Dinitrogen and Ammonia

Dinitrogen and Ammonia – The p-Block Elements – Class 12

Dinitrogen

1) It is the first member of group 15 of the periodic table. 

2) It has the electronic configuration 1s² 2s² 2p³ and therefore, has five electrons in its valence shell.

3) In the molecular form it exists as a diatomic molecule (N₂) having triple bond between nitrogen atoms (NΞN). Therefore, it is also referred to as dinitrogen.

 

Preparation of Dinitrogen

In the laboratory dinitrogen is prepared by heating an aqueous solution containing an equivalent amount of ammonium chloride and sodium nitrate.

NH₄Cl(aq) + NaNO₃ (aq) ——-> N₂ (g) + 2H₂O(l) + NaCl(aq)

 

It can be prepared by :

 

1) Thermal decomposition of ammonium dichromate.

(NH₄)₂ Cr₂O₇ ——> Cr₂O₃ + N₂ + 4H₂O

 

2) By the action of ammonia with cupric oxide or bleaching powder

2NH₃ +3CuO –> 3 Cu + N₂ + 3H₂O

2NH₃ +3CaOCl₂ –> 3 CaCl₂ + N₂ + 3H₂O

 

3) Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide.

Ba(N₃)₂ —–> Ba + 3 N₂ 

Commercial Isolation of Dinitrogen from Air

Commercially dinitrogen is prepared by the liquefaction and fractional distillation of air.  When the liquid air is allowed to boil, dinitrogen with lesser boiling point gets distilled first leaving behind dioxygen.

Properties of Dinitrogen

Physical Properties of Dinitrogen

1) Dinitrogen is a colourless, odourless and tasteless gas.

2) It has two stable isotopes ¹⁴N and ¹⁵N.

3) It is a non-toxic gas.

4) It is slightly lighter than air and its vapour density is 14.

5) It has very low solubility in water. 

6) Its melting and boiling points are low; 63.2 K and 77.2 K respectively.

 

Dinitrogen is chemically unreactive at room temperature. It is neither combustible nor it supports combustion. The low reactivity of nitrogen is due to very small size of the molecule and high bond dissociation enthalpy of the molecule. However, reactivity increases rapidly with the rise in temperature. At higher temperature dinitrogen combines with some metals to form predominantly ionic nitrides and with non-metals to form covalent nitrides.

 

1) Action with litmus: Dinitrogen is neutral towards litmus because it has no acțion on blue or red litmus.

 

2) Action with metals: It combines with a number of active metals on strong heating to form their respective nitrides. For example,

 

6Li + N₂ ——-> 2Li₃N

3Mg + N₂ ——–> Mg₃N₂

Ca + N2 —-> Ca₃N2

2Al + N₂ –> 2 AlN

 

3) Combination with non-metals

 

Dinitrogen combines with non-metals like dioxygen and dihydrogen.

 

(i) Combination with dihydrogen: Dinitrogen reacts with dihydrogen about 700K under a pressure of 200 atm in the presence of iron as catalyst (Haber process). A small amount of molybdenum is also used as a promotor.

N₂ + 3 H₂ ——> 2 NH₃

 

(ii) Combination with dioxygen: Dinitrogen combines with dioxygen during lightning or at high temperature of about 2000 K and form nitric oxide.

 

N₂ + O₂ ——> 2 NO

Nitric oxide is a colourless gas but is very unstable. It immediately reacts with more of oxygen to form nitrogen dioxide.

 

2NO + O₂ → 2NO₂

 

4) Combination with compounds: Dinitrogen also combines with certain compounds on strong heating. For example,

a) With calcium carbide

CaCl₂ + N₂ ——-> CaCN₂ + C 

Calcium carbide       Calcium cyanamide

 

b) With alumina in the presence of carbon

Al₂O₃ + N₂ + 3C —–> 2 AlN + 3CO

Aluminium oxide            Aluminium nitride

 

Both these compounds are hydrolysed on boiling with water to give ammonia.

CaCN₂ + 3H₂O –> CaCO₃  + 2NH₃

AlN + 3 H₂O –> Al(OH)₃ + NH₃ 

 

Therefore, calcium cyanamide is used as a fertilizer under the name nitrolim (CACN2 + C).

 

Uses of Dinitrogen

 

(i) Dinitrogen is used in the manufacture of compounds like ammonia, nitric acid, calcium cyanamide, etc.

 

(ii) It is used in providing inert atmosphere in iron and steel industries.

 

(iii) Liquid nitrogen is used as refrigerant to preserve biological specimens, in freezing food stuffs and also in cryosurgery.

 

(iv) It is used in gas-filled thermometers used for measuring high temperatures.

 

Ammonia

Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea.

 

NH₂CONH₂ + 2H₂O —–> (NH₄)₂CO₃ ⇔ 2NH₃ + CO₂ + H₂O

Urea

 

Laboratory Preparation of Ammonia

 

Ammonia is prepared in the laboratory by heating ammonium salt: NH₄Cl or (NH₄)₂SO₄ with a strong alkali like NaOH.

 

(NH₄)₂SO₄ + 2NaOH —->  2NH₃ + 2H₂O  + Na₂SO₄

 

NH₄Cl +NaOH ——> NH₃ + H₂O + NaCl 

1) It can also be prepared by heating ammonium chloride with slaked lime

 

2NH₄Cl + Ca(OH)₂ —–> CaCl₂ + 2NH₃ + 2H₂O

 

2) It can also be prepared by the hydrolysis of magnesium nitride.

 

Mg₃N₂ + 6H₂O →3Mg(OH)₂ + 2NH₃

 

Ammonia can be dried by passing over quicklime (CaO). However, it cannot be dried with dehydrating agents such as conc. H₂SO₄. P₂O₅ and anhydrous CaCl₂ because ammonia reacts with these compounds.

 

Manufacturing of Ammonia

 

Ammonia can be manufactured by Haber’s process which involves the reaction :

 

N₂ (g) + 3H₂ (g) ⇔ 2NH₃ (g)

The favourable conditions for high yield of ammonia can be understood by applying Le Chatelier’s principle.

 

(i) Low temperature: Since the forward reaction is exothermic, therefore, low temperature will favour the formation of ammonia. However, an optimum temperature of about 700 K is necessary.

 

(ii) High pressure: High pressure of the order of 200 atmospheres or 200 x 10⁵ Pa is required to favour the forward reaction.

 

(iii) Presence of catalyst: The use of catalyst such as iron oxide containing a small amounts of molybdenum or potassium oxide (K₂O) and aluminium oxide (Al₂0₃) as promoter, increases the rate of attainment of equilibrium of ammonia.

 

Details of the process

 

 

 

1) In this method a mixture of N₂ and H₂ in the molar ratio of 1:3 is compressed to about 200 atmosphere pressure.

 

2) The compressed gases are then cooled and passed through soda lime tower to free them from moisture and carbon dioxide.

 

3) Then these are fed into catalyst chamber packed with iron oxide with small amount of K₂0 and Al₂O₃ or molybdenum.

 

4) The chamber is heated electrically to a temperature of 700 K when the two gases combine to form ammonia.

 

5) The reaction being exothermic, the heat evolved maintains the desired temperature and further electrical heating is not required.

 

6) The gases which escape from the chamber contain about 15-20% ammonia and the remaining are unreacted N₂ and H₂.

 

7) These are passed through condensing pipes where ammonia gets liquefied and is collected in the receiver.

 

8) The unreacted gases are pumped back to the compression pump where they are mixed with fresh gaseous mixture.

 

Structure of Ammonia 

 

1) Ammonia is expected to have a tetrahedral geometry because the central nitrogen atom involves sp³ hybridisation. 

 

2) It has one position occupied by a lone pair. The lone pair distorts its geometry and the molecule has pyramidal geometry with nitrogen atom at the apex. 

 

3) The N-H bond length is 101.7 pm and HNH bond angle is 107.8°.

 

Properties of Ammonia

 

Physical properties of Ammonia

 

1)  Ammonia is a colourless gas with a characteristic pungent smell called ammoniacal smell.

 

2) It is lighter than air.

 

3)  It is highly soluble in water and its solution is basic in nature due to the formation of OH¯ ions.

 

NH₃ (g) + H₂O (l) ⇔ NH₄⁺ (aq) + OH¯

 

4) Ammonia can be easily liquefied under pressure. Liquid ammonia has b.p. 239.7 K and f.p. 198.4 K.

 

Chemical properties of Ammonia

 

1) Basic nature: Ammonia gas is highly soluble in water. Its aqueous solution turns red litmus blue indicating its weakly basic character. The basic character of ammonia is due to the formation of OH ions in aqueous solution.

 

NH₃ (aq) + H₂O (aq) ⇔ NH₄⁺ (aq) + OH¯ (aq)

 

Being basic ammonia forms salts with both weak and strong acids such as NH₄Cl , (NH₄)₂SO₄ ,(NH₄)₂CO₃ etc.

 

NH₃ (aq) + HCl (aq) —-> 2NH₄Cl (s)

 

2 NH₃ (aq) + H₂SO₄ —–> (NH₄)₂SO₄ (s) 

 

2) Tendency to form complexes. Ammonia acts as a Lewis base due to the presence of lone pair of electrons on the nitrogen atom. Therefore, it can form coordinate bond with a number of transition metal cations forming complex compounds. 

 

For example,

Ag⁺ (aq) + 2NH₃(aq) ——-> [Ag(NH₃)₂]⁺

 

Cu²⁺(aq) + 4 NH₃ (aq) ——> [Cu (NH3)4]²⁺ (aq)
 

Cd²⁺ + 4NH₃ ——–> [ Cd(NH₃)₄]²⁺Cl (aq) 

 

Since ammonia forms complexes, the white precipitate of silver chloride dissolves in excess of ammonium hydroxide to form a soluble complex.

 

AgCl + 2NH₄OH ⇔ [Ag(NH₃)₂]Cl + + 2H₂0

White ppt.

Similarly, copper sulphate dissolves in excess of ammonium hydroxide to form deep blue coloured complex.

 

CuSO₄ + 4NH₄OH ⇔ [Cu(NH₃)₄]SO₄ + 4H₂O

 

                                     Tetraammine copper (II) sulphate (Blue)

 

3. Action with halogens.

 

(i) Chlorine reacts with ammonia in two ways depending upon whether ammonia is in excess or chlorine is in excess.

 

Excess NH₃

8NH₃+ 3Cl₂ ——-> 6NHCl₄ + N₂

 

Excess Cl₂

NH₃ + 3Cl₂ ——> NCl₃ + 3HCl

 

Excess of ammonia

8NH₃+ Br₂ ——-> 2NHBr₄ + 3N₂

 

Excess Br₂

NH₃ + 3Br₂ ——> NBr₃ + 3HCl

 

(ii) Iodine reacts with liquid ammonia to form a dark brown complex.

2NH₃ + 3 I₂ —> NH₃NI₃ + 3

 Nitrogen tri-iodide ammoniate

 

The complex is very explosive when dry. It explodes violently when rubbed against a hard surface as follows :

 

8NH₃NI₃ ——->  5N₂ + 9I₂ + 6NH₄I

4) Oxidation: Ammonia is oxidised to dinitrogen with oxidising agents like CuO, NaClO, CaOCl₂, etc. 

For example: When ammonia is passed over heated Copper oxide, it gets oxidised to dinitrogen.

 

3CuO + 2NH₃ —> 3 Cu + N₂ + 3 H₂O

 

When vapours of NH₃ and O₂ are passed over red hot platinum gauze at 1075 K, it is oxidised to nitric oxide.

4NH₃ + 5 O₂ ——-> 4 NO + 6H₂O

 

This reaction forms the basis of Ostwarld’s process for the manufacture of nitric acid.

5) Combustion: Ammonia is neither combustible nor supporter of combustion.However, it burns in the presence of oxygen to form dinitrogen and water.

 

4 NH₃ + 3O₂ –> 2 N₂ + 6H₂O

 

6) Formation of amides: When ammonia is passed over heated sodium or potassium at 575 K. it forms amides with the liberation of dihydrogen.

 

2Na + 2 NH₃ ——-> 2 NaNH₂ + H₂

 

7) Precipitation of heavy metal ions from the aqueous solution of their salts

 

Heavy metal ions such as Fe³⁺, Cr³⁺, Al³⁺, etc. are precipitated from their aqueous salt solutions as hydroxides with ammonia.

 

FeCl₃(aq) + 3 NH₄OH ——–> Fe(OH)₃ (s) or Fe₂O₃.xH₂O(s)+3NH₄Cl (aq)
Brown ppt.

 

AlCl₃ (aq) + 3NH₄OH(aq) ——> Al(OH)₃ (s) + 3NH₄Cl (aq)

 

CrCl₃ (aq) + 3NH₄OH(aq) ——–>  Cr(OH)₃(s) + 3NH₄Cl (aq)

  Green ppt.

 

ZnSO₄ (aq) + 2NH₄OH(aq) ——> Zn(OH)₂(s) + (NH₄)₂SO₄(aq)

White ppt.

Uses of Ammonia

(i) It is used in the manufacture of nitric acid and sodium carbonate.

(ii) Liquid ammonia is used as a refrigerant.

(iii) It is used in the manufacture of rayon.

(iv) It is commonly used for preparing various nitrogenous fertilizers such as ammonium nitrate, ammonium sulphate, ammonium phosphate and urea.

(v) It is an important reagent and is used as a solvent in the laboratory.