Page 253 - Chemistry ICSE Class X

Study of Compounds–Nitric Acid 239
>200°C
4HNO (l) o 4NO (g) + O (g) + 2H O(g)
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3
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nitric acid nitrogen dioxide oxygen
y the residue (K SO ) left in the retort would form a hard crust
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which sticks to the glass. Its removal becomes a problem.
ƒ Conc. hydrochloric acid [HCl(aq)] should not be used in place of conc.
sulphuric acid (H SO ). This is because HCl is volatile and therefore
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the HNO vapour will also contain HCl vapour.
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How is nitric acid obtained on industrial scale
On industrial (or commercial) scale, nitric acid can be obtained by Ostwald’s
process described below.
In this process, nitric acid is produced by the oxidation of ammonia.
According to the Le-Chatelier’s principle, the favourable conditions
for this reaction are,
ƒ Temperature. 750–900°C (800°C is the optimum value)
ƒ Pressure. 6–7 atm
ƒ Ammonia : Air ratio. 1 : 8 (by volume)
ƒ Catalyst. Pt (90%) + Rh (10%)
About 90–95% yield is achieved in this reaction.

Description of the plant
The plant used in the process is shown in Fig. 10.2 and
consists of the following parts:
ƒ Catalytic converter. %CVCN[VKE EQPXGTVGT KU ſVVGF YKVJ
a platinum-rhodium gauze. The gauze is initially
heated to 800°C electrically. As the oxidation of
ammonia is an exothermic reaction, therefore, no
external heating is required subsequently.
A mixture of ammonia and clean air (volume ratio
1 : 8) is passed through the gauze. Here, ammonia gets Fig. 10.2 The Ostwald’s process for the manufacture
oxidised to nitric oxide (NO). of nitric acid
Pt–Rh
4NH (g) + 5O (g) o 4NO(g) + 6H O(g)
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2
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800°C
ƒ Oxidation tower. The nitric oxide containing nitrogen and some
water vapour coming out of the converter are cooled to about 100°C
by passing through coolers. In oxidation tower, nitric oxide is mixed
with more air and it gets converted to NO .
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2NO(g) + O (g) o 2NO (g)
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nitric oxide from air nitrogen dioxide
ƒ Absorption tower. Nitrogen dioxide is allowed to enter the absorption
tower from the lower end. Water is sprinkled from the top of the tower ǀĞƌLJ ϭϬϬϬ Đŵ ĂƌĞĂ ŽĨ ƚŚĞ ŐĂƵnjĞ
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packed with quartz pieces. Here, NO is absorbed into water in the ƉƌŽĚƵĐĞƐ ĂďŽƵƚ ϱϬϬ ŬŐ ŽĨ ŶŝƚƌŝĐ ŽdžŝĚĞ
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presence of air to yield nitric acid. ĞǀĞƌLJ Ϯϰ ŚŽƵƌƐ͘
4NO (g) + 2H O(l) + O (g) o 4HNO (aq)
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2
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nitrogen dioxide from air nitric acid
How to increase the concentration of nitric acid ŽŵŵĞƌĐŝĂů ŶŝƚƌŝĐ ĂĐŝĚ ŚĂƐ Ă ďƌŽǁŶ
The aqueous nitric acid obtained by this method can be concentrated by ĐŽůŽƵƌ ĚƵĞ ƚŽ ƚŚĞ ĚŝƐƐŽůǀĞĚ EK ͘
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ŽůŽƵƌůĞƐƐ ĂĐŝĚ ĐĂŶ ďĞ ŽďƚĂŝŶĞĚ
distillation to ~ 68.5% (by mass). Further concentration to 98% acid (called ĨƌŽŵ ƚŚĞ ďƌŽǁŶŝƐŚ ĂĐŝĚ ďLJ ďƵďďůŝŶŐ
fuming nitric acid) can be achieved by dehydration with concentrated ĚƌLJ Ăŝƌ ƚŚƌŽƵŐŚ ǁĂƌŵ ĐŽŵŵĞƌĐŝĂů
sulphuric acid. ŶŝƚƌŝĐ ĂĐŝĚ͘ dŚŝƐ ƉƌŽĐĞĚƵƌĞ ĚƌŝǀĞƐ
Anhydrous nitric acid can be obtained by distillation of concentrated ĂǁĂLJ ƚŚĞ ĚŝƐƐŽůǀĞĚ ŶŝƚƌŽŐĞŶ ĚŝŽdžŝĚĞ
aqueous nitric acid with phosphorus pentoxide (P O or P O ). ĂŶĚ ƚŚĞ ĂĐŝĚ ďĞĐŽŵĞƐ ůŝŐŚƚ ŝŶ ĐŽůŽƵƌ͘
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