9.1 Important compounds of Nitrogen

  p Block Elements

1. Ammonia

Nitrogen forms three well-known hydrides with hydrogen:

(i) Ammonia, $NH_3$

(ii) Hydrazine, $N{H_2}.N{H_2}({N_2}{H_4})$

(iii) Hydrozoic acid, $N_3H$.

Occurrence: $NH_3$ is found in traces in the atmosphere. Ammonium salts such as ammonium chloride and ammonium sulphate are found in small amounts in the soil.

Discovery: It was first isolated by Priestly by the action of ammonium chloride and lime. It was named alkaline air.

Preparation:

(i) Ammonium is obtained on a small scale from ammonium salts which evolve it when heated with caustic soda or lime.\[\begin{gathered} N{H_4}Cl + NaOH \to N{H_2} + NaCl + {H_2}O \hspace{1em} \\ N{H_4}Cl + Ca{(OH)_2} \to 2N{H_2} + CaC{l_2} + 2{H_2}O \hspace{1em} \\ \end{gathered} \]

(ii) Ammonia is formed when ammonium chloride is heated with litharge.$$2N{H_4}Cl + PbO \to 2N{H_2} + PbC{l_2} + {H_2}O$$

(iii) By reacting nitrides with water, ammonia is obtained.$$\begin{equation} \begin{aligned} AlN + 3{H_2}O \to Al{(OH)_3} + N{H_3} \\ M{g_{{3_{}}}}{N_2} + 6{H_2}O \to 3Mg{(OH)_2} + 2N{H_3} \\\end{aligned} \end{equation} $$

(iv) Ammonium can also be formed by doing reduction of nitrate with Zinc and caustic soda. Zinc and caustic soda produce nascent hydrogen which reacts with nitrates and nitrites to form ammonia, \[\begin{gathered} NaN{O_3} + 8H\xrightarrow{{Zn/NaOH}}NaOH + N{H_3} + 2{H_2}O \hspace{1em} \\ NaN{O_2} + 6H\xrightarrow{{Zn/NaOH}}NaOH + N{H_3} + {H_2}O \hspace{1em} \\ \end{gathered} \]

(v) Calcium cyanamide is also obtained by heating ammonium compounds\[\begin{gathered} {(N{H_4})_2}S{O_4}\xrightarrow{{Heat}}N{H_3} + N{H_4}HS{O_4} \hspace{1em} \\ \hspace{1em} \\ N{H_4}{H_2}.P{O_4}\xrightarrow{{Heat}}N{H_3} + HP{O_3} + {H_2}O \hspace{1em} \\ \end{gathered} \]

(vi) Urea on treatment with caustic soda forms ammonia.\[\begin{gathered} N{H_2}CON{H_2} + 2NaOH \to N{a_2}C{O_3} + 2N{H_3} \hspace{1em} \\ urea \hspace{1em} \\ \end{gathered} \]

Drying of Ammonia gas:

The common dehydrating agents like sulphuric acid or $CaCl_2$ or $P_2O_5$ cannot be used as these react with ammonia.$$2N{H_3} + {H_2}S{O_4} \to {(N{H_4})_2}S{O_4}(Ammonium\,sulphate)$$$$CaC{l_2} + 8N{H_3} \to CaC{l_2}.8N{H_3}(Addition\,product)$$$${P_2}{O_5} + 6N{H_3} + 3{H_2}O \to 2{(N{H_4})_3}P{O_4}(Ammonium\,phosphate)$$

From dying quick lime is used as it does not react with ammonia but reacts readily with moisture.$$CaO + {H_2}O \to Ca{(OH)_2}$$

Manufacture of Ammonia:

(i) Haber's Process: The method involves the direct combination of nitrogen and hydrogen according to the following reaction:$${N_2} + 3{H_2} \rightleftarrows 2N{H_3} + 24.0\;Kcal$$ Raw materials: Nitrogen and hydrogen are the chief raw materials. Nitrogen is obtained from air by liquefication followed by fractional evaporation of liquid air. Hydrogen is obtained by fractional evaporation of liquid air. Hydrogen is obtained by electrolysis of water.

(ii) Bosch Process: From Powder gas & water gas.

(iii) Cyanamide process:\[\begin{gathered} CaC{N_2} + 3{H_2}O\xrightarrow[{(steam)}]{{180^\circ C}}CaC{O_3} + 2N{H_3} \hspace{1em} \\ \quad \quad \quad \quad \quad \quad \quad \quad 3 - 4{\kern 1pt} {\kern 1pt} atm \hspace{1em} \\ \end{gathered} \]

(iv) From ammoniacal liquor obtained during coal distillation: Large quantities of ammonia obtained as a by-product in the manufacture of coal gas.

Physical properties:

(i) Ammonia is a colorless gas with a characteristic pungent odor it brings tears to the eyes.

(ii) It is highly soluble in water. This high solubility is due to the hydrogen bonding. The solubility of ammonia increases with increases in pressure and decreases with increase in temperature.

(iii) It can be easily liquefied at room temperature by the application of pressure.

(iv) Ammonia molecule links together to form associated molecules through hydrogen bonding.

Higher melting point in comparison to other hydrides of $V$ group is due to hydrogen bonding.

Chemical Properties:

(i) Stability: It is highly stable. It decomposes into nitrogen and hydrides at red heat or when electric sparks are passed through it.$$2N{H_3} \rightleftarrows {N_2} + 3{H_2}$$.

(ii) Combustion: Ordinary, ammonia is neither combustible nor a supporter of combustion. However, it burns in the presence of oxygen to form nitrogen and water. $$4N{H_3} + 3{O_2} \to 2{N_2} + 6{H_2}O$$

(iii) Basic nature: Ammonia is a Lewis base, accepting a proton to form ammonium as it has a tendency to donate an electron pair.

It forms salts with acids,\[\begin{gathered} N{H_3} + HCl \to N{H_4}Cl\;\quad \quad \quad \quad \quad (Ammonium\;Chloride) \hspace{1em} \\ \ \ \ \ \ \ Thick{\kern 1pt} white{\kern 1pt} fumes \hspace{1em} \\ 2N{H_3} + {H_2}S{O_4} \to {(N{H_4})_2}S{O_4}\;\quad \quad (Ammonium{\mkern 1mu} \;sulphate) \hspace{1em} \\ \end{gathered} \]

It's solution is a weak base, the solution is described as aqueous ammonia. It's ionisation in water is represented as:\[N{H_3} + {H_2}O \to N{H_4}OH \rightleftarrows NH_4^ + + O{H^ - }\]The solution turns red litmus to blue phenophthalein pink.

(iv) Oxidation: It is oxidised to nitrogen when apssed over heated $CuO$ or $PbO$$$\begin{equation} \begin{aligned} 3CuO + 2N{H_3} \to 3Cu + {N_2} + 3{H_2}O \\ 3PbO + 2N{H_3} \to 3Pb + {N_2} + 3{H_2}O \\ \\\end{aligned} \end{equation} $$Both chloride and bromine oxidised ammonia.\[\frac{\begin{gathered} 2N{H_3} + 3C{l_2} \to {N_2} + 6HCl \hspace{1em} \\ 6N{H_3} + 6HCl \to 6N{H_4}Cl \hspace{1em} \\ \end{gathered} }{{8N{H_3} + 3C{l_2} \to {N_2} + N{H_4}Cl}}\]When chlorine is in excess an explosive substance nitrogen trichloride is formed.$$N{H_3} + 3C{l_2} \to NC{l_3} + 3HCl$$Iodine flake when rubbed with liquor ammonia form a dark brown precipitate of ammoniated nitrogen iodide which expodes readily on drying.$$2N{H_3} + 3{I_2} \to N{H_3}.N{I_3} + 3HI$$Hypochlorite and hypobromite oxidise ammonia to nitrogen.$$2N{H_3} + 3NaClO \to {N_2} + 3Nacl + 3{H_2}O$$The oxidation of ammoina with bleaching powder occours on warming.$$3CaOC{l_2} + 2N{H_3} \to 3CaC{l_2} + {N_2} + 3{H_2}O$$The ammonia acts as a reducing agent.

The restricted oxidation of $NH_3$ can be done with air, when the mixture is passed over heated platinum gauze at $700 - 800^\circ C$

$$4N{H_3} + 5{O_2} \to 4NO + 6{H_2}O$$

This is the Ostwald's process and used for manufacture of $HNO_3$

(v) Formation of amides: When dry ammonia is passed over heated sodium or potassium, amide are formed wth evolution of hydrogen.$$2Na + 2N{H_3} \to 2NaN{H_2}(Sodamide) + {H_2}$$

(vi) Reaction of aqueous ammonia: Many metal hydroxides are formed which they may be precipitated or remain in the form of complex compounds in excess of $NaOH$.$$\begin{equation} \begin{aligned} FeC{l_3} + 3N{H_4}OH \to Fe{(OH)_2} + 3N{H_4}Cl \\ AlC{l_3} + 3N{H_4}OH \to Al{(OH)_3}(ppt.) + 3N{H_4}Cl \\ CrC{l_3} + 2N{H_4}OH \to Cr{(OH)_3}(ppt.) + 3N{H_4}Cl \\ CuS{O_4} + 2N{H_4}OH \to Cu{(OH)_2}(Blue{\kern 1pt} {\kern 1pt} ppt.) + {(N{H_4})_2}S{O_4} \\ Cu{(OH)_2} + {(N{H_4})_2}C{O_4} + 2N{H_4}OH \to \left[ {Cu{{(N{H_3})}_4}} \right]S{O_4}(Tetramine\,copper\;sulphate) + 4{H_2}O \\ CdS{O_4} + 4N{H_4}OH \to \left[ {Cd{{(N{H_3})}_4}} \right]S{O_4}(Cadmium\;tetramine\;sulphate) + 4{H_2}O \\ AgN{O_3} + 4N{H_4}OH \to AgOH(White\;ppt.) + N{H_4}N{O_3} \\ AgOH + 2N{H_4}OH \to \left[ {Ag{{(N{H_3})}_2}} \right](OH)(soluble) + 2{H_2}O \\\end{aligned} \end{equation} $$ $AgCl$ also dissolves in $NH_4OH$ solution$$\begin{equation} \begin{aligned} AgCl + 2N{H_4}OH \to \left[ {Ag{{(N{H_3})}_2}Cl} \right] + 2{H_2}O \\ \quad \quad \quad \quad \quad \quad \quad \quad (Diamine\;silver\;chloride) \\ ZnS{O_4} + 2N{H_4}OH \to Zn{(OH)_2} + {(N{H_4})_2}S{O_4} \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad (ppt.) \\ Zn{(OH)_2} + {(N{H_4})_2}S{O_4} + 2N{H_4}OH \to \left[ {Zn{{(N{H_3})}_4}} \right]S{O_4} + 4{H_2}O \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (Tetramin e\;zinc\;sulphate) \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad (soluble)\;colourless \\\end{aligned} \end{equation} $$

Nickel salt first give a green precipitate which dissolves in excess of $NH_4OH$$$\begin{equation} \begin{aligned}
NiC{l_2} + 2N{H_4}OH \to Ni{(OH)_2} + 2N{H_4}Cl \\
Ni{(OH)_2} + 2N{H_4}Cl + 4N{H_4}OH \to \left[ {Ni{{(N{H_3})}_6}} \right]C{l_2} + 6{H_2}O \\\end{aligned} \end{equation} $$

It forms a grey precipitate with mercurous chloride.$$H{g_2}C{l_2} + 2N{H_4}OH \to \underbrace {Hg + HgN{H_2}Cl}_{Grey} + N{H_4}Cl + {H_2}O$$

(vii) Reaction with Nessler's reagent: A reedish brown ppt. is formed.$$\begin{equation} \begin{aligned} 2KI + HgC{l_2} \to Hg{I_2} + 2KCl \\ 2KI + HgC{l_2} \to {K_2}Hg{I_4} \\\end{aligned} \end{equation} $$

Alkaline solution of ${K_2}Hg{I_4}$ is called Nessler's reagent.

This gives brown ppt. with $NH_3$ called iodide of Million's base.$$\begin{equation} \begin{aligned} {K_2}Hg{I_4} + N{H_3} + 3KOH \to {H_2}NHgOHgI + 7KI + 2{H_2}O \\ \quad \quad \quad \quad\quad \quad \quad \quad \quad \quad \quad \quad Brown\;ppt. \\\end{aligned} \end{equation} $$

Uses:

(i) Liquid hydrogen is not safe to transport in cylinders. Ammonia can be easily liquefied and transported safely in cylinders. Ammonia can be decomposed into hydrogen and nitrogen by passing over a heated metallic catalyst. Thus, ammonia is the source for the production of hydrogen ay any destination.

(ii) Ammonia is also used in the manufacture of urea which is an excellent fertilizer of nitrogen.