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Production
Hydrogen can be prepared in several different ways but the economically most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700–1100 °C), steam (water vapor) reacts with methane to yield carbon monoxide and H2.

CH4 + H2O → CO + 3 H2
This reaction is favored at low pressures but is nonetheless conducted at high pressures (20 atm) since high pressure H2 is the most marketable product. One of the many complications to this very optimized technology is the formation of coke or carbon:

CH4 → C + 2 H2
Consequently, steam reforming typically employs an excess of H2O.

Additional hydrogen from steam reforming can be recovered from the carbon monoxide through the Water gas shift reaction:

CO + H2O → CO2 + H2
Other important methods for H2 production include partial oxidation of hydrocarbons:

CH4 + 0.5 O2 → CO + 2 H2
and water electrolysis.

In the laboratory, H2 can be generated by treatment of many metals with acids or base.

Zn + 2 H+ → Zn2+ + H2
2 Al + 6 H2O → 2 Al(OH)3 + 3 H2
Compounds
Hydrogen forms compounds with most other elements, although interestingly H2 does not directly react with most common elements. For example, millions of hydrocarbons are known, but none arise from direct reactions of hydrogen and carbon. Hydrogen with an electronegativity of 2.2 (Pauling's scale) forms compounds with elements that are both more electronegative such as halogens (F, Cl, Br, I) and chalcogens (O, S, Se). It also forms compounds with elements that are less electronegative, such as the metals and metalloids.

Hydrides
Many compounds of hydrogen are called hydrides, but the term is used fairly loosely. To chemists, the term "hydride" usually implies that the H atom has acquired a negative charge, H−-like. The hydride ion itself, H−, exists only in few compounds such as alkali metals hydrides. In fact in 1920, K. Moers demonstrated that electrolysis of molten lithium hydride LiH (m.p. 692 °C) produced the stoichiometric quantity of hydrogen at the anode. Well known hydrides include NaH, an ionic solid, and lithium aluminum hydride, a salt containing the AlH4− complex anion. Palladium hydride contains insterstitial hydrogen atoms, i.e. the H atoms are bonded to multiple Pd atoms without perturbing the overall Pd framework. Hydrogen forms hydrides with all main group elements with the exception of the noble gases and Indium and Thallium.

Protons
Oxidation of H2 formally gives the proton, H+. The proton is central to discussions of acids and the term proton is loosely used to refer to hydrogen with H+-like character. Being a bare nucleus, H+ cannot exist in solution; it would have a strong tendency to attach itself to atoms or molecules with electrons. In acknowledgement of the non-existence of H+, chemists sometimes discuss acidic aqueous solutions in the context of hydronium (H3O+). Even the hydronium ion is a poor representation of the "solvated proton"; H9O4+ is a better description.

Although exotic on earth, one of the most common ions in the universe is the H3+ ion.

H2 reacts with oxygen to form water, H2O. Considerable energy is released in this process. No reaction occurs between H2 and O2 in the absence of a catalyst or a flame. Deuterium oxide, or D2O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and their study is called organic chemistry.


First tracks observed in liquid hydrogen bubble chamber.See also hydrogen compounds.

Forms
Under normal conditions, hydrogen gas is a mixture of two different kinds of molecules which differ from one another by the relative spin of the nuclei.[4] These two forms are known as ortho- and para-hydrogen (this is different from isotopes, see below). In ortho-hydrogen the nuclear spins are parallel and form a triplet, whereas in the para form. the spins are antiparallel, giving rise to a singlet. At standard conditions hydrogen is composed of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depends on temperature, but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. At low temperatures (around boiling point), the equilibrium state is comprised almost entirely of the para form.

The interconversion between para and orther H2 is slow. Rapidly condense if H2 contains large quantities of the ortho form. The ortho/para ratio is important in the preparation and storage of liquid H2, since the ortho-para conversion produces more heat than the heat of its evaporation, and a lot of hydrogen can be lost by evaporation in this way during several days after liquefying. Therefore, some catalysts for the ortho-para interconversion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.

Elemental hydrogen can exist in over 50 different forms, arising from either ionized species such H+, H−, H2+…H1- , or from the three isotopes: H-1, H-2(D), H-3(T), and their corresponding ions which also include H with different nuclear spin isomers.