properties of ammonia and nitric acid

Ammonia (NH3) is formed by decomposing nitrogenous organic matter, like urea. It is present in small quantities in air and soil, and it is also a common nitrogenous waste among aquatic organisms. Ammonia serves as a precursor to 45% of the world’s food, thus contributing significantly to the nutritional needs of terrestrial organisms. Nitric acid (HNO3) is an oxoacid of nitrogen. It is also known as aqua fortis in Latin, meaning ‘strong water’. It is a type of mineral acid.

Ammonia

Ammonia is a compound of nitrogen and hydrogen, i.e., a single nitrogen atom is covalently bonded to 3 hydrogen atoms. It has the formula NH3. It is one of the most abundant hydrides found in our atmosphere. Its IUPAC name is Azane, and it has no colour and a distinct pungent smell.

At the apex, an ammonia molecule shows a trigonal pyramidal shape along with a nitrogen atom. It has one single electron pair and three bond pairs.

Preparation of ammonia

• By the reaction of ammoniated salts with a strong base

NH4Cl + NaOH → NH3 + NaCl + H2O

NH4Cl + KOH → NH3 + KCl + H2O

• By hydrolysis of urea

NH2-CO-NH2 + H2O → 2NH3 + CO2

• Haber’s process

• The raw materials used here are nitrogen and hydrogen.
• Scrubbing removes impurities from the gases.
• The amalgamated gases are passed through a converter at a temperature of 450°C and 200atm pressure.
• Nitrogen reacts with hydrogen to form ammonia.
• Catalysts such as iron (Fe) and molybdenum (Mo) are used.

N2 + 3H2 → 2NH3

Physical properties of ammonia

• Ammonia has a molar mass of 17.031 g/mol.
• Ammonia solutions appear clear and colourless.
• It is lighter than air.
• It is corrosive to metals and tissues.
• It has a sharp, pungent smell.
• It has a boiling point of -33.34 °C.
• Its melting point is −77.73 °C.
• Its density is 0.769 kg/m3 at Standard Temperature and Pressure (STP).
• It is trigonal pyramidal in shape.

Chemical properties of ammonia

• Its aqueous solution is weakly basic due to a lone pair of electrons.
• It works as the Lewis base.
• Reaction with halogens:

1. Ammonia on reaction with excess chlorine forms nitrogen trichloride and hydrochloric acid.

NH3 + 3Cl2 (excess) → NCl3 + 3HCl

1. Excess ammonia on reaction with chlorine forms ammonia chloride and liberates nitrogen gas

8NH3 (excess) + 3Cl2 → 6NH4Cl + N2

• When ammonia is heated and decomposed, it emits nitrogen oxides and toxic fumes.
• The pH of the 1N aqueous solution of ammonia is 11.6.

Absorption, excretion and distribution of ammonia in the human body

• Ammonia can be absorbed by inhalation. The inhaled ammonia gets collected in the upper respiratory tract and is subsequently exhaled out.
• The ammonia produced by the intestinal tract is also absorbed. It is converted to urea and glutamine by the liver.
• Excretion of urea is mainly through urinary urea, followed by faeces and exhalation.

Uses

• Ammonia is used to produce various nitrogenous fertilisers (ammonium nitrate, ammonium phosphate, urea).
• It is used in the manufacturing of inorganic nitrogen compounds like nitric acid.
• Liquid ammonia can be utilised as a refrigerant.

Nitric acid

Nitric acid (HNO3) is also known as aqua fortis, Latin for ‘strong water.’ It is bonded to one -OH group and two oxygen atoms. It is a mineral acid, and it is highly corrosive. It is pale yellow or reddish-brown in colour and has a suffocating odour. If the nitric acid concentration in an aqueous solution is more than 86%, then it is called red fuming nitric acid. If the concentration is above 95%, it is referred to as white fuming acid.

Manufacturing of nitric acid

On a large scale, it is mainly prepared by Ostwald’s process. It is based on catalytic oxidation of NH3 by oxygen and involves the following steps:

• Formation of NO in the presence of a catalyst

4NH3 + 5O2 → 4NO + 6H20

• Formation of NO2

2NO + O2 → 2NO2

• Dissolving in water to form nitric acid

3NO2 + H2O → 2HNO3 + NO

Laboratory synthesis of nitric acid involves the thermal decomposition of copper (II) nitrate. Nitrogen dioxide and oxygen are formed, which in reaction with water, give nitric acid.

Another alternative method is reacting a nitrate salt (such as sodium nitrate) with sulphuric acid (H2SO4):

NaNO3 + H2SO4 → HNO3 + NaHSO4

Physical properties of nitric acid

• The molar mass of nitric acid is 63.012 g/mol.
• It is colourless but turns yellow on decomposition.
• It is a very corrosive acid.
• It is toxic. Prolonged exposure may lead to adverse health effects.
• Its boiling point is 355.6K.
• It has a freezing point of 231.4K.
• Its density is 1.51 g cm−3.
• It has a planar structure and shows resonance.

Chemical properties of nitric acid

• On decomposition, nitric acid gives nitrogen dioxide.
• It is one of the best oxidising agents.
• It is monoprotic.
• Only concentrated nitric acid reacts with non-metals; dilute does not.

Oxidation of non-metals by nitric acid

1. C + HNO3 → CO2 + NO2
2. S + HNO3 → H2SO4 + NO2
3. P4 + HNO3 → H3PO4 + NO2

Oxidation of metals by nitric acid

• Very dilute nitric acid

Out of all metals, only magnesium and manganese react with very dilute nitric acid. They liberate hydrogen gas and metal nitrates.

Mg + 2HNO3 → Mg(NO3)2 + H2

Mn + 2HNO3 → Mn(NO3)2 + H2

• Cold dilute nitric acid

Metals like magnesium, zinc and iron react with cold dilute nitric acid to liberate ammonium nitrate and metal nitrate.

4Mg + 10HNO3 → 4Mg(NO3)2 + 3H2O + NH4NO3

4Zn + 10HNO3 → 4Zn(NO3)2 + 3H2O + NH4NO3

4Fe + 10HNO3 → 4Fe(NO3)2 + 3H2O + NH4NO3

• Hot dilute nitric acid

When metals react with hot dilute nitric acid, nitrous oxide (N2O) is formed.

4Mg + 10HNO3 → 4Mg(NO3)2 + 5H2O + N2O

4Zn + 10HNO3 → 4Zn(NO3)2 + 5H2O + N2O

4Fe + 10HNO3 → 4Fe(NO3)2 + 5H2O + N2O

• Concentrated nitric acid

In reaction with concentrated nitric acid, metals give off NO2 gas.

Zn + 4HNO3 → 4Zn(NO3)2 + 2H2O + NO2

Mg + 4HNO3 → 4Mg(NO3)2 + 2H2O + NO2

Uses

• Nitric acid is essential for the manufacturing of fertilisers.
• Nitric acid is used for making various organic nitrogen compounds, like derivatives of aniline.
• It is also used in woodwork.
• It is commonly used for cleaning and dairy equipment.
• It helps pick stainless steel and clean silicon wafers in electronics.
• Aqua regia is formed when concentrated hydrochloric acid and nitric acid are mixed in the ratio of 3:1.

Conclusion

Ammonia and nitric acid are the two major compounds of nitrogen. On a large scale, ammonia is produced by Haber’s process. Ammonia acts as a Lewis base due to the presence of a lone pair of electrons. Ostwald’s process is used to manufacture nitric acid. Nitric acid is an excellent oxidising agent, which is monoprotic and corrosive. It is toxic if inhaled.