogen, liquid hydrogen, and slush hydrogen composed of liquid and solid , as well as solid and metallic forms.
Hydrogen gas H 2 is highly flammable and will burn in air at a very wide range of concentrations between 4 percent and 75 percent by volume. Hydrogen gas can also explode in a mixture of chlorine from 5 to 95 percent.
These mixtures can explode in response to a spark, heat, or even sunlight. Pure hydrogen-oxygen flames emit ultraviolet light and are invisible to the naked eye. As such, the detection of a burning hydrogen leak is dangerous and requires a flame detector. Because hydrogen is buoyant in air, hydrogen flames ascend rapidly and cause less damage than hydrocarbon fires. H 2 reacts with oxidizing elements, which in turn react spontaneously and violently with chlorine and fluorine to form the corresponding hydrogen halides.
H 2 does form compounds with most elements despite its stability.
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