🧪 Nitric Acid (HNO₃) – Full Breakdown
1) Preparation of Nitric Acid
a) Laboratory Method
Reactants: KNO₃ (or NaNO₃) + conc. H₂SO₄
| Lab Preparation of HNO₃ |
|---|
| KNO₃ + H₂SO₄(conc.) → KHSO₄ + HNO₃↑ |
| NaNO₃ + H₂SO₄(conc.) → NaHSO₄ + HNO₃↑ |
| NOTES: Heat the mixture gently (below 200°C). HNO₃ vapours are condensed in a water-cooled receiver. The flask is made of glass — HNO₃ attacks rubber/cork. KHSO₄ or NaHSO₄ remains in the flask. |
b) Industrial Method — Ostwald Process
| Ostwald Process for HNO₃ (Step-wise) |
|---|
| Step 1 (Catalytic Oxidation): 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt/Rh catalyst, 800°C) |
| Step 2 (Oxidation): 2NO + O₂ → 2NO₂ (brown fumes) |
| Step 3 (Absorption): 3NO₂ + H₂O → 2HNO₃ + NO (NO is recycled back) |
| NOTES: Pt-Rh gauze catalyst at ~800°C. NH₃ must be pure — impurities poison the catalyst. The NO produced in Step 3 is recycled to Step 2. |
2) Physical Properties of HNO₃
- Pure HNO₃: Colourless liquid (but often yellowish-brown due to dissolved NO₂)
- Odour: Pungent, choking smell
- Density: 1.52 g/cm³ (heavier than water)
- Boiling point: 86°C (low — evaporates easily)
- Miscibility: Completely miscible with water (with evolution of heat — exothermic)
- Fuming HNO₃: Concentrated HNO₃ with dissolved NO₂ — appears yellow to reddish-brown. Also called fuming nitric acid.
3) Chemical Properties of HNO₃
a) Acidic Nature (General Acid Reactions)
| HNO₃ as an Acid |
|---|
| With bases: HNO₃ + NaOH → NaNO₃ + H₂O |
| With metal oxides: 2HNO₃ + CuO → Cu(NO₃)₂ + H₂O |
| With carbonates: 2HNO₃ + Na₂CO₃ → 2NaNO₃ + H₂O + CO₂↑ |
| With NH₃: HNO₃ + NH₃ → NH₄NO₃ (ammonium nitrate — fertiliser) |
| NOTES: These are all standard acid reactions. HNO₃ is a strong acid — completely dissociates in water. |
b) Decomposition on Heating
| Thermal Decomposition of HNO₃ |
|---|
| 4HNO₃ → 4NO₂↑ + O₂↑ + 2H₂O (brown fumes of NO₂) |
| NOTES: This decomposition is accelerated by light and heat. The brown fumes (NO₂) dissolve in the remaining HNO₃ giving it a yellow colour. |
c) Oxidising Agent (THE most important property)
HNO₃ is a powerful oxidising agent. This is the biggest topic in HNO₃ for ICSE. It oxidises non-metals and metals, getting reduced itself.
i) Action on Metals
Rule of thumb: Conc. HNO₃ usually gives NO₂. Dilute HNO₃ usually gives NO. Very dilute HNO₃ can give N₂O, NH₃, etc.
| HNO₃ + Metals — Oxidation-Reduction |
|---|
| Cu + conc. HNO₃: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂↑ + 2H₂O (brown fumes, blue solution) |
| Cu + dil. HNO₃: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO↑ + 4H₂O (colourless NO, turns brown in air) |
| Zn + conc. HNO₃: Zn + 4HNO₃ → Zn(NO₃)₂ + 2NO₂↑ + 2H₂O |
| Zn + dil. HNO₃: 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + N₂O↑ + 5H₂O |
| NOTES: H₂ is NEVER produced when HNO₃ reacts with metals — because any H₂ formed would immediately be oxidised to H₂O by the strong oxidising HNO₃. |
"Concentrated = NO₂ (2 oxygens — think 'Concentrated', more oxygen around)",
"Dilute = NO (fewer oxygens)".
Copper is the most commonly asked metal for HNO₃ reactions.
ii) Action on Non-Metals
| HNO₃ + Non-metals — Oxidation |
|---|
| Sulphur: S + 6HNO₃(conc.) → H₂SO₄ + 6NO₂↑ + 2H₂O |
| Carbon: C + 4HNO₃(conc.) → CO₂↑ + 4NO₂↑ + 2H₂O |
| Phosphorus: P₄ + 20HNO₃(conc.) → 4H₃PO₄ + 20NO₂↑ + 4H₂O |
| Iodine: I₂ + 10HNO₃(conc.) → 2HIO₃ + 10NO₂↑ + 4H₂O |
| NOTES: In all cases, HNO₃ is reduced to NO₂ (brown fumes). The non-metal is oxidised to its highest oxyacid. |
d) Passivity of Iron and Aluminium
Conc. HNO₃ makes Fe and Al passive — it forms a thin, invisible, protective oxide layer on the metal surface that prevents further reaction. This is why conc. HNO₃ is stored in iron/aluminium containers (but not in copper containers!).
| Passivity of Fe/Al with Conc. HNO₃ |
|---|
| Fe + conc. HNO₃: Forms protective Fe₂O₃/Fe₃O₄ layer → no further reaction |
| Al + conc. HNO₃: Forms protective Al₂O₃ layer → no further reaction |
| NOTES: This is not a chemical reaction — it's a physical passivation. Once passivated, the metal does NOT react with dil. HNO₃ or dil. H₂SO₄ either (until the layer is removed). |
e) Nitration Reactions
| Nitration — Introduction of -NO₂ group |
|---|
| Benzene → Nitrobenzene: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (conc. H₂SO₄ catalyst) |
| Toluene → TNT: C₆H₅CH₃ + 3HNO₃ → C₆H₂(NO₂)₃CH₃ + 3H₂O (trinitrotoluene — explosive) |
| NOTES: Conc. H₂SO₄ acts as a dehydrating agent — removes water and prevents dilution of HNO₃. |
4) Brown Ring Test (Test for Nitrate Ion)
| Brown Ring Test for NO₃⁻ |
|---|
| Add FeSO₄ solution to the test solution, then slowly add conc. H₂SO₄ along the test tube wall |
| A brown ring forms at the junction of the two liquids |
| NOTES: The brown colour is due to the formation of [Fe(H₂O)₅NO]²⁺ — a complex of NO with Fe²⁺. Reaction: NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O (then Fe²⁺ + NO → brown complex). |
5) Uses of Nitric Acid
- Fertilisers: NH₄NO₃, Ca(NO₃)₂
- Explosives: TNT, nitroglycerine, dynamite
- Aqua regia: 3 parts conc. HCl + 1 part conc. HNO₃ — dissolves gold and platinum
- Nitration: In manufacturing of dyes, drugs, perfumes (nitrobenzene → aniline → dyes)
- Purification of Ag and Au: Impurities dissolve in HNO₃, leaving pure metal
- Rocket propellant: As an oxidiser
6) Top ICSE Reasoning Questions
Q1: Why is conc. HNO₃ stored in brown glass bottles?
Answer: HNO₃ undergoes photochemical decomposition in sunlight: 4HNO₃ → 4NO₂ + O₂ + 2H₂O. Brown glass blocks UV light and slows this decomposition.
Q2: Why does conc. HNO₃ appear yellow?
Answer: Due to dissolved NO₂ gas (brown fumes). NO₂ forms from the decomposition of HNO₃: 4HNO₃ → 4NO₂ + O₂ + 2H₂O. The NO₂ dissolves in the acid giving it a yellow colour.
Q3: Why is H₂ gas not evolved when metals react with HNO₃?
Answer: HNO₃ is a strong oxidising agent. Any H₂ gas that might form would immediately be oxidised to H₂O by HNO₃. So the products are nitrogen oxides (NO, NO₂, N₂O) instead of H₂.
Q4: Why is HNO₃ called a strong oxidising agent?
Answer: HNO₃ easily gives up oxygen atoms (it is reduced from +5 oxidation state of N to lower states like +4 in NO₂, +2 in NO, etc.). It can oxidise non-metals like S, C, P, I to their oxyacids. The HNO₃ itself gets reduced to NO₂ (with conc.) or NO (with dil.).
Q5: Compare reactions of Cu with conc. HNO₃ and dil. HNO₃.
Answer: Cu + conc. HNO₃ → Cu(NO₃)₂ + 2NO₂↑ + 2H₂O (brown fumes of NO₂, blue-green solution). Cu + dil. HNO₃ → 3Cu(NO₃)₂ + 2NO↑ + 4H₂O (colourless NO which turns brown on exposure to air). The difference is in the product — NO₂ vs NO — based on concentration.
Q6: Why is dil. H₂SO₄ not used in the lab preparation of HNO₃?
Answer: Dil. H₂SO₄ is not strong enough to displace HNO₃ from nitrates. Also, HNO₃ would dissolve in the water of dil. H₂SO₄ and would not come out as vapours. Conc. H₂SO₄ is a strong dehydrating agent that drives the reaction forward.
Q7: What happens when HNO₃ is heated with sulphur?
Answer: S + 6HNO₃(conc.) → H₂SO₄ + 6NO₂↑ + 2H₂O. Sulphur is oxidised to H₂SO₄, HNO₃ is reduced to NO₂ (brown fumes). This proves HNO₃ is an oxidising agent.
Q8: What is passivity? Which metals show passivity with conc. HNO₃?
Answer: Passivity is the formation of a thin protective oxide layer on the metal surface when treated with conc. HNO₃. Iron (Fe) and aluminium (Al) show passivity. This allows conc. HNO₃ to be stored in iron/aluminium containers.
Q9: Describe the brown ring test for nitrates. Why is conc. H₂SO₄ used?
Answer: FeSO₄ solution + test solution + conc. H₂SO₄ added slowly along test tube wall → brown ring at junction. Conc. H₂SO₄ is used because it supplies H⁺ ions (acidic medium) and absorbs water (dehydrating agent) — necessary conditions for the reduction of NO₃⁻ to NO.
Q10: In the Ostwald process, why is NH₃ purified before use?
Answer: Impurities in NH₃ would poison the Pt-Rh catalyst. The catalyst must be kept active for the oxidation of NH₃ to NO.
1. NO₂ vs NO: Conc. HNO₃ → NO₂ (brown). Dil. HNO₃ → NO (colourless, turns brown in air). This comes every year.
2. No H₂ with HNO₃: It's an oxidising agent, kills any H₂. MCQ gold.
3. Brown ring test: The complex is [Fe(H₂O)₅NO]²⁺ — just know it involves Fe²⁺, conc. H₂SO₄, and a brown ring.
4. Brown bottle: Blocks sunlight → prevents decomposition of HNO₃.
5. Aqua regia: 3HCl + HNO₃ → dissolves Au, Pt. The actual reaction: HNO₃ + 3HCl → NOCl + Cl₂ + 2H₂O — the Cl₂ is what dissolves gold.
6. Passivity: Fe and Al become passive with conc. HNO₃ — they're still reactive metals normally, but the oxide layer stops them.