Group 15 Elements - Nitrogen Family

Real-Life Connection: From Fertilizers to Explosives

The air you breathe is 78% nitrogen! Yet plants can’t use it directly - they need it “fixed” as ammonia in fertilizers. The same nitrogen that helps crops grow can also form explosives like TNT and nitroglycerin. Phosphorus lights up matches, glows in the dark (white phosphorus), and is essential for DNA and bones. Red phosphorus is on matchbox sides, while white phosphorus was used in early matches and bombs!

Group 15 Elements Overview

Members: Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), Bismuth (Bi)

Old Name: Pnictogens (from Greek “pniktos” = to choke, referring to nitrogen)

Electronic Configuration Pattern

• General configuration: ns² np³
• Nitrogen: [He] 2s² 2p³
• Phosphorus: [Ne] 3s² 3p³
• Arsenic: [Ar] 3d¹⁰ 4s² 4p³

Memory Trick - “No Poor Ass Should Be”: Nitrogen, Phosphorus, Assenic, Sb, Bismuth

Key Trends Down the Group

Memory Trick for Oxidation States: “Negative Three, Positive Three and Five” - Group 15 shows -3, +3, +5

Interactive Demo: Visualize Group 15 in Periodic Table

Explore the position and trends of Group 15 elements in the periodic table.

Dinitrogen (N₂) - The Inert Giant

Why is N₂ so unreactive?

• Triple bond (N≡N) is very strong (946 kJ/mol)
• High bond energy due to small size and effective overlap
• Breaking this bond requires high temperature/pressure or catalysts

Preparation:

Laboratory:
NH₄Cl + NaNO₂ → N₂ + 2H₂O + NaCl (pure N₂)
NH₄NO₂ → N₂ + 2H₂O (if available)

Industrial (from air):
Fractional distillation of liquid air (b.p. -196°C)

Reactions (at high temperature/pressure):

1. With hydrogen (Haber process):

N₂ + 3H₂ ⇌ 2NH₃ (450°C, 200 atm, Fe catalyst)

2. With oxygen:

N₂ + O₂ ⇌ 2NO (at 3000°C or lightning)

3. With metals (forms nitrides):

3Mg + N₂ → Mg₃N₂ (magnesium nitride)
6Li + N₂ → 2Li₃N (lithium nitride)

Uses: Inert atmosphere, liquid N₂ as coolant, fertilizer production

Ammonia (NH₃) - The Alkaline Gas

Structure and Properties

Structure:

• Pyramidal shape (sp³ hybridization)
• Bond angle: 107° (less than 109.5° due to lone pair repulsion)
• Polar molecule (μ = 1.46 D)

Physical Properties:

• Colorless gas, pungent smell
• Highly soluble in water (1:700 volumes)
• Forms NH₃·H₂O (ammonium hydroxide)
• Liquid NH₃ boils at -33°C

Memory Trick: “N Has 3 Hydrogens” - NH₃, pyramidal, 107° (10-7 = 3!)

Preparation

Laboratory (heating ammonium salt with base):

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

Industrial (Haber Process):

N₂ + 3H₂ ⇌ 2NH₃ (ΔH = -92 kJ/mol)

Conditions:
- Temperature: 450-500°C
- Pressure: 200 atm
- Catalyst: Fe with K₂O, Al₂O₃ promoters
- Le Chatelier: High pressure favors product (4 moles → 2 moles)
              Low temperature favors product (exothermic)
              But 450°C used for reasonable rate

Memory Trick: “HABER = High Atm, Both Elements React” - 200 atm, N₂ + H₂

Chemical Reactions of Ammonia

1.Basicity (proton acceptor - Lewis base):

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (weak base, Kb = 1.8 × 10⁻⁵)
NH₃ + HCl → NH₄Cl (white fumes - test for NH₃)
NH₃ + H₂SO₄ → (NH₄)₂SO₄

2. Reducing agent:

2NH₃ + 3CuO → 3Cu + N₂ + 3H₂O (at 800°C)
4NH₃ + 3O₂ → 2N₂ + 6H₂O (combustion)
4NH₃ + 5O₂ → 4NO + 6H₂O (Pt catalyst, 800°C - Ostwald process)

3. Complex formation (ligand):

AgCl + 2NH₃ → [Ag(NH₃)₂]Cl (soluble complex)
CuSO₄ + 4NH₃ → [Cu(NH₃)₄]SO₄ (deep blue complex)

4. Reaction with halogens:

8NH₃ + 3Cl₂ → N₂ + 6NH₄Cl (controlled)
NH₃ + 3Cl₂ → NCl₃ + 3HCl (excess Cl₂, explosive NCl₃)

Memory Trick: “ABCR” - Acid neutralization, Base in water, Complexes, Reducing agent

Uses of Ammonia

• Fertilizers (80% of production): (NH₄)₂SO₄, NH₄NO₃, urea
• Nitric acid production (Ostwald process)
• Refrigerant
• Cleaning agents
• Explosives (via HNO₃)

Oxides of Nitrogen

Key Points:

1. N₂O (Laughing gas):

   • Used as anesthetic
   • Decomposes on heating: 2N₂O → 2N₂ + O₂ (supports combustion)

2. NO (Colorless):

   • Paramagnetic (odd electron)
   • Readily oxidized: 2NO + O₂ → 2NO₂
   • Important signaling molecule in body

3. NO₂ (Brown, toxic):

   • Paramagnetic
   • Dimerizes: 2NO₂ ⇌ N₂O₄ (colorless)
   • Acidic: 2NO₂ + H₂O → HNO₃ + HNO₂

Memory Trick for Colors: “N₂O, NO = None, NO₂ = Brown”

Nitric Acid (HNO₃)

Preparation

Laboratory (from nitrate salt):

NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃ (below 200°C)

Industrial (Ostwald Process):

Step 1: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt-Rh catalyst, 800°C)
Step 2: 2NO + O₂ → 2NO₂
Step 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃

Overall: NH₃ + 2O₂ → HNO₃ + H₂O

Memory Trick: “OSTWALD = Oxygen Stirs The Water, Ammonia Liberates, Dioxide”

Properties of Nitric Acid

Physical:

• Colorless liquid (pure)
• Brown color when decomposed (NO₂ dissolves)
• 68% conc. HNO₃, 98% fuming HNO₃

Chemical:

1. Acidic nature:

HNO₃ + NaOH → NaNO₃ + H₂O

2. Oxidizing agent (conc. > dil.):

With metals:

3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 2NO + 4H₂O
Cu + 4HNO₃(conc) → Cu(NO₃)₂ + 2NO₂ + 2H₂O
4Zn + 10HNO₃(very dil) → 4Zn(NO₃)₂ + N₂O + 5H₂O
8Zn + 10HNO₃(very very dil) → 8Zn(NO₃)₂ + NH₄NO₃ + 3H₂O

Memory Trick for Products: “DCN” - Dilute gives NO, Concentrated gives NO₂, very dilute gives N₂O or NH₄⁺

3. Aqua Regia (King’s water) - dissolves noble metals:

3HCl + HNO₃ = 2H₂O + NOCl + Cl₂ (nascent)
Au + 4HCl + HNO₃ → HAuCl₄ + NO + 2H₂O
3Pt + 16HCl + 4HNO₃ → 3H₂PtCl₆ + 4NO + 8H₂O

Ratio: 3:1 (HCl:HNO₃) - “Three to One” for aqua regia

4. With non-metals:

S + 6HNO₃(conc) → H₂SO₄ + 6NO₂ + 2H₂O
P₄ + 20HNO₃(conc) → 4H₃PO₄ + 20NO₂ + 4H₂O
C + 4HNO₃(conc) → CO₂ + 4NO₂ + 2H₂O

5. Passivation (with Fe, Al, Cr):

Conc. HNO₃ forms protective oxide layer
Makes Fe, Al, Cr passive (inert)

6. Decomposition (on heating or light):

4HNO₃ → 4NO₂ + O₂ + 2H₂O (brown fumes)

Common Mistake: Mg and Mn do NOT show passivation with conc. HNO₃ (only Fe, Al, Cr)

Ring Test for Nitrate (NO₃⁻)

NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O
NO + [Fe(H₂O)₆]²⁺ → [Fe(H₂O)₅(NO)]²⁺ + H₂O
                      (brown ring complex)

Procedure: Add FeSO₄ to test solution, then add conc. H₂SO₄ along the sides Observation: Brown ring at junction of two layers

Phosphorus - The Light Bearer

Allotropes of Phosphorus

1. White Phosphorus (P₄)

Structure:

• Tetrahedral P₄ molecules
• P-P-P bond angle: 60° (highly strained)
• Discrete molecular structure

Properties:

• White waxy solid (or pale yellow)
• Poisonous
• Glows in dark (chemiluminescence)
• Soluble in CS₂, insoluble in water
• Spontaneously ignites at 35°C (pyrophoric)
• Stored under water

Preparation:

2Ca₃(PO₄)₂ + 6SiO₂ + 10C → P₄ + 6CaSiO₃ + 10CO (1775K, electric furnace)

2. Red Phosphorus

Structure:

• Polymeric chain structure
• Less reactive than white
• No P₄ discrete molecules

Properties:

• Red powder
• Non-poisonous
• Does not glow
• Insoluble in CS₂
• Ignites at 260°C
• More stable than white P

Conversion:

P₄(white) → P(red) (at 573K in inert atmosphere)

Uses: Matchbox sides (safety matches), smoke screens

3. Black Phosphorus

Structure:

• Layered structure (like graphite)
• Most stable allotrope

Properties:

• Black solid
• Least reactive
• Conductor (unlike white and red)

Memory Trick: “WRB = White Reacts Badly, Red is Reliable, Black is Best (stable)”

Comparison: White vs Red Phosphorus

Phosphine (PH₃)

Preparation:

Laboratory:
P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂ (Ca(OH)₂ also used)

Pure PH₃:
Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃

Properties:

• Colorless gas, rotten fish smell
• Poisonous
• Less basic than NH₃ (90° bond angle vs 107°)
• Reducing agent

Structure:

• Pyramidal (like NH₃)
• Bond angle: 93.5° (less than NH₃ due to less s-character)

Why is PH₃ less basic than NH₃?

• Larger P atom, more diffuse lone pair
• Less effective overlap with H⁺
• Lower electronegativity of P

Reactions:

PH₃ + 3HCl → PH₄Cl (phosphonium chloride - decomposes easily)
2PH₃ + 4O₂ → P₂O₅ + 3H₂O
PH₃ + 8AgNO₃ + 4H₂O → 8Ag + H₃PO₄ + 8HNO₃ (reducing agent)

Memory Trick: “Phosphine = Poor base, Pungent smell, Pyramid 93°”

Oxides of Phosphorus

Phosphorus Trioxide (P₄O₆)

Structure:

• P₄ tetrahedron with O bridging each edge
• 6 P-O-P bridges

Preparation:

P₄ + 3O₂ → P₄O₆ (limited oxygen)

Reaction:

P₄O₆ + 6H₂O → 4H₃PO₃ (phosphorous acid)

Phosphorus Pentoxide (P₄O₁₀)

Structure:

• P₄O₆ structure + 4 terminal P=O bonds
• Most stable oxide of phosphorus

Preparation:

P₄ + 5O₂ → P₄O₁₀ (excess oxygen)

Properties:

• White powder
• Excellent dehydrating agent (absorbs water vigorously)
• Forms metaphosphoric acid, then orthophosphoric acid with water

Reactions:

P₄O₁₀ + 2H₂O → 4HPO₃ (metaphosphoric acid - glacial)
P₄O₁₀ + 6H₂O → 4H₃PO₄ (orthophosphoric acid)

Dehydrating action:
2HNO₃ + P₄O₁₀ → N₂O₅ + "HPO₃"
2HClO₄ + P₄O₁₀ → Cl₂O₇ + "HPO₃"
H₂SO₄ + P₄O₁₀ → SO₃ + "HPO₃"

Uses: Powerful desiccant in laboratories

Phosphoric Acids

1. Orthophosphoric Acid (H₃PO₄)

Structure:

• Tetrahedral with one P=O and three P-OH
• Tribasic (three OH groups)

Preparation:

Industrial:
P₄ + 5O₂ → P₄O₁₀
P₄O₁₀ + 6H₂O → 4H₃PO₄

Laboratory:
P₄O₁₀ + 6H₂O → 4H₃PO₄
or
Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄

Properties:

• Colorless syrupy liquid
• Weak tribasic acid
• Non-oxidizing (unlike HNO₃)
• Forms three series of salts

Reactions:

H₃PO₄ + NaOH → NaH₂PO₄ + H₂O (monobasic salt)
H₃PO₄ + 2NaOH → Na₂HPO₄ + 2H₂O (dibasic salt)
H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O (tribasic salt)

Heating:
2H₃PO₄ --heat--> H₄P₂O₇ + H₂O (pyrophosphoric acid)

Uses:

• Food industry (soft drinks)
• Fertilizers
• Rust remover
• Detergents

2. Phosphorous Acid (H₃PO₃)

Structure:

• One P-H bond (reducing property)
• Two P-OH and one P=O
• Dibasic (only 2 ionizable H, not 3!)

Preparation:

P₄O₆ + 6H₂O → 4H₃PO₃
PCl₃ + 3H₂O → H₃PO₃ + 3HCl

Key Point:

• Formula H₃PO₃ but only dibasic
• P-H bond doesn’t ionize
• Structure: (HO)₂P(=O)H

Reactions:

H₃PO₃ + 2NaOH → Na₂HPO₃ + 2H₂O (max neutralization)
4H₃PO₃ → 3H₃PO₄ + PH₃ (disproportionation on heating)
H₃PO₃ + 2AgNO₃ + 2H₂O → 2Ag + H₃PO₄ + 2HNO₃ (reducing agent)

Memory Trick: “Three-Three = Two basic” - H₃PO₃ is dibasic (has P-H)

3. Hypophosphorous Acid (H₃PO₂)

Structure:

• Two P-H bonds
• One P-OH and one P=O
• Monobasic (only 1 ionizable H!)

Preparation:

P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃
Ba(H₂PO₂)₂ + H₂SO₄ → 2H₃PO₂ + BaSO₄

Properties:

• Strong reducing agent
• Monobasic acid

Reactions:

H₃PO₂ + NaOH → NaH₂PO₂ + H₂O (max neutralization)
4H₃PO₂ → 3H₃PO₃ + PH₃ (disproportionation)
H₃PO₂ + 4AgNO₃ + 2H₂O → 4Ag + H₃PO₄ + 4HNO₃ (strong reducing)

Memory Trick: “Three-Two = One basic” - H₃PO₂ is monobasic (has 2 P-H)

Basicity Summary of Phosphorus Acids

Memory Rule: Only H attached to O (as OH) is ionizable, P-H doesn’t ionize!

Common Mistakes to Avoid

1. Mistake: N₂ is very reactive

   • Correct: N₂ is very unreactive (triple bond, 946 kJ/mol)

2. Mistake: NH₃ is acidic

   • Correct: NH₃ is basic (proton acceptor, forms NH₄⁺)

3. Mistake: All HNO₃ reactions produce NO₂

   • Correct: Dilute HNO₃ gives NO, concentrated gives NO₂

4. Mistake: H₃PO₃ is tribasic

   • Correct: Dibasic (only 2 OH groups ionize, P-H doesn’t)

5. Mistake: White P and red P have same reactivity

   • Correct: White P much more reactive (strained P₄, 35°C ignition)

6. Mistake: PH₃ is as basic as NH₃

   • Correct: PH₃ much less basic (larger atom, diffuse lone pair)

7. Mistake: Aqua regia ratio is 1:1

   • Correct: 3:1 (HCl:HNO₃)

8. Mistake: All metals show passivation with conc. HNO₃

   • Correct: Only Fe, Al, Cr (not Mg, Mn)

Practice Problems

Level 1: JEE Main Basics

1. Why is N₂ less reactive than P₄?

2. Write balanced equations for: a) Haber process b) Ostwald process c) Ring test for nitrate

3. Why is white phosphorus stored under water?

4. Arrange in decreasing order of basicity: NH₃, PH₃, AsH₃

5. What products form when Cu reacts with: a) Dilute HNO₃ b) Concentrated HNO₃

Level 2: JEE Main Advanced

6. Explain why bond angle in PH₃ (93.5°) is less than NH₃ (107°).

7. H₃PO₃ is dibasic while H₃PO₄ is tribasic. Explain with structures.

8. Calculate the oxidation state of N in: a) NH₄NO₃ b) N₂O₅ c) N₂O₃ d) NO

9. Why doesn’t conc. HNO₃ react with Fe, Al, Cr?

10. White phosphorus has a garlic odor and glows in the dark. Explain both phenomena.

Level 3: JEE Advanced

11. In Haber process, high pressure and low temperature are favorable according to Le Chatelier’s principle. Yet the process is carried out at 450°C and 200 atm. Explain the optimization.

12. When H₃PO₃ is heated, it undergoes disproportionation. Write the balanced equation and explain the change in oxidation states of P.

13. A colorless gas X reacts with oxygen to form a brown gas Y. Y dissolves in water to give an acid Z which acts as an oxidizing agent. Identify X, Y, Z with equations.

14. Explain why: a) NH₃ forms hydrogen bonds but PH₃ doesn’t significantly b) N doesn’t form pentahalides but P does c) Bond energy of N₂ > P₄ but bond energy of N-N single bond < P-P single bond

15. Complete and balance: a) P₄ + NaOH + H₂O → b) NH₃ + CuO (heat) → c) Zn + very dilute HNO₃ → d) Au + HCl + HNO₃ →

Cross-Links to Other Topics

Related to Periodic Classification

• Periodic Trends - Basicity trends in hydrides
• Inert Pair Effect - Bi(III) stability

Related to Chemical Bonding

• Hybridization - NH₃, PH₃ pyramidal structures
• Bond Angles - 107° in NH₃, 93.5° in PH₃
• Hydrogen Bonding - High b.p. of NH₃

Related to Chemical Equilibrium

• Le Chatelier’s Principle - Haber process optimization
• Equilibrium Constants - NH₃ + H₂O equilibrium

Related to Other Chapters

• Redox Reactions - HNO₃ as oxidizing agent
• Qualitative Analysis - Ring test for NO₃⁻
• Coordination Compounds - [Cu(NH₃)₄]²⁺ complex

Memory Palace for Group 15

Imagine a Fertilizer Factory:

Gate: Sign says “No Poor Ass Should Be” (N, P, As, Sb, Bi)

Air Separation Unit:

• Giant N₂ tank (78% of air)
• Triple-locked vault (triple bond symbolism)
• Requires high energy to open (946 kJ/mol)

Haber Hall:

• 450°C thermometer on wall
• 200 atm pressure gauge (both glowing)
• Fe catalyst statues
• NH₃ fountain (pungent smell warning!)

Ostwald Tower (3 floors):

• Floor 1: NH₃ + O₂ → NO (platinum catalyst chandelier)
• Floor 2: NO + O₂ → NO₂ (brown smoke)
• Floor 3: NO₂ + H₂O → HNO₃ (acid rain simulator)

Phosphorus Wing:

• White P room (dark, glowing material under water tank)
• Red P room (matchboxes on shelves, 260°C ignition point sign)
• Black P vault (most stable, conductor label)

Acid Library:

• H₃PO₄ shelf: “3 books = Tribasic”
• H₃PO₃ shelf: “3 books, 2 usable = Dibasic”
• H₃PO₂ shelf: “3 books, 1 usable = Monobasic”

Quick Revision Checklist

• Group 15 configuration: ns² np³
• N₂ unreactive (triple bond 946 kJ/mol)
• Haber process: N₂ + 3H₂ ⇌ 2NH₃ (450°C, 200 atm, Fe)
• NH₃ structure: pyramidal, 107°, basic
• Ostwald process: NH₃ → NO → NO₂ → HNO₃
• Dilute HNO₃ → NO, Conc. HNO₃ → NO₂
• Aqua regia: 3 HCl : 1 HNO₃
• Passivation: Fe, Al, Cr with conc. HNO₃
• Ring test: Brown ring of [Fe(H₂O)₅(NO)]²⁺
• White P: P₄, 60°, 35°C ignition, stored under water
• Red P: polymeric, 260°C ignition, matchbox sides
• PH₃ less basic than NH₃ (93.5° vs 107°)
• H₃PO₄ tribasic, H₃PO₃ dibasic, H₃PO₂ monobasic
• P₄O₁₀ excellent dehydrating agent

Important Equations Summary

1. N₂ preparation: NH₄Cl + NaNO₂ → N₂ + 2H₂O + NaCl
2. Haber: N₂ + 3H₂ ⇌ 2NH₃ (450°C, 200 atm, Fe)
3. NH₃ lab: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃
4. Ostwald 1: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt, 800°C)
5. Ostwald 2: 2NO + O₂ → 2NO₂
6. Ostwald 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃
7. Cu + dil HNO₃: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O
8. Cu + conc HNO₃: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
9. White P: 2Ca₃(PO₄)₂ + 6SiO₂ + 10C → P₄ + 6CaSiO₃ + 10CO
10. White to Red: P₄(white) → P(red) (573K, inert atmosphere)
11. PH₃: P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂
12. P₄O₁₀: P₄ + 5O₂ → P₄O₁₀
13. H₃PO₄: P₄O₁₀ + 6H₂O → 4H₃PO₄
14. H₃PO₃: P₄O₆ + 6H₂O → 4H₃PO₃
15. Disproportionation: 4H₃PO₃ → 3H₃PO₄ + PH₃

Last updated: July 2025 Previous: Group 14 Elements | Next: Group 16 Elements