Hydrogen and the Hydrides

Elemental hydrogen is a colorless, odorless, tasteless diatomic gas with the lowest molecular weight and density of any known substance. Discovery of the element is attributed to the Englishman Henry Cavendish (1731-1810), who prepared it in 1766 by passing steam through a red-hot gun barrel (mosdy iron) and by the reaction of acids with reactive metals. The latter is still the method commonly used for the preparation of small amounts of H2 in the laboratory. 

Hydrogen is very flammable it was responsible for the Hindenburg airship disaster in 1937. A spark is all it takes to initiate the combustion reaction, which is exothermic enough to provide the heat necessary to sustain the reaction. 

Hydrogen is prepared by the water gas reaction, which results from the passage of steam over white-hot coke (impure carbon, a nonmetal) at 1500°C. The mixture of products commonly called water gas is used industrially as a fuel. Both components, CO and H2, undergo combustion. 

Vast quantities of hydrogen are produced commercially each year by a process called steam cracking. Methane reacts with steam at 830°C in the presence of a nickel catalyst. 

The mixture of H2 and CO gases is also referred to as s)mthesis gas. It can be used to produce a wide variety of organic chemicals like methanol (CH3OH) and hydrocarbon mixtures for gasoline, kerosene, and related fuels. 

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Reactions similar to these provide convenient syntheses of hydrides of such elements as phosphorus, arsenic, tellurium, and selenium, because these elements do not react directly with hydrogen and the hydrides are unstable. 

The ability of metallic palladium to dissociatively absorb hydrogen reversibly is well known. Hydrogen is first chemisorbed at the surface of the metal at increased pressures hydrogen enters the palladium lattice and a- and p-phase palladium hydrides are formed. INS has been used to characterise both chemisorbed hydrogen and the hydrides (see Chapter 6). 

The alkylborane (RBH2) formed in the first step of the reaction reacts with another molecule of alkene to form a dialkylborane (R2BH), which then reacts with yet another molecule of alkene to form a trialkylborane (R3B). In each of these reactions, boron adds to the sp carbon bonded to the most hydrogens and the hydride ion adds to the other sp carbon. 

In Chapter 9 we established the first five components of our interconnected network of ideas for understanding the periodic table. These included the periodic law, the uniqueness principle, the diagonal effect, the inert-pair effect, and the metal-nonmetal line. These components are summarized individually and collectively in colored figures located on the front inside cover of the book The icons for each component are shown there as well as on the bookmark pullout in the back of the text. In Chapter 10 we discussed hydrogen and the hydrides (as well as basic nuclear processes). In Chapter 11 we discussed the chemistry of oxygen, reviewed and extended our knowledge of the nature of water and aqueous solutions, and added a sixth component to our network the acid-base character of oxides and their corresponding hydroxides and oxoacids. The network with this additional component is shown in color on the top left side of the back inside cover of the book. 

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