Nitric acid : manufacture

Nitric acid

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Nitric Acid:

Manufacture :

It is manufactured by two methods :

1. Birkeland and Eyde process or arc process :

Principle :

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

Process :

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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 150^o 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 50^oC.

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 :

Principle :

NH₃ is oxidized into NO by air at 8oo ^oC in presence of Pt (Platinum)catalyst.

Method :

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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.