Answer the question on the basis of passage given below:
Nitrogen forms a large number of oxoacids such as \(H_2N_2O_2\) (hyponitrous acid), \(HNO_2\) (nitrous acid) and \(HNO_3\) (nitric acid). Amongst these, \(HNO_3\) is the most important. It is used in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives. It is also used to prepare many important organo-nitro compounds like nitroglycerine, trinitrotoluene etc. It acts as a strong oxidizing agent and used as an oxidiser in rocket fuels.
In the laboratory, \(HNO_3\) is made by the action of sulphuric acid on nitrate salts. However, on a commercial scale, it is prepared by Ostwald's process that involves following steps:
A. Dissolution of \(NO_2\) gas in water.
B. Conversion of nitric oxide to nitrogen dioxide.
C. Heterogenous catalysis in which \(NH_3\) is oxidised by atmospheric oxygen.
D. Distillation of nitric acid followed by dehydration with concentrated sulphuric acid.
Arrange the above steps in the correct order.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C, B, A and D
In Ostwald’s process nitric acid is made by the catalytic oxidation of ammonia by atmospheric oxygen. The steps follow one another in this order.
First (C), ammonia is oxidised by air over a platinum–rhodium gauze catalyst at about 500 K and 9 bar; the gases react on the solid catalyst, so this is heterogeneous catalysis:
\( \displaystyle \ce{4NH3(g) + 5O2(g)} \xrightarrow[\text{500 K, 9 bar}]{\text{Pt/Rh gauze}} \ce{4NO(g) + 6H2O(g)} \)
Next (B), the nitric oxide formed combines with oxygen to give nitrogen dioxide:
\( \ce{2NO(g) + O2(g) -> 2NO2(g)} \)
Then (A), nitrogen dioxide dissolves in water to give nitric acid; the NO formed is recycled:
\( \ce{3NO2(g) + H2O(l) -> 2HNO3(aq) + NO(g)} \)
Finally (D), the aqueous acid is concentrated by distillation to about 68% and then by dehydration with concentrated H2SO4 to 98%.
So the order is C, B, A and D.
Shortcut: ammonia must be oxidised before anything else happens, so the sequence starts with C; only Options (3) and (4) do, and \( \ce{NO2} \) (step A) cannot be dissolved before it is made (step B), which leaves Option (3).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Nitric Acid – “… In the laboratory, nitric acid is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 in a glass retort. \(\ce{NaNO3 + H2SO4 -> NaHSO4 + HNO3}\) On a large scale it is prepared mainly by Ostwald’s process. This method is based upon catalytic oxidation of NH3 by atmospheric oxygen. \(\ce{4NH3(g) + 5O2(g)} \xrightarrow[\text{500 K, 9 bar}]{\text{Pt/Rh gauge catalyst}} \ce{4NO(g) + 6H2O(g)}\) (from air) Nitric oxide thus formed combines with oxygen giving NO2. \(\ce{2NO(g) + O2(g) <=> 2NO2(g)}\) Nitrogen dioxide so formed, dissolves in water to give HNO3. \(\ce{3NO2(g) + H2O(l) -> 2HNO3(aq) + NO(g)}\) NO thus formed is recycled and the aqueous HNO3 can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4.”