HYDROGEN Many gases glow with a characteristic color when electricity is passed through an evacuated tube containing the gas. For hydrogen, a violet-pink glow is observed. The spectrum is actually composed of four different lines, arising from four energy transitions within the atom.

Atomic Number: 1
Atomic Weight: 1.0079
Electron Configuration: 1s¹
First Ionization Energy: 1312 kJ/mol
Atomic Radius: 37 pm
Ionic Radius: 35 pm (H⁺), 208 pm (H^-)

Hydrogen is the most abundant element in the universe and forms more compounds than any other element. While some hydrogen does exist in the upper atmosphere, most has long since escaped into space and almost all of the hydrogen found on Earth is combined with other elements.

Hydrogen is the simplest element and has an atomic number of one. At room temperature, hydrogen is a colorless and tasteless gas and occurs as diatomic nonpolar molecules.
 

The term "hyride" is sometimes applied to all hydrogen compounds. By this definition, compounds such as H2O, NH3, or CH4 could all be classified as hydrides. In the strictest sense, however, the term "hyride" refers to a compound in which hydrogen has a negative charge. Such hydrides fall into three categories; saline hyrides, covalent hydrides, and interstitial hydrides.

Saline Hydrides

The saline hydrides are formed with the alkali metals and the heavier members of the alkaline earth metals. These are are best described as ionic substances consisting of positive metal cations and negative hydride ions. Sodium hydride, for example, as the same structure as sodium chloride but with the hydride ion taking the place of the chloride ion. Saline hydrides will dissolve in molten alkali halides and electrolysis of these solutions results in the formation of hydrogen at the anode, or site of oxidation. This is in contrast to the electrolysis of aqueous solutions, where hydrogen is observed at the cathode, or site of reduction. Metal hydrides are moisture sensitive and will react explosively with water to yield hydrogen gas and the corresponding metal hydroxide. Sodium hydride reacts explosively with water; calcium hydride is less reactive and can be used as a source of hydrogen.

NaH + H₂O = H₂ + NaOH

CaH₂ + 2H₂O = 2H₂ + Ca(OH)₂

Covalent Hydrides

Hydrogen forms bonds to nearly all of the nonmetallic elements. Such compounds can be divided into two categories, those in which hydrogen is bonded to an element of greater electronegativity (such as nitrogen, oxygen, or fluorine), and those in which hydrogen is bonded to an element of lesser electronegativity (boron and silicon being the most common examples). When hydrogen is bonded to an element of greater electronegativity, the hydrogen possesses an oxidation state of +1. When hydrogen is bonded to an element of lesser electronegativity, it possesses an oxidation state of -1; such compounds are hydrides. A good example of of a covalent hydride is the borohydride (BH4-) ion; in this ion the hydrogen atoms carry an oxidation state of -1. Like the saline hydrides, the covalent hydrides are moisture-sensitive and will react with water.

When assigning oxidation numbers, the bonding electrons are assigned to the more electronegative element. In this case, hydrogen is the more electronegative element. The bonding electrons pairs are therefore assigned to the hydrogen atoms, giving each an oxidation state of -1. The boron atom loses all its electrons, and has an oxidation state of +3.

Interstitial Hydrides

The interstitial hydrides are quite different. In these compounds the hydrogen atoms may occupy the intersticies between the atoms in the metal crystal. There is no actualy chemical bonding involved, and as a result such compounds are often nonstoichiometric.

Laboratory Preparation of Hydrogen

Hydrogen can be readily produced in the laboratory by several different means.

1. The moderately reactive metals such as Mg, Ca, Al, and Zn will react with acids to produce hydrogen gas and the corresponding metal salt. For example, zinc metal reacts with even dilute hydrochloric acid to give zinc chloride and hydrogen gas.

Zn + 2HCl = ZnCl₂ + 2H₂

2. Many of the alkali metals and alkaline earth metals will react with water to produced hydrogen gas and the corresponding metal hydroxide. For most of the alkali metals, this reaction occurs too quickly and is too dangerous to be used as a source of hydrogen. Magnesium reacts slowly with water but the reaction with calcium is more vigorous.

Na + H₂O = NaOH + H₂

Ca + 2H₂O = Ca(OH)₂ + 2H₂

The reaction of calcium metal with water. The photo was taken almost instantly after the addition of the metal; the solution quickly turns white due to formation of insoluble calcium hydroxide

3. Hydrogen can be prepared by the reaction of hydrides with water. For example, the reaction of calcium hydride with water produces calcium hydroxide and hydrogen gas.

CaH₂ + 2H₂O = 2H₂ + Ca(OH)₂

4. Hydrogen can also be prepared between certain metals such as aluminum and a solution containing the hydroxide ion.

5. Hydrogen can be prepared by the electrolysis of aqueous solutions. In the case of the electrolysis of pure water with inert eelctrodes, water is broken down into hydrogen and oxygen gases.

2H₂O = 2H₂ + O₂

_{Industrial Preparation of Hydrogen}

_{The above methods are too expensive for the large-scale production of hydrogen. Industrially, hydrogen is prepared by the steam reformer process. This involves the reaction of methane or coke (a form of carbon) with steam at high temperature and pressure. Some of the reactions are listed below.}

CH₄(g) + 2H₂O(g) = CO(g) + 3H₂(g)

C(s) + H₂O = CO(g) + H₂(g)

The mixture of gases produced (carbon monoxide and hydrogen) is called water gas or synthesis gas. This mixture is often put through a second reaction, called a shift reaction, in which the mixture of gases is reacted with additional steam over a catalyst, which causes converts the carbon monoxide into carbon dioxide.

CO(g) + H₂O(g) = CO₂(g) + H₂(g)

The Chemistry of Hydrogen at Purdue University