source : tutorvista.com
It is manufactured by two methods :
1. Birkeland and Eyde process or arc process :
N2 and O2 is present in the air are used for the manufacture of nitric acid.
N2 + O2 ——-> 2NO – 43.2 K.Calorie
Formation of NO is a reversible reaction . According to Le chatelier’s principle, high temperature is required to get more NO because reaction is endothermic. There is no effect of pressure on the reaction because there is no change in volume. The temperature for the reaction is 3000 oC. NO is oxidized to NO2 (nitrogen dioxide) which on dissolving in water gives HNO3.
2NO + O2 ——-> 2NO2
3NO2 + H2O —–> 2HNO3 + NO
source : sardanatutorials.com
CO2 and dust free air is heated in an arc furnace at 3000 o C. Nitric oxide is formed. Gases coming out from the furnace are cooled at 1000 oC by passing through cooler then gases are passed in a boiler and cooled at 150o C. Cold water is circulated around the boiler to cool the gases. Gaseous mixture is now passed through aluminum tubes. The temperature of gaseous mixture becomes 50oC.
Gaseous mixture is passed in oxidizing tower where NO is oxidized into NO2.
2NO + O2 ——–> 2NO2
NO2 is absorbed in water in absorption tower then dilute HNO3 is obtained.
3NO2 + H2O ——-> 2 HNO3 + NO
Now HNO3 is not manufactured by this method because consumption of electricity is very high to maintain the temperature 3000 o C. Even at this temperature 1% NO is form.
Ostwald’s method :
NH3 is oxidized into NO by air at 8oo oC in presence of Pt (Platinum)catalyst.
source : edurite.com
Mixture of NH3 and O2 in the ratio of 1:9 is passed through catalyst chamber containing Pt- gauge. Temperature of the catalyst is kept at 800 oC. About 95 % NH3 is converted into NO.
4NH3 + 5 O2 ——–> 4NO + 6 H2O + 21.6 K.Calorie
Heat liberated maintains the temperature of the catalyst. The gases coming out are cooled to about 50 oC and mixed with more O2. This gaseous mixture is passed in oxidizing tower where NO is oxidized to NO2.
2NO + O2 ——> 2NO2
NO2 is passed in absorption tower. The dilute solution of HNO3 is obtained.
3NO2 + H2O ——> 2HNO3 + NO
Question : HNO3 is usually yellow in color . why?
Solution : It is due to the dissolved NO2 present in HNO3.
Question: What is fuming HNO3?
Solution : Concentrated nitric acid containing dissolved oxide of nitrogen like NO2 and N2O3 is called fuming HNO3. It gives out brown fumes of NO2. It is light yellow fuming liquid and is a strong oxidizing agent.