Ammonia (NH₃)

Ask a Question, Get an Answer!

Type your question here

Hundreds of tutors are online and ready to help you right now!

Ammonia (NH₃) is an important compound of nitrogen and hydrogen. It is produced by the natural decomposition of animal and vegetable bodies. The death and decay of plants and animals cause the nitrogen compounds present in them to get decomposed, giving ammonia. Ammonia also occurs in the soil in the form of ammonium salts.

Preparation of Ammonia

Ammonia is prepared by the following methods:

From ammonium chloride

Ammonia gas is usually prepared in the laboratory by gently heating ammonium chloride (NH₄Cl) and slaked lime [Ca(OH)₂].

Fig: 13.3 - Preparation of ammonia

Ammonia gas is lighter than air, necessitating its collection by the downward displacement of air. Because it is highly soluble in water it cannot be collected over it. Passing ammonia gas over quicklime (CaO) dries it. Being a basic gas, ammonia cannot be dried by passing it through concentrated sulphuric acid or phosphorus pentoxide (P₂O₅), as it reacts with them to form ammonium sulphate or ammonium phosphate respectively.

Calcium chloride also cannot be used for drying ammonia gas as it forms ammoniates with CaCl₂.

By the hydrolysis of metal nitrides

Hydrolyzing metal nitrides like magnesium and aluminium nitrides, with water or alkalies, can also produce ammonia gas.

Manufacture of Ammonia

Haber's process

The manufacture of ammonia by Haber's process involves the direct combination of nitrogen and hydrogen.

This reaction is, (a) reversible, (b) exothermic, and (c) proceeds with a decrease in volume. According to the Le Chatelier's principle, the favorable conditions for the formation of ammonia are,

Low temperature

The temperature should be remain as low as possible, (although at unusually low temperatures, the rate of reaction becomes slow). It has been found that the temperature, which optimizes the yield of ammonia for the reaction, is maximum at about 500°C.

High pressure

Since Haber's process proceeds with a decrease in volume, it is favored by high pressure. In actual practice, a pressure of 200 - 900 atmospheres is employed.

Catalyst

A catalyst is usually employed to increase the speed of the reaction. Finely divided iron containing molybdenum or alumina is used as a catalyst. Molybdenum or alumina (Al₂O₃) acts as a promoter and increases the efficiency of the catalyst. A mixture of iron oxide and potassium aluminate has been found to work more effectively.

Fig: 13.4 - Manufacturing plant employed in Haber's process

Source of raw materials

The nitrogen and hydrogen gases used as the raw material in Haber's process are obtained as follows.

• Nitrogen is obtained from the liquid air and hydrogen from water by electrolysis.

• Hydrogen may be obtained from water gas (mixture of CO and H₂) by Bosch process.

• Water gas can be obtained by passing steam over red hot coke.

By bubbling the mixture through water, CO₂ is removed.

• A mixture of nitrogen and hydrogen may be obtained by treating a mixture of producer gas (CO + N₂), water gas (CO + H₂) with steam in the presence of ferric oxide - chromium oxide catalyst at 450°C.

Carbon dioxide is removed by bubbling through water under pressure.

Plant

The plant, which manufactures ammonia, has the following components and processes.

Compressor

A mixture of nitrogen and hydrogen is compressed to 200-900 atmosphere pressure, in the ratio 1:3 (by volume). The compressed gas is sent to ammonia converter.

Fig: 13.5 - Ammonia converter

Converter

Ammonia converter is made from chrome-vanadium steel. It is usually 1.3 meter high and 1 meter in diameter. The converter is provided with a heat exchanger in the upper portion and the catalyst is packed in the central portion of the converter. There is an arrangement for heating the gas mixture. After the gas mixture enters through the inlet at the bottom, the gases circulate around the catalyst maintained at 450-500°C and then pass through to the heat exchanger. The gases finally enter the catalyst chamber to give ammonia. Before entering the condensers the product as well as the unreacted gases pass through the pipes of the heat exchanger and transfer their heat to the incoming gas mixture containing nitrogen and hydrogen.

Condensers

This cools and liquefies ammonia. The condensed ammonia, called 'liquor ammonia' is filled into cylinders under pressure.

Re-circulating pump

Some of the nitrogen and hydrogen gases escape condensation and are re-circulated through the converter.

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.