JEE Chemistry Hydrogen Notes Set 01

Position and Isotopes of Hydrogen

Hydrogen has the simplest atom, 1s1. It resembles alkali metals (loses one electron to give H+) and also halogens (gains one electron to give H-, and forms a diatomic molecule), so its position is anomalous; it is placed in group 1.
Isotopes: protium (1H, 99.98 percent), deuterium (2H or D, 0.015 percent, stable) and tritium (3H, radioactive, beta emitter, half-life about 12.3 years). They differ in mass, so properties dependent on mass differ; reactions of deuterium compounds are slower because the D-X bond is stronger than H-X (isotope effect).

Preparation of Dihydrogen

Laboratory: Zn + dil. H2SO4 gives ZnSO4 + H2. Amphoteric metals (Zn, Al, Be) also liberate H2 with NaOH; Mg does not.
Industrial: electrolysis of water, reaction of steam with hot coke (water gas, CO + H2), and the water gas shift reaction, CO + H2O over iron chromate at 673 to 773 K giving CO2 + H2, with CO2 removed by scrubbing. Steam reforming of methane over nickel at about 1270 K gives CO + 3H2.

Properties and Uses

Dihydrogen is the lightest gas, colourless, odourless and insoluble in water. It is combustible and acts as a reducing agent. It combines with many elements to give hydrides, with nitrogen (Haber process) to give ammonia, and with unsaturated oils in presence of nickel to give saturated fats (hydrogenation). Nascent hydrogen is freshly generated atomic hydrogen with excess energy and is a strong reductant. Palladium occludes large volumes of hydrogen.
Uses: ammonia and methanol manufacture, hydrogenation of oils, rocket fuel, fuel cells, and welding (oxy-hydrogen flame).

Hydrides

Ionic or saline hydrides (s-block metals, such as NaH and CaH2) are crystalline, non-volatile, non-conducting solids containing the H- ion. Covalent or molecular hydrides are formed by p-block elements, such as CH4, NH3, H2O and HCl. Metallic or interstitial hydrides are formed by d- and f-block metals (like PdH). The hydride gap is the group 7, 8 and 9 region, whose elements do not form hydrides.