The Chemistry of Hydrogen

Physical Properties

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

Occurrence and Properties

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

Hydrides

The term hydride is sometimes applied to all hydrogen compounds. By this definition, compounds such as H2O, NH3, and CH4 could all be classified as hydrides. In the strictest sense, however, the term hydride refers to a compound in which hydrogen has a negative charge. Hydrides fall into three categories; saline hyrides, covalent hydrides, and interstitial 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
In the covalent hydrides hydrogen is covalently bodnded to another element. Such compounds can be divided into two categories. In the first category of compounds hdrogen is bound to aelement of greater electronegativity, such as nitrogen, oxygem, or fluorine. In these compounds, the hydrogen is in the +1 oxidation state. In the second category of compounds, hydroge is bound to an element ot lesser electronegativity, usually boron or silivon. In these compounds, hydrogen is in the -1 oxidation statge; they are hydrides. An example is the borohydride (BH4-) ion. Like the saline hydrides, the covalent hydrides are moisture-sensitive and will react with water.

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.They tend to be very hard materials with very high melting points.

Hydrogen Bonding

Whenever hydrogen is bonded directly to an electronegative element (usually nitrogen, oxygen, or fluorine) the hydrogen acquires a parial positive charge and the electronegative element acquires a partial negative charge. In such cases a force of attraction exists between the molecules; this is referred to as hydrogen bonding. Molecules capable of hydrogen bonding usually have increased melting and boiling points. For example, the unusually high boiling point of water is due to hydrogen bonding.

Hydrogen bonding is one of the reasons water expands as it freezes. Ice has a larger volume than an equivalent amount of water. In an ice crystal, the molecules are locked into a rigid framework due to the hydrogen bonding, precenting the structure from collapsing into a random but slightly smaller volume. In the structue of ice, each oxygen atom has off-center tetrahedral geometry, connected to two hydrogen atoms by covalent bonds and to two other water molecules by hydrogen bonds.

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.
    2Na + 2H2O = 2NaOH + H2
    Ca + 2H2O = Ca(OH)2 + 2H2

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)

External Links
The Chemistry of Hydrogen at Purdue University

Figure 1. A gas dischage tube filled with hydrogen emits a characteristic pink light
Figure 2. A dot structure of the borohydride ion
Figure 3. Calcium metal reacting with water to produce hydrogen gas
Figure 4. The structure of ice. Modeled with POV-Ray.
Figure 5.. A ballon filled with hydrogen gas produces a spectacular (but brief) fireball when ignited