Nitrogen: Properties, Compounds, and Uses

TAKEAWAYS:

• Nitrogen is a diatomic gas (N₂) with unique triple bonds, low reactivity, and key roles in fertilizers and explosives.
• Ammonia (NH₃) is basic, soluble, and used in fertilizers (e.g., urea) and cleaning agents.
• Nitric acid (HNO₃) is a strong oxidizing acid used in explosives (TNT) and fertilizers (ammonium nitrate).
• The Haber process (N₂ + H₂ → NH₃) and Ostwald process (NH₃ → HNO₃) are critical industrial reactions.
• Nitrogen oxides (NOₓ) cause acid rain and photochemical smog, harming the environment.
• NEB exam focus: Stoichiometry of reactions, industrial processes, and environmental impacts of nitrogen compounds.

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### **1. Physical and Chemical Properties of Nitrogen**
Nitrogen is a **colorless, odorless gas** that makes up **78% of Earth’s atmosphere**. It exists as a **diatomic molecule (N₂)** with a **triple bond** between the two nitrogen atoms.

#### **Key Properties**

- **Boiling point**: −196°C (liquid nitrogen is used in cryogenics).
- **Low reactivity**: N₂ is **inert** at room temperature due to its strong triple bond.
- **Density**: Lighter than air (used in food packaging to prevent spoilage).

#### **Why is N₂ Unreactive?**
The **triple bond** (one σ and two π bonds) requires **a lot of energy** to break. This makes N₂ **resistant to reactions** unless under extreme conditions (e.g., lightning, industrial catalysts).

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### **2. Allotropes of Nitrogen**
Nitrogen has **two main allotropes**:
1. **Dinitrogen (N₂)** – Most common, stable form.
2. **Monatomic nitrogen (N)** – Highly reactive, found in **plasma** (e.g., lightning).

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### **3. Chemical Compounds of Nitrogen**
Nitrogen forms **many important compounds**, including:
- **Ammonia (NH₃)**
- **Nitric acid (HNO₃)**
- **Nitrogen oxides (NO, NO₂, N₂O)**
- **Nitrates (NO₃⁻)**

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### **4. Ammonia (NH₃)**
Ammonia is a **colorless gas** with a **pungent smell**. It is **highly soluble in water** and forms **ammonium hydroxide (NH₄OH)**.

#### **Preparation of Ammonia**
1. **Laboratory method** (using ammonium chloride and calcium hydroxide):

2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O

2. **Industrial method (Haber process)**:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) (ΔH = −92 kJ/mol, exothermic)

- **Conditions**: 450°C, 200 atm pressure, **iron catalyst**.
- **Why these conditions?**
  - High pressure → More NH₃ (Le Chatelier’s principle).
  - Moderate temperature → Balances yield and speed.
  - Catalyst → Speeds up reaction without being consumed.

#### **Properties of Ammonia**
- **Basic in nature** (turns red litmus blue).
- **Forms salts with acids** (e.g., NH₃ + HCl → NH₄Cl).
- **Used in**:
  - **Fertilizers** (urea, ammonium nitrate).
  - **Cleaning agents** (window cleaners).
  - **Refrigeration** (liquid ammonia).

#### **Solved Example: Calculating NH₃ Yield**
**Question**: If 10 moles of N₂ and 30 moles of H₂ react, what is the **maximum NH₃** that can be formed?
**Solution**:
1. **Balanced equation**: N₂ + 3H₂ → 2NH₃
2. **Limiting reagent**:
- 10 moles N₂ → Needs 30 moles H₂ (exactly available).
- **N₂ is limiting**.
3. **NH₃ produced**:
- 1 mole N₂ → 2 moles NH₃
- 10 moles N₂ → 20 moles NH₃.

**Answer**: **20 moles of NH₃** can be formed.

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### **5. Nitric Acid (HNO₃)**
Nitric acid is a **strong oxidizing acid** used in **fertilizers, explosives (TNT), and dyes**.

#### **Preparation (Ostwald Process)**
1. **Ammonia oxidation**:

4NH₃ + 5O₂ → 4NO + 6H₂O

2. **NO to NO₂**:

2NO + O₂ → 2NO₂

3. **NO₂ dissolved in water**:

3NO₂ + H₂O → 2HNO₃ + NO

- **Conditions**: Platinum catalyst, 900°C.

#### **Properties of Nitric Acid**
- **Colorless liquid** (turns yellow on exposure to light due to NO₂ formation).
- **Strong oxidizing agent** (can oxidize metals like Cu to NO₂).
- **Used in**:
  - **Fertilizers** (ammonium nitrate).
  - **Explosives** (TNT, nitroglycerin).
  - **Laboratory reagent** (for nitration reactions).

#### **Solved Example: Reaction with Copper**
**Question**: What happens when copper reacts with dilute nitric acid?
**Solution**:

3Cu + 8HNO₃ (dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O

- **Observation**: Copper dissolves, **brown NO gas** evolves.
- **Why NO?** Dilute HNO₃ reduces to NO (not NO₂).

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### **6. Nitrogen Oxides (NOₓ)**
Nitrogen oxides (**NO, NO₂, N₂O**) are **pollutants** formed by:
- **Combustion of fuels** (cars, factories).
- **Lightning** (N₂ + O₂ → NO).

#### **Effects of NOₓ**
| Oxide | Formula | Effect |
|-------|---------|--------|
| Nitric oxide | NO | Forms **smog** (reacts with O₂ → NO₂) |
| Nitrogen dioxide | NO₂ | Causes **acid rain** (forms HNO₃) |
| Nitrous oxide | N₂O | **Greenhouse gas** (laughing gas) |

#### **Prevention of NOₓ Pollution**
- **Catalytic converters** in cars.
- **Using cleaner fuels** (CNG instead of diesel).

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### **7. Nitrates (NO₃⁻)**
Nitrates are **highly soluble salts** found in:
- **Fertilizers** (KNO₃, Ca(NO₃)₂).
- **Explosives** (NH₄NO₃ in TNT).

#### **Solved Example: Fertilizer Calculation**
**Question**: How much **KNO₃** (potassium nitrate) is needed to provide **50 kg of nitrogen** to a field?
**Solution**:
1. **Molar mass of KNO₃** = 39 (K) + 14 (N) + 3×16 (O) = **101 g/mol**.
2. **Nitrogen content** in KNO₃ = 14/101 ≈ **13.86%**.
3. **Calculation**:
   - 13.86% of KNO₃ = 50 kg N
   - Total KNO₃ needed = (50 kg) / (0.1386) ≈ **360.8 kg**.

**Answer**: **~361 kg of KNO₃** is required.

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### **8. Environmental Impact of Nitrogen Compounds**
| Compound | Environmental Effect |
|----------|----------------------|
| NH₃ | **Eutrophication** (algae blooms in water) |
| NOₓ | **Acid rain**, **smog** |
| N₂O | **Global warming** (300× worse than CO₂) |

**Solutions**:
- **Use nitrogen-efficient fertilizers**.
- **Control industrial emissions**.

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### **9. NEB Board-Style Questions**
#### **Short Answer Questions**
1. **Why is N₂ less reactive than O₂?**
   - N₂ has a **triple bond** (stronger than O₂’s double bond), requiring more energy to break.

2. **Write the equation for the Haber process and state the conditions.**
   - **Equation**: N₂ + 3H₂ ⇌ 2NH₃
   - **Conditions**: 450°C, 200 atm, Fe catalyst.

3. **How is NO₂ formed from NO?**
   - **2NO + O₂ → 2NO₂**

4. **What is the role of nitric acid in explosives?**
   - It reacts with organic compounds to form **nitro derivatives** (e.g., TNT).

#### **Long Answer Questions**
5. **Describe the Ostwald process with a flowchart. How is HNO₃ used in fertilizers?**
   - **Flowchart**: (See above)
   - **Use in fertilizers**: Forms **ammonium nitrate (NH₄NO₃)**, a key nitrogen source for crops.

6. **Explain the environmental effects of nitrogen oxides. How can they be controlled?**
   - **Effects**: Acid rain, smog, global warming.
   - **Control**: Catalytic converters, cleaner fuels.

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### **Exam Tip**
✅ **NEB Exam Focus Areas**:
- **Stoichiometry**: Calculate yields in Haber/Ostwald processes.
- **Industrial processes**: Conditions, catalysts, and equations.
- **Environmental impact**: Effects of NOₓ, NH₃, and N₂O.
- **Applications**: Fertilizers, explosives, and cleaning agents.

🔹 **Common Mistakes to Avoid**:
- Forgetting **conditions** (temperature, pressure, catalyst) in industrial processes.
- Mixing up **NO and NO₂** in reactions.
- Incorrect **limiting reagent** calculations.

📌 **Quick Revision Trick**:
- **"Haber = NH₃, Ostwald = HNO₃"** (Remember the processes by their products).
- **"NOₓ = Pollution"** (Nitrogen oxides are harmful).

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![nitrogen cycle diagram**](/media/665d635345a8eaa87d4b.jpg "A simplified nitrogen cycle showing fixation, nitrification, and denitrification. (Image: Paul A. O'Brien, Kathleen M. Morrow, Bette L. Willis and Dav, CC BY-SA 4.0, via Wikimedia Commons)")