Ammonia Gas: Preparation, Properties, uses

Chemical Formula

  • Calcium cyanamide- CaCN2
  • Magnesium Nitride:- Mg3N2
  • Sodium zincate:- Na2Zn(OH)4
  • Diamine Silver Chloride: [Ag(NH3)2Cl]
  • Urea:- NH2CONH2

  • Calcium cyanamide:- CaCN2
  • Silver Nitrate: AgNO3

  • Mercurous Chloride:- Hg2Cl2

Ammonia Gas

Ammonia gas was first obtained by Joseph Priestley in 1771 by heating the ammonium and lime together and named it Alkaline Air.

In some quantities it is found in the atmosphere. In places where animals rot, its quantity is more than expected in the air. In nature it is also found as its salts.

Example:

In nature, it is found as ammonium chloride (NH4Cl) and ammonium sulphate [(NH4)2SO4].

Preparation Normal Methods

Ammonization salts: Ammonia is obtained by heating ammonium salts with alkalis. Some ammonium salts give ammonia only on heating alone.

2NH4Cl + Ca(OH)2 CaCl2 + 2H2O + 2NH3

(NH4)2SO4 + 2NaOH Na2SO4 + 2H2O + 2NH3

(NH4)2SO4 H2SO4 + 2NH3

Addition of Nitrogen and Hydrogen: Ammonia is obtained by adding nitrogen and hydrogen at about 500°C temperature and 200 atmospheric pressure in the presence of
catalyst (Iron).

N2 + 3H2 2NH3

From Calcium Cyanamide: Ammonia is obtained by the action of water vapor on calcium
cyanamide.

CaCN2 + 3H2O 2NH3 + CaCO3

By the action of water on Magnesium Nitride:

Mg3N2 + 6H2O 3Mg(OH)2 + 2NH3

From Metallic Nitrate: Ammonia gas is obtained by the reaction of metallic nitrate with nascent hydrogen. The nascent hydrogen used in this activity is obtained by heating zinc or aluminum with sodium hydroxide.

4Zn + 7NaOH + NaNO3 4Na2ZnO2 + 2H2O + NH3

From Carbonic Compounds: When some nitrogen containing organic compounds are heated with soda lime, ammonia is obtained.

NH2CONH2 + NaOH Na2CO3 + 2NH3

Preparation in Laboratory

In the laboratory, ammonia is made by heating the ammonium chloride with a quenched lime.

2NH4Cl + Ca(OH)2 CaCl2 + 2H2O + 2NH3

In a rigid glass tube, a mixture of ammonium and quenched lime is heated. The received ammonia gas is collected from the gas jar by downward displacement of air.

Before being collected in a gas jar, it is passed onto CaO, which removes the moisture of this gas.

ammonia preparation in laboratory

Note: Ammonia cannot be dried by concentrated sulfuric acid H2SO4, calcium chloride, phosphorus penta oxide because ammonia undergoes chemical reactions with these compounds.

2NH3 + H2SO4 (NH4)2SO4

2NH3 + CaCl2 + 2H2O 2NH4Cl + Ca(OH)2

6NH3 + P2O5 + 3H2O 2(NH4)3PO4

Industrial Manufacture

The industrial manufacture of ammonia is done by the following methods:

Haber's Process

In this method, nitrogen and hydrogen combine to form ammonia gas.

N2 + 3H2 ⇌ 2NH3 + 24,000 calorie

In this activity, heat is generated and heat is increased. This is an exothermic reversible reaction. In this reaction the volume of the reactants decreases.

According to the principle of le-chatelier, it is necessary to have low temperature and high pressure in this action. In this way more ammonia can be obtained and the action
velocity can be increased.

The presence of suitable catalysts also helps in increasing the velocity of action. But the optimum temperature of the reaction is about 500°C in the presence of the catalyst used in the reaction. 

Therefore, in this reaction the temperature is kept at around 500°C. The pressure value is kept equal to about 200 atmospheres.

In this method any of the following catalysts can be used –

  • ·      Iron granular powder containing a small amount of malibdanam
  • ·      Ferric oxide containing soda, silica and potassium
  • ·      Fine powder of nickel

 

Haber Process for Ammonia Synthesis

In the industrial manufacture of ammonia by the Haber method, a mixture of iron and molybdenum is often used as a catalyst. Molybdenum catalyst acts as a promotor.

Nitrogen gas obtained from the atmosphere and hydrogen from the water gas are mixed in dry state in a ratio of 1: 3 and flow into a chamber at 200 atmospheric pressure, where the catalyst is kept at around 500°C.

In this chamber, both these gases are reacted together to produce ammonia. The remaining gases are re-circulated in this chamber by a circulatory pump.

Haber's Process

A mixture of CaCN2 and carbon is obtained when CaC2 and nitrogen are heated to almost 1100°C temperature. Also called Nitrolime and which is also used as fertilizer.

CaC2 + N2 CaCN2 + C

Ammonia gas is obtained by its reaction at 180°C and 3-4 atmospheric pressure with water vapor.

CaCN2 + 3H2O CaCO3 + 2NH3

In the manufacture of coal gas: As a side product, ammonia-rich liquor is obtained when making coal gas, which contains ammonium salts in large quantities. Ammonia gas is
obtained by boiling it with lime water.

Physical Properties

It is a colorless and intensely smelling gas. Due to this, tears start coming out of the eyes. This can also be fatal due to an overdose.

The molecular weight of ammonia is 17 and density is 8.5. Hence it is lighter than air. This is why in the laboratory it is collected by downward displacement of air.

It is soluble in water. It also dissolves in large quantities in water. At normal temperature, its water content is 89.9 grams per 100 ml. Like other gases, its solubility in water decreases at higher temperatures.

Ammonia dissolves in water to form ammonium hydrosidae –

NH3 + H2O NH4OH ⇌ NH4+ + OH

Ammonium hydrosidae is a valid additive. Hence, aqueous solution of ammonia is alkaline and conductor of electricity.

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Ammonia gas does not burn itself nor is it helpful in burning other substances. It does not burn when ammonia gas is ignited in the air. But it starts burning due to excess of oxygen. She extinguishes when a burning candle is carried in a jar of ammonia gas.

The critical temperature of ammonia is 133°C. Therefore, it keeps on increasing pressure at normal temperature.

The boiling point of liquid ammonia is -33.4°C and freezing point -77.7°C. Ammonia has a higher boiling point than phosphine(PH3). The reason for this is that the negative efficiency of nitrogen is higher than the negative efficiency of phosphorus. Hence intermolecular hydrogen bonding occurs in ammonia. And more energy is required to separate its molecules.

Chemical Properties

Reaction with water: Ammonia dissolves in water to form ammonium hydroxide-

NH3 + H2O NH4OH ⇌ NH4+ + OH

Alkaline properties: It reacts with acids to form salts.

2NH3 + H2SO4 (NH4)2SO4

NH3 + HCl NH4Cl

NH3 + HNO3 NH4NO3

Combustion: Ammonia does not burn itself or helps in the burning of other substances. It burns with yellow flame in excess of oxygen.

4NH3 + 3O2 6H2O + 2N2

Nitric oxide and water are obtained when a mixture of ammonia and oxygen gas flows through a hot plate of platinum.

4NH3 + 5O2 6H2O + 4NO

Thermal Decomposition: Ammonia is a permanent additive but dissociates into components at high temperatures or when electric sparks flow.

2NH3 N2 + 3H2

Action with halogens: If more amount of ammonia is taken with less amount of chlorine, nitrogen gas is obtained.

2NH3 + 3Cl2 N2 + 6HCl

6NH3 + 6HCl 6NH4Cl

8NH3 + 3Cl2 N2 + 6NH4Cl

If an excess amount of chlorine is treated with a low amount of ammonia, then nitrogen tri-chloride is formed, which is a highly explosive substance.

3Cl2 + NH3 NCl3 + 3HCl

Ammonia has similar action with bromine and iodine.

Reaction with Magnesium: It reacts with magnesium metal at high temperature to form magnesium nitride (Mg3N2).

3Mg + 2NH3 Mg3N2 + 3H2

Action with alkaline metals: It reacts with alkaline metals at high temperature to form amide[Sodium amide (NaNH2)].

2Na + 2NH3 2NaNH2 + H2

Action with copper oxide: It is oxidized to nitrogen on red hot copper oxide.

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2CuO + 2NH3 3Cu + 3H2O + N2

Its aqueous solution (NH4OH) combines with some salts to form complex compounds.

1) When a white precipitate of silver chloride (AgCl) is added in large amount with aqueous solution of ammonia, ie ammonium (NH4OH), dissolves due to the formation of complex compounds called diamine silver chloride[Ag(NH3)2Cl].

AgCl + 2NH4OH [Ag(NH3)2]Cl + 2H2O

2) Aqueous solution of copper sulfate (CuSO4) is blue due to Cu++ ion. It forms Copper hydroxide Cu(OH)2 by first passing ammonia or adding an aqueous solution of ammonia.

Copper hydroxide is insoluble in water and its color is greenish blue. Therefore, greenish blue precipitate is obtained.

A dark blue solution is obtained when a large amount of ammonium hydroxide is flowed or an excess amount of ammonium hydroxide is formed by the formation of complex compounds called tetra amine cupric sulphate. The thick blue color of tetra amine cupric sulphate is due to Cu(NH3)4++ ion.

CuSO4 + 4NH4OH Zn(NH3)4Cl2 + 4H2O

3) A white precipitate of zinc hydroxide is obtained when ammonium hydroxide solution is added to aqueous solution of zinc chloride(ZnCl2). It dissolves a large amount of ammonium hydroxide to form a complex compound called tetra amine zinc chloride[Zn(NH3)4Cl2] which is merged with water.

ZnCl2 + 4NH4OH Zn(NH3)4Cl2 + 4H2O

Anhydrous zinc chloride also absorbs the ammonia gas to make this complex compound.

ZnCl2 + 4NH3 Zn(NH3)4Cl2

Reaction with Silver Nitrate: Aqueous solution of ammonia reacts with silver nitrate (AgNO3) to form a white brown precipitate of silver oxide(Ag2O) which dissolves in excess of ammonium to form a merged complex salt. The solution thus obtained is called tollen’s reagent. 

2AgNO3 + 2NH4OH 2NH4NO3 + H2O + Ag2O

Ag2O + 2NH4NO3 + 2NH4OH 2[Ag(NH3)2]NO3 + 2H2O

Reaction with mercurous chloride: A dilute aqueous solution of ammonia in mercuric chloride solution forms a mixture of mercury and mercuric amino chloride, which is black in color.

Hg2Cl2 + 2NH4OH Hg + Hg(NH2)Cl + 2H2O + NH4Cl

Reaction with Mercuric Chloride: White precipitate of mercuric amino chloride is obtained by adding aqueous solution of ammonia to mercuric chloride solution.

HgCl2 + 2NH4OH NH4Cl + 2H2O + Hg(NH2)Cl

Action with Nessler’s reagent: The alkaline solution of Potassium tetraiodomercurate(II) is called Nessler’s reagent. On reaction with ammonia, Iodide of Millon’s base and a brown precipitate is obtained. This reaction is also used to test ammonia and ammonium ions. 

2K2HgI4 + 3NaOH + NH3 4KI + 3NaI + 2H2O + HgOHg(NH2)I

Uses of Ammonia

  • It is used as a reagent in the laboratory.
  • In the industrial manufacture of nitric acid
  • In making urea, ammonium salts and fertilizers.
  • Artificial silk tear gas is also used in making explosive material, ice, etc.

 

Molecular structure of ammonia

Following is the bond formula, electron point formula and orbital diagram of ammonia-

The hybridization of nitrogen atom in ammonia is of the sp3 type and has a pyramidal shape. The Basic property of ammonia is due to the lone pair of electrons present on nitrogen in it.

Molecular structure of ammonia

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