a) Write down the principle and procedure with self explanatory diagram for the manufacture of nitric acid by catalytic oxidation of ammonia. How does Conc. HNO 3 react with : [3+3] i) KI ii) Zn b)…

4NH₃+5O₂Pt-Rh gauge 900 K4NO+6H₂O

2NO+O₂Cooling Oxidation2NO₂

3NO₂+H₂OAbsorption in water Ostwald process2HNO₃+NO

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a) Principle and Procedure for the Manufacture of Nitric Acid by Catalytic Oxidation of Ammonia

Principle

The industrial manufacture of nitric acid by the Ostwald process involves the catalytic oxidation of ammonia with atmospheric oxygen over a platinum-rhodium (Pt-Rh) catalyst at high temperatures (~900 K). The process can be summarized in three main steps:

Nitric acidNitric acid

1. Oxidation of ammonia (NH₃) to nitric oxide (NO): $$4\mathrm{NH}{}_3+5\mathrm{O}{}_2\underset{\text{900 K}}{\overset{\text{Pt-Rh gauge}}{\to }}4\mathrm{NO}+6\mathrm{H}{}_2\mathrm{O}$$

2. Oxidation of nitric oxide (NO) to nitrogen dioxide (NO₂): $$2\mathrm{NO}+\mathrm{O}{}_2\to 2\mathrm{NO}{}_2$$

3. Absorption of NO₂ in water to form nitric acid (HNO₃): $$3\mathrm{NO}{}_2+\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{HNO}{}_3+\mathrm{NO}$$ The NO produced is recycled back into the process.

Procedure

1. Purification of Ammonia: Ammonia gas is purified to remove impurities like phosphine (PH₃) and arsine (AsH₃), which poison the catalyst.

2. Catalytic Oxidation: The purified ammonia is mixed with air (containing oxygen) and passed over a Pt-Rh catalyst at 900 K. This converts ammonia into nitric oxide (NO) and water.

3. Oxidation of NO to NO₂: The NO produced is cooled and further oxidized to nitrogen dioxide (NO₂) by atmospheric oxygen.

4. Absorption in Water: The NO₂ gas is dissolved in water under pressure to form dilute nitric acid (68% HNO₃). The reaction also regenerates NO, which is recycled.

5. Concentration of Nitric Acid: The dilute nitric acid is concentrated by distillation to obtain concentrated nitric acid (98%).

Self-Explanatory Diagram

The three key reactions are illustrated above. The process is continuous, with NO being recycled to maximize efficiency.

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Reactions of Concentrated Nitric Acid (HNO₃)

i) Reaction with Potassium Iodide (KI)

Concentrated nitric acid is a strong oxidizing agent. When it reacts with potassium iodide (KI), it oxidizes iodide ions (I⁻) to iodine (I₂), which appears as a brown vapor or purple solution in excess KI.

$$8\mathrm{HNO}{}_3+6\mathrm{KI}\to 8\mathrm{NO}{}_2+3\mathrm{I}{}_2+2\mathrm{K}{}_2\mathrm{O}+4\mathrm{H}{}_2\mathrm{O}$$ Observation: Brown fumes of NO₂ and liberation of iodine (purple coloration in excess KI).

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ii) Reaction with Zinc (Zn)

Concentrated nitric acid reacts with zinc to produce nitrogen dioxide (NO₂), which appears as reddish-brown fumes. The reaction is highly exothermic.

$$4\mathrm{Zn}+10\mathrm{HNO}{}_3\to 4\mathrm{Zn}(\mathrm{NO}{}_3){}_2+\mathrm{NH}{}_4\mathrm{NO}{}_3+3\mathrm{H}{}_2\mathrm{O}$$ Note: Unlike dilute nitric acid (which produces NO), concentrated nitric acid oxidizes zinc further to produce NO₂ instead of NO.

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b) Explanation of Observations

i) Why is Concentrated Nitric Acid Stored in Dark Bottles?

Concentrated nitric acid decomposes in sunlight due to its photochemical instability. When exposed to light, it undergoes self-oxidation-reduction, producing nitrogen dioxide (NO₂), which is a brown gas and a pollutant. This decomposition weakens the acid and reduces its purity.

$$4\mathrm{HNO}{}_3\stackrel{\text{Light}}{\to }4\mathrm{NO}{}_2+2\mathrm{H}{}_2\mathrm{O}+\mathrm{O}{}_2$$ Preventive Measure: Storing in dark brown bottles prevents light-induced decomposition.

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ii) Why Does Iron Become Passive with Concentrated Nitric Acid?

When iron (Fe) reacts with dilute nitric acid, it dissolves to form iron(II) nitrate (Fe(NO₃)₂) and nitric oxide (NO). However, with concentrated nitric acid, a thin, impermeable layer of iron(III) oxide (Fe₂O₃) forms on the surface of the iron. This layer prevents further reaction, making the iron passive (unreactive).

Reaction: $$\mathrm{Fe}+4\mathrm{HNO}{}_3(\mathrm{conc}\cdot )\to \mathrm{Fe}(\mathrm{NO}{}_3){}_3+\mathrm{NO}{}_2+2\mathrm{H}{}_2\mathrm{O}$$ The Fe(NO₃)₃ hydrolyzes to form Fe₂O₃, which acts as a protective coating.

Passivation: This phenomenon is called passivation, where the metal becomes resistant to further corrosion due to the formation of an oxide layer. This is why concentrated nitric acid does not react further with iron after initial contact.