Physical properties, chemical properties and uses of ammonia

Physical Properties of Ammonia

• Ammonia is a colorless gas.

• It has a pungent odor with and an alkaline or soapy taste. When inhaled suddenly, it brings tears into the eyes.

• It is lighter than air and is therefore collected by the downward displacement of air.

• It is highly soluble in water: One volume of water dissolves about 1300 volumes of ammonia gas. It is due to its high solubility in water that the gas cannot be collected over water.

• It can be easily liquefied at room temperature by applying a pressure of about 8-10 atmosphere.

• Liquid ammonia boils at 239.6 K (- 33.5°C) under one atmosphere pressure. It has a high latent heat of vaporization (1370 J per gram) and is therefore used in refrigeration plants of ice making machines.

• Liquid ammonia freezes at 195.3 K (-77.8°C) to give a white crystalline solid.

Structure of ammonia

Ammonia is a covalent molecule as is shown by its dot structure. The ammonia molecule is formed due to the overlap of three sp³ hybrid orbitals and orbitals of three hydrogens. The fourth sp³ hybrid orbital is occupied by a lone-pair. This gives a trigonal pyramidal shape to ammonia molecule. The H-N-H bond angle is 107.3°, which is slightly less than the tetrahedral angle of 109°28. This is because the lone pair - bond pair repulsions tend to push the N-H bonds slightly inwards. In liquid and solid states, ammonia is associated through hydrogen bonds.

Chemical Properties of Ammonia

Thermal stability

Ammonia is highly stable. However, it can be decomposed into hydrogen and nitrogen by passing over heated metallic catalysts or when electric discharge is passed through it.

Combustibility

Ammonia is combustible in air. However, it will burn in an atmosphere of oxygen

Nitric oxide is obtained when a mixture of ammonia and air is passed over platinum - rhodium catalyst at 800°C

Basic character

Ammonia molecule has a strong tendency to donate its lone pair of

electrons of nitrogen to other molecules. Thus, it acts like a strong Lewis base. In aqueous solutions, NH₃ ionizes in accordance with the reaction.

The equilibrium constant for this reaction at 298 K is 1.8 x 10^-5. Thus, ammonia ionizes to a very small extent in aqueous solution. The aqueous solution of ammonia acts as a weak base due to the presence of OH^- ions therein. Therefore, ammonia turns red litmus blue and reacts with acids to form salts.

For example,

With metal oxides

Ammonia gets oxidized to nitrogen, when passed over heated metal oxides.

With halogens

Ammonia reacts with halogens but the type of halogen and reaction conditions determine the nature of products.

Chlorine

Nitrogen and ammonium chloride are formed with a limited amount of chlorine. In the presence of excess of chlorine, nitrogen trichloride is formed.

Bromine

It gives ammonium bromide and nitrogen

Iodine

When rubbed with solid iodine, a dark colored precipitate of nitrogen

tri-iodide is obtained

After drying, if NH₃.NI₃ is struck against a hard surface or hit with a hammer, it explodes producing iodine vapors.

With carbon dioxide (formation of urea)

Ammonia when heated under pressure with CO2 gives urea.

With alkali metals

When ammonia is passed over heated sodium or potassium, amides are formed and hydrogen is set free.

Alkali metal dissolved in liquid ammonia gives a blue solutions, which upon standing slowly, liberate hydrogen. The blue color of such solutions is due to the presence of solvated electrons (e^- (NH₃)_n). For instance, with sodium

Action with heavy metal ions

Ammonia forms metal hydroxides, which are insoluble and form precipitates, with the metal ions of Al, Fe, Cr, and Zn.

Formation of complex compounds

Ammonia forms complex compounds with the soluble salts of copper, silver etc. With copper sulphate solution, it gives a deep blue colored complex compound, tetramminecopper(II) sulphate.

Uses of ammonia

• In the manufacture of rayon and urea

• In the manufacture of fertilizers such as urea diammonium phosphate, ammonium nitrate, ammonium sulphate etc.

• In ice plants, as a refrigerant

• In furniture industry, as a cleansing agent for furniture and glass surfaces.

• In the manufacture of nitric acid by Ostwald's process.

• In the manufacture of sodium carbonate by Solvay's process.

Tests of ammonia

The following tests of any sample confirm the presence of ammonia.

• The ammoniacal smell of ammonia is easily detectable having a characteristic pungent smell.

• Ammonia turns moist red litmus blue, and moist turmeric paper brown.

• When added to a solution of copper sulphate, ammonia turns the solution deep blue.

• A glass rod dipped in concentrated HCl when brought close to ammonia, causes dense white fumes.

• When added to Nessler's reagent (alkaline solution of K₂[HgI₄] ammonia gives brown precipitate.