Chemistry of Nitric Acid

1. Introduction

Nitric Acid (HNO₃) is a strong monobasic acid and a powerful oxidizing agent. It is also known as aqua fortis (strong water).

Appearance: Colourless liquid when pure; often yellowish due to dissolved NO₂
Molecular Formula: HNO₃
Molar Mass: 63.01 g/mol
Oxidation State of Nitrogen: +5

2. Preparation of Nitric Acid

a) Laboratory Method

NaNO₃ + H₂SO₄ (conc.) → NaHSO₄ + HNO₃ (on heating)

b) Industrial Method (Ostwald Process)

1. Oxidation of Ammonia:
   4NH₃ + 5O₂ → 4NO + 6H₂O (Pt-Rh catalyst, 800–900°C)
2. Oxidation of Nitric Oxide:
   2NO + O₂ → 2NO₂
3. Absorption in Water:
   4NO₂ + 2H₂O + O₂ → 4HNO₃

3. Physical Properties

• Colourless fuming liquid (pure)
• Boiling point: 83°C (181 °F)
• Melting point: -42 °C (-44 °F)
• Density: 1.51 g/cm³ (conc. HNO₃)
• Highly hygroscopic and corrosive
• Miscible with water in all proportions

4. Chemical Properties

a) Acidic Nature

HNO₃ ⇌ H⁺ + NO₃⁻ (Strong acid, completely ionized in water)

Reacts with bases to form nitrates:

NaOH + HNO₃ → NaNO₃ + H₂O

b) Oxidizing Properties

Nitric acid acts as a strong oxidizing agent due to high oxidation state of nitrogen (+5).

Reactions with Metals:

• With Copper (dil. HNO₃):
  3Cu + 8HNO₃ (dil.) → 3Cu(NO₃)₂ + 2NO + 4H₂O
• With Copper (conc. HNO₃):
  Cu + 4HNO₃ (conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O
• With Zinc (dil. HNO₃):
  4Zn + 10HNO₃ (very dil.) → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O

c) Oxidation of Non-Metals

C + 4HNO₃ (conc.) → CO₂ + 4NO₂ + 2H₂O

S + 6HNO₃ (conc.) → H₂SO₄ + 6NO₂ + 2H₂O

P₄ + 20HNO₃ (conc.) → 4H₃PO₄ + 20NO₂ + 4H₂O

d) Reaction with Organic Compounds

• Nitration: Used in the nitration of benzene, toluene, etc. to form nitro compounds.
• Oxidation of Alcohols and Aldehydes

5. Uses of Nitric Acid

• Manufacture of fertilizers (Ammonium nitrate, Calcium ammonium nitrate)
• Manufacture of explosives (TNT, Nitroglycerine, RDX)
• Pickling of stainless steel and etching of metals
• Preparation of aqua regia (3:1 HCl : HNO₃)
• In the manufacture of dyes, drugs, and plastics
• As a laboratory reagent

6. Structure of Nitric Acid

Nitric acid has a planar structure with nitrogen at the center, bonded to three oxygen atoms. One oxygen is attached with a hydrogen atom (–OH group). The oxygen-nitrogen-oxygen bond angles are approximately 120 degrees, and the N-O bond lengths are about 1.22 Å.

Key Point: Concentrated nitric acid (68–70%) is commonly used in laboratories. Fuming nitric acid contains dissolved NO₂ and is more oxidizing.

7. The Brown Ring Test

1. Procedure

1. Add a freshly prepared solution of Iron(II) sulfate (FeSO₄) to the substance being tested.
2. Carefully pour concentrated sulfuric acid (H₂SO₄) down the side of the test tube so that it forms a layer at the bottom.
3. A brown ring forms at the junction (interface) of the two layers.

2. Chemical Mechanism

The reaction occurs in two main steps. First, the nitrate ion is reduced to nitric oxide (NO) by the Fe²⁺ ions in an acidic medium:
NO₃^- + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O

Then, the nitric oxide reacts with the remaining Fe²⁺ ions (which are hydrated in the solution) to form a complex nitrosyl compound, which is the "brown ring":
[Fe(H₂O)₆]²⁺ + NO → [Fe(H₂O)₅(NO)]²⁺ + H₂O

3. Key Observations

The Complex: The brown color is specifically due to the formation of the pentaaquanitrosyliron(II) complex.

Freshness Matters: You must use freshly prepared FeSO₄. If it sits out, it reacts with oxygen in the air and oxidizes to Fe³⁺, which won't work for the test.

Don't Shake: The ring is quite fragile. If you shake the test tube, the heat generated by the acid-water mixing will decompose the complex, and the ring will disappear.