Introduction
All elements in the periodic table have been classified as metals and non-metals. The non-metals are placed on the right hand side of the periodic table.
Dihydrogen
Hydrogen is the first element in the periodic table and is the lightest known element. It was discovered by Henry Cavendish in 1766. It exists as a diatomic molecule H2. Its name hydrogen was proposed because it produces water on burning with oxygen (in Greek; hydro means water, gene means producing).
Occurrence of Hydrogen
Hydrogen is the most abundant element in the universe. About half the mass of sun and stars is due to hydrogen. Jupiter and Saturn planets consist mainly of hydrogen. It is common on earth in water, coal, petroleum, clay, animal and vegetable matter that constitutes 0.9% by weight of Earth's crust. It is the ninth element in order of abundance.
Preparation of Dihydrogen
By the action of water on metals
Various metals differ in their reactivity towards water.
For example, very active metals like Na, K and Ca react at room temperature.
Industrial Preparation of Dihydrogen
Hydrocarbon steam process
Dihydrogen is prepared by the action of steam on hydrocarbon.
Carbon monoxide is oxidized to carbon dioxide and dissolved in water while dihydrogen is collected.
Properties of Dihydrogen
Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated above 2000 K.
Its bond dissociation energy is very high (DH = 435.9 kJ mol-1) which makes it less reactive. However, it combines with many elements or compounds.
Uses of Dihydrogen
It is used for welding purposes in both the oxy-hydrogen torch and the atomic-hydrogen torch, when temperatures of the order of 2500°C and 4000°C are required respectively.
Ortho and Para Hydrogen
A molecule of dihydrogen contains two atoms, in which the nuclei of both the atoms are spinning. Depending upon the direction of the spin of the nuclei.
Atomic and Nascent Hydrogen
Langmuir ,in 1915, obtained atomic hydrogen by dissociating on a hot filament of tungsten or platinum. The dissociation of molecular hydrogen is an endothermic process.
Hydrogen Fuel
Our present day life highly banks upon the use of fossil fuels such as coal, natural gas, LPG, gasoline, etc. for the production of energy. Our reserves of fossil fuels are getting depleted at an alarming rate and they may get exhausted by the middle of 21st century. The reason for this shortage is that the rate of consumption of fossil fuels is much larger than the rate of their renewal.
Hydrides
The formation of binary compounds by hydrogen with other elements, are called hydrides. Hydrogen combines with most of the metals and non-metals (except noble gases). Hydrides are classified into three main types, depending upon their physical and chemical properties.
Polymeric Hydrides and Complex Hydrides
Polymeric hydrides are formed by elements having electronegativity ranging between 1.4 and 2.0. They consist of molecules held together in two or three dimensions by hydrogen bridges. These are usually solids. The common examples of polymeric hydrides are (BeH2)n, (MgH2)n, (AlH3)n, (lnH3)n, (SiH4)n etc.
Water
In the Earth's biosphere, water is essential to all forms of life. Interestingly, it is also the biospheres most abundant compound and a unique substance from the chemical point of view.
Physical Properties of Water
Pure water is transparent, tasteless, colourless and odourless. It readily picks up the flavour of any substance dissolved in it. Water in thick layers appears greenish blue.
Chemical Properties of Water
Water shows a remarkable and versatile chemical behaviour. It acts as an acid, a base, an oxidizing agent, a reducing agent, and as a ligand to metal ions.
Hard and Soft Water
Water is described as being 'hard' if it does not lather readily with soap. 'Soft water', on the other hand, is described as the one, which lathers readily with soap. Chemically, natural water is never pure and contains varying amounts of the dissolved impurities absorbed from the natural or man made environment. When appreciable concentrations of calcium ions (Ca2+) or the magnesium ions (Mg2+) in the form of their dissolved salts are contained in water, it becomes hard.
Methods of Removal of Hardness in Water
Temporary hardness in water can be easily removed by boiling. On boiling, calcium/magnesium bicarbonate decomposes to give calcium/magnesium carbonate, which is insoluble in water. Therefore, it precipitates out.
Heavy Water
Multistage electrolysis of ordinary water containing NaOH gives heavy water. The cell used for electrolysis, contains a cylindrical vessel made of steel as cathode while a perforated cylindrical sheet acts as the anode. The electrolysis is carried out in different stages.
Hydrogen Peroxide
Hydrogen peroxide is present in the juice of certain plants and traces of it are also found in the atmosphere and water.
Chemical Properties of Hydrogen Peroxide
Hydrogen peroxide has an interesting chemistry because of its ability to function as oxidant as well as a reductant in both acid and alkaline solutions. The oxidation state of oxygen in hydrogen peroxide is -1. It can therefore be oxidized to O2 (zero oxidation state) or reduced to H2O or OH- (-2 oxidation state for oxygen). On the whole, hydrogen peroxide is a very powerful oxidizing agent and a poor reducing agent.
