Contents
Manufacture of nitric acid by Ostwald’s process
Principle :
1. Catalytic oxidation of NH3 to NO : When ammonia is oxidized with oxygen in presence of catalyst platinum and rhodium in 9 : 1 ratio at about 8000C and 5 atmospheric pressure, nitric oxide is formed.
2. Oxidation of NO to NO2 : Nitric oxide is cooled to about 500C to 1000C and air oxidized to nitrogen dioxide.
3. Oxidation of nitrogen dioxide(NO2) to nitric acid in presence of air and water.
Working of the plant or process :
The working of the plant involves following steps :
1. Catalyst converter : Ammonia gas and air in the ratio of 1:8 by volume is first compressed and passed through converter where ammonia is oxidized into nitric oxide in the presence of platinum as catalyst at temperature 8000C.
2. Cooler : The nitric oxide formed is cooled using refrigerator. The cooled nitric oxide is passed in oxidation chamber.
3. Oxidation chamber : In this chamber, nitric oxide combines with oxygen to form nitrogen dioxide.
4. Absorption tower : In absorption tower water is sprayed from its roof and tower is packed with acid proof(resistant) flint (stone). Here, nitrogen dioxide and oxygen {present in air} react with water to form nitric acid.
Thus obtained nitric acid is about 60% concentrated. On distillation, {nitric acid vaporize first, i.e. b.pt. 860C)} the concentration of nitric acid can be increased upto 68%. This is the commercial nitric acid available.
{68% nitric acid forms azeotropic mixture(constant boiling mixture) with water. }
Preparation of 98% nitric acid : For the preparation of conc. HNO3, it is treated with conc. H2SO4, which acts as dehydrating agent. So it absorbs water molecules strongly. The resulting mixture is distilled where nitric acid vaporizes first{ b.pt. 860C}. The vapour is condensed to form 98% nitric acid.{ This form of nitric acid is further crystallized at -420C which is separated and melted to get conc. HNO3.}
Fuming nitric acid : when some starch solution is kept in conc. (98%) HNO3, conc. HNO3 decomposes into nitrogen dioxide (NO2) forming fume. This form of nitric acid is fuming nitric acid. Fuming nitric acid is stronger oxidizing agent than conc. HNO3.
Physical properties of nitric acid :
1. It is colourless and odourless liquid. But on long storage, sometimes yellowish colour is seen due to slight decomposition into NO2 ( and O2).
2. It is soluble in water. { 68% nitric acid forms a constant boiling mixture which is termed as azeotropic mixture.}
3. Nitric acid is corrosive to skin.
4. Melting point of nitric acid is -420C and its boiling point is 860C.
Chemical Properties of nitric acid (HNO3)
1. Decomposition (dissociation) of nitric acid :
In presence of sunlight, nitric acid slightly decomposes and solution turns slightly to yellow or brown due to formation of NO2 (nitrogen dioxide) .
Concentrated nitric acid decomposes on heating. At higher temperature, the decomposition is rapid.
2. Acidic nature :
{It changes : blue litmus to red, phenolphthalein- colourless, methyl orange- pink}
- Nitric acid is monobasic acid. It completely dissociates in water:
- It reacts with basic hydroxides, oxides,{ carbonates, ammonia,} etc to give salt and water.
- Mg and Mn reacts with very dilute nitric acid to produce hydrogen gas.
3. Oxidizing property of nitric acid :
Nitric acid acts as strong oxidizing agent at it’s all concentration. The reactivity and products in chemical reaction involving acid depends upon the following factors:
i. Concentration of nitric acid
ii. Reactivity of metal
iii. Temperature
i. Concentration of acid : Higher the concentration of nitric acid, higher will be the oxidation state of product formed. These reduced products are formed in general :
| Concentration | Reduced product | Oxidation no. of N |
| Concentrated HNO3 | NO2 | +4 |
| Moderately conc. HNO3 | NO | +2 |
| Dilute HNO3 | N2O | +1 |
| Very dilute HNO3 | NH3 then NH4NO3 | -3 |
ii. Reactivity of metals : Metals which are more electropositive than hydrogen react with HNO3 to produce nascent hydrogen and metals which are less electropositive than hydrogen react with HNO3 to produce nascent oxygen.
Electrochemical series : The series of various elements obtained by arranging the elements in their increasing order of standard reduction potential in the vertical column is called electrochemical series.
Nascent hydrogen formation theory :
Nascent oxygen formation theory :
iii. Temperature : The increase in temperature of the reactant increases the rate of the reaction .
A. Oxidation of metals more electropositive than hydrogen :-
→ According to nascent hydrogen formation theory.
1. Action with zinc :
- With conc. HNO3 :
- With moderately conc (1:1) HNO3 :
- With dilute HNO3 :
- With very dilute HNO3 :
2. Action with magnesium :
- With conc. HNO3 :
- With moderately conc and dilute HNO3 :
- With very dilute HNO3 :
3. Action with iron :
- With hot and conc. HNO3 : When a highly conc. HNO3 is added on Fe, a protective layer of ferrosoferric oxide (Fe3O4) is formed and the iron becomes passive i.e. prevents the further action of acid on the iron metal.
- With moderately conc.(1:1) HNO3 : Fe gives ferric salt and NO2 gas.
- With very dilute HNO3 : Fe gives ferrous salt and NH4NO3
B. Oxidation of metals less electropositive than hydrogen :
→ According to nascent oxygen formation theory.
1. Action with copper :
- With conc. HNO3 :
- With moderately conc. or dilute HNO3 :
2. Action with mercury :
- With conc. HNO3 : Hg gives mercuric salt and NO2
- With moderately conc. or dilute HNO3 : Hg gives mercurous salt and NO.
C. Reaction of HNO3 with non-metals :
Dilute HNO3 has no action with non-metals and metalloids {eg. As, Sb, etc}. Conc. HNO3 oxidizes non-metals like carbon, sulphur, phosphorus, iodine, etc into their oxides or oxyacids and itself gets reduced to NO2.
1. With carbon : Carbon is oxidized to CO2.
2. With sulphur : sulphur is oxidized to sulphuric acid.
3. With phosphorus : Phosphorus is oxidized to phosphoric acid.
4. With iodine : Iodine is oxidized to iodic acid.
D. Reaction of HNO3 with inorganic compounds (reducing agents) like H2S, SO2, HI, HBr, FeSO4, etc.
1. Conc. HNO3 oxidizes H2S to S.
2. Conc. HNO3 oxidizes KI to I2 .
3. Conc. HNO3 oxidizes SO2 to H2SO4 .
Action with FeSO4 : ( Test for nitric acid or nitrate ion (NO3– ) :
Presence of nitric acid ( or nitrate ion ) in solution can be detected by ring test in wet test analysis. In laboratory, about 1 ml of sample solution is taken and its double volume of concentrated sulphuric acid is added (cautiously). The solution is cooled and few ml of freshly prepared ferrous sulphate solution is added from the side of test tube. Formation of brown ring between two layers of liquid indicates the presence of nitric acid ( or nitrate ).
Here, nitric acid (or nitrate ) reacts with conc. H2SO4 and ferrous sulphate to form ferric sulphate. Some ferrous sulphate molecules absorb nitric oxide from the solution and form nitrosyl ferrous sulphate which forms brown ring between two layers of liquid.
Chemical reaction of nitric acid with noble metals in presence of conc. HCl :
Noble metals like gold (Au), platinum (Pt), palladium (Pd), etc. neither react with conc. nitric acid nor with conc. hydrochloric acid. But these noble metals get dissolved in a mixture of conc. HNO3 and conc. HCl in 1 : 3 ratio by volume. This mixture is called aqua- regia { Aqua= water, regia = king,i.e. kingly water} .
Aqua- regia dissolves such metals due to formation of highly reactive nascent chlorine.
Eg. reaction of gold:
Structure of nitric acid (HNO3)
Uses of nitric acid :
- It is used in the manufacture of fertilizers, dyes and drugs.
- Used to manufacture explosive like TNT ( trinitro toluene),nitro glycerin etc.
- Used in the purification of gold and silver.
- Used as oxidizing and nitrating reagent in the laboratory.
References :
- Lee, j. D., Concise Inorganic Chemistry, Fifth Edition, Joh, Wiley and Sons, Inc., 2007.
- Sarkar, R., General and Inorganic Chemistry, Second Edition, New Central Book Agency(P) Ltd., India, 2007.
- Shriver, D. F., Atkins, P. W., Inorganic Chemistry, Fifth Edition, Oxford university Press, 2010.
- Agrawal, S. K., Lal, K., Advanced Inorganic Chemistry, Fifth Revised Edition, Pragati Prakashan, Meerut, 2001.
- Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, Fifth edition, John Wily and Sons, Singapore, 1995.
- https://www.tandfonline.com/doi/pdf/10.1080/00022470.1964.10468252
- https://doubtnut.com/question-answer-chemistry/draw-the-lewis-structure-of-nitric-acid-hno3-12973480
- https://en.wikipedia.org/wiki/Ostwald_process
