What are interhalogen compounds?

Interhalogen compounds are formed between two different halogens, such as `ClF3`, `BrF5`, and `IF7`.

Class 12

Tamil Nadu Class 12 Chemistry p-Block Elements II — Samacheer Kalvi

Plus Two Chemistry notes on nitrogen, phosphorus, oxygen, sulphur, halogens, noble gases, oxoacids, interhalogens, and periodic trends.

Updated 6 June 20263 min read

Quick Summary

This unit surveys groups 15 to 18. The scoring areas are anomalous trends, hydrides, oxoacids, preparation and properties of important compounds, interhalogens, and noble-gas chemistry.

Group 15 and 16

Nitrogen differs from phosphorus because of small size, high electronegativity, and strong N≡N bond.
Ammonia is basic and forms hydrogen bonds.
Phosphorus shows allotropy and forms several oxoacids.
Oxygen and sulphur differ in catenation and oxidation states.
Sulphuric acid is a strong dehydrating and oxidising agent.

Halogens and Noble Gases

Halogens are highly reactive non-metals; oxidising power decreases down the group.
Interhalogen compounds are often more reactive than parent halogens except fluorine.
Noble gases are mostly inert, but xenon forms compounds such as XeF2, XeF4, and XeF6.

Key Reactions & Preparations

Ammonia (Haber process):
   N₂ + 3H₂ ⇌(Fe catalyst, 200 atm, 700 K) 2NH₃

Nitric acid (Ostwald process):
   4NH₃ + 5O₂ →(Pt) 4NO + 6H₂O
   2NO + O₂ → 2NO₂ ;  3NO₂ + H₂O → 2HNO₃ + NO

Sulphuric acid (Contact process):
   2SO₂ + O₂ ⇌(V₂O₅) 2SO₃ ;  SO₃ + H₂SO₄ → H₂S₂O₇ →(H₂O) 2H₂SO₄

Xenon fluorides (direct combination, varying Xe:F₂ ratio):
   Xe + F₂ → XeF₂ ;  Xe + 2F₂ → XeF₄ ;  Xe + 3F₂ → XeF₆

Oxoacid Basicity (count P–OH groups)

H₃PO₄ (orthophosphoric) : 3 P–OH → tribasic
H₃PO₃ (phosphorous)     : 2 P–OH (+1 P–H) → dibasic
H₃PO₂ (hypophosphorous) : 1 P–OH (+2 P–H) → monobasic

Basicity = number of replaceable –OH hydrogens, NOT the total H atoms.

Structure Shapes (VSEPR)

XeF₂ : linear        XeF₄ : square planar     XeF₆ : distorted octahedral
ClF₃ : T-shaped      BrF₅ : square pyramidal

High-Value Comparisons

Topic	Key point
`NH3` vs `PH3`	ammonia is more basic and hydrogen-bonded
`H2O` vs `H2S`	water has stronger hydrogen bonding
`HF` vs `HCl`	HF is weaker acid because H-F bond is strong
Fluorine vs chlorine	fluorine shows only `-1` oxidation state

Important Questions

One Mark

1. Why is N2 relatively inert? 2. Name one oxoacid of phosphorus. 3. Write one interhalogen compound. 4. Which halogen is the strongest oxidising agent? 5. Name one xenon fluoride.

Short Answer

1. Compare ammonia and phosphine. 2. Explain anomalous behaviour of fluorine. 3. Write a note on contact-process chemistry of sulphuric acid. 4. Distinguish orthophosphorous acid and orthophosphoric acid by basicity.

Long Answer

1. Discuss important oxoacids of phosphorus or sulphur. 2. Explain trends in hydrides of group 16 or group 17. 3. Write notes on interhalogen compounds and xenon fluorides.

Common Mistakes

Giving nitrogen the same catenation behaviour as phosphorus
Forgetting that oxidising power and acid strength trends are different
Treating all noble gases as completely unreactive
Confusing basicity of oxoacids with number of oxygen atoms

Revision Checklist

[ ] Group 15-18 trends
[ ] Nitrogen and fluorine anomalies
[ ] Oxoacids and their basicity
[ ] Halogen hydrides and oxidising power
[ ] Interhalogens and xenon compounds

Frequently Asked Questions

Nitrogen exists as `N2` with a very strong triple bond, so bond breaking requires high energy. Phosphorus forms weaker single bonds in `P4`.