Nitrogen was discovered in 1772 by Daniel Rutherford, a Scottish physician, and chemist. Nitrogen occurs in the free state in the air and constitutes four-fifth of its volume (78%). Apart from it, it occurs in certain natural gases issuing out in volcanic regions.
Many compounds are known to contain nitrogen e.g nitre (NaNO3), salt petre (KNO3), ammonium chloride and ammonium sulphate. Nitrogen is an essential constituent of an important class of organic substances called the proteins and is present in many other vegetable animals and biological waste products. Manure and guano (bird droppings) are effective nitrogenous fertilizers.
Nitrogen lies on the group VA. It is P-block element.
Symbol = N
Atomic number = 7
Molecular weight = 28
Valency = 3
Prestly prepared gaseous ammonia in 1775 by heating lime with sal ammoniac. He called it alkaline air. This is hydride of nitrogen, NH3 and it can be made in very small amounts by heating nitrogenous organic materials such as hoofs and horns of animals. Its old name was, in-fact, spirit of harts horn.
Atmospheric air and natural water contain small quantities of ammonia. In combined state, it occurs in nature as NH4Cl and (NH4)2SO4. Because of the great solubility of ammonia in water, it rapidly finds its way to the soil by the action of rain and snow where it may be converted into other compounds.
Molecular formula: NH3
Molecular weight: 17
Ammonia is prepared in the laboratory by heating any ammonium salt (Sal ammoniac) with an alkali. Usually, a mixture of ammonium chloride and calcium hydroxide (slaked lime, the cheapest alkali) is used. Both are solids so they must be thoroughly grinded first to give a very fine mixture in which the reaction can occur satisfactorily. An excess of the slaked lime is preferable.
Gas evolved is dried by a rather unusal drying agent, quicklime CaO because it reacts with all the usual drying agents like conc. H2SO4, CaCl2 and P2O5. For example,
Quick lime readily absorbs moisture to give slaked lime as:
but they don't react with ammonia because both of them are basic in nature.
Ammonia is lighter than air and very soluble in water, it can't be collected over water. It may be collected over mercury or by the downward displacement of air. If required, moist ammonia gas obtained by heating the mixture is dried by passing through lime tower and finally collected by the downward displacement of air of an inverted gas jar. Apart from this, ammonia gas can also be prepared by heating any ammonium salt with base. For example,
Ammonia is involved the direct combination between nitrogen and hydrogen.
The reaction is reversible, exothermic and is accompanied by the decrease in volume. According to L e-Chatelier's principle to the most favourable condition, maximum yield are low temperature, high pressure, the high concentration of reactant, catalyst and purity of gases.
Since the forward reaction is exothermic itself favoured by low temperature. However at low temperature, the rate of reaction is very slow. So the optimum temperature is about 500°C, the pressure of 200 to 900 atm and catalyst and which consists of finely divided iron containing molybdenum as the promoter.
Purified N2 and H2 led to the compressed pump where it is compressed by a pressure of 200 to 900 atm then the mixture goes to catalyst chamber packed with finely divided iron as catalyst and molybdenum as promoters. The chamber is heated to about 450°C to 500°C to imitate the reaction. Under these condition about 15% of gases are converted into ammonia.
The resulting reaction mixture containing ammonia N2 & H2 is passed through a condenser of condensed ammonia. The liquefied ammonia is removed from time to time. The uncombined N2 & H2 are recycled and reprocessed.
Gewali, Mohan Bikram and Rishi Tiwari. principles of chemistry. second edition. buddha academic publishers and distributers pvt. ltd,