Study of Compounds — Nitric Acid
Introduction
Nitric acid (HNO₃) is one of the most important mineral acids, known for its powerful oxidising properties. In ICSE Class 10 Chemistry, you study its industrial manufacture by Ostwald's process, its characteristic oxidising behaviour, and the brown ring test for identification.
Ostwald's Process (Manufacture of HNO₃)
Principle: Catalytic oxidation of ammonia.
Steps
Step 1: Catalytic oxidation of ammonia:
4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) + Heat
Conditions: Pt-Rh catalyst (platinum gauze), ~800°C.
Step 2: NO is oxidised to NO₂:
2NO(g) + O₂(g) → 2NO₂(g)
Step 3: NO₂ is absorbed in water:
3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)
The NO formed is recycled.
Concentration
Dilute HNO₃ (50−60%) is concentrated by distillation with concentrated H₂SO₄ (which absorbs water).
Physical Properties
| Property | Observation |
|---|---|
| Colour | Colourless liquid (turns yellow on storage due to dissolved NO₂) |
| Odour | Pungent, suffocating |
| Density | 1.52 g/mL (for concentrated acid) |
| Solubility | Completely miscible with water |
| Boiling point | 86°C (forms an azeotrope with water at 68% concentration) |
| Fuming nature | Concentrated HNO₃ 'fumes' in moist air |
Oxidising Property
Nitric acid is a strong oxidising agent because it readily decomposes to give nascent oxygen:
2HNO₃ → 2NO₂ + H₂O + [O]
Reactions Showing Oxidising Nature
With non-metals:
C + 4HNO₃ (conc.) → CO₂ + 4NO₂ + 2H₂O S + 6HNO₃ (conc.) → H₂SO₄ + 6NO₂ + 2H₂O P₄ + 20HNO₃ (conc.) → 4H₃PO₄ + 20NO₂ + 4H₂O
With metals (except Au, Pt): Most metals are oxidised by HNO₃. The products depend on concentration.
| Concentration | Reaction example | Product of nitrogen |
|---|---|---|
| Very dilute (with reactive metals) | 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O | NH₄⁺ |
| Dilute | 3Zn + 8HNO₃ → 3Zn(NO₃)₂ + 2NO + 4H₂O | NO |
| Concentrated | Zn + 4HNO₃ → Zn(NO₃)₂ + 2NO₂ + 2H₂O | NO₂ |
Note: HNO₃ reacts with metals to produce H₂O and nitrogen oxides — NOT hydrogen gas (except with Mg and Mn in very dilute form).
Brown Ring Test
Test for nitrate ion (NO₃⁻):
- Add freshly prepared FeSO₄ solution to the sample.
- Carefully add concentrated H₂SO₄ along the side of the test tube (do not mix).
- A brown ring forms at the junction of the two liquids.
Chemistry:
NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O Fe²⁺ + NO → [Fe(H₂O)₅(NO)]²⁺ (brown ring complex)
Uses of Nitric Acid
- Manufacture of fertilisers (ammonium nitrate).
- Production of explosives (TNT, nitroglycerine).
- Manufacture of dyes, drugs, and paints.
- Pickling of stainless steel.
- As a laboratory reagent.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Thinking HNO₃ produces H₂ with metals | HNO₃ is oxidising — produces NO/NO₂/NH₄⁺, not H₂ |
| Confusing Ostwald's and Haber's processes | Ostwald: NH₃ → HNO₃; Haber: N₂ + H₂ → NH₃ |
| Forgetting the brown ring test uses FeSO₄ | FeSO₄ + conc. H₂SO₄ are essential for the test |
| Writing wrong products for metal-HNO₃ reactions | Product depends on concentration and reactivity of metal |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: Ostwald's process, oxidising reactions (with C, S, P), brown ring test, reactions with metals.
Marks Blueprint: Ostwald's process — 2 marks, Oxidising property — 2 marks, Brown ring test — 1 mark.
Self-Test Questions
-
Describe Ostwald's process for the manufacture of HNO₃. Write all the equations.
-
Why is HNO₃ stored in brown bottles?
-
Write equations for the reaction of concentrated HNO₃ with (a) carbon, (b) sulphur, (c) zinc.
-
Describe the brown ring test for nitrate ions.
-
Why does HNO₃ not produce hydrogen gas when reacting with most metals?
-
State three important uses of nitric acid.
