Halogens and Their Compounds
Objectives
- Describe the properties and reactivity trends of halogen elements
- Understand the differences among hydrogen halides
- Know the preparation and reactions of chlorine
Properties of Halogen Elements
Going down Group 17 (halogens):
- Oxidizing power decreases: F₂ > Cl₂ > Br₂ > I₂
- Melting/boiling points increase (greater molecular mass → stronger intermolecular forces)
- Color deepens: F₂ (pale yellow) → Cl₂ (yellow-green) → Br₂ (red-brown) → I₂ (purple-black)
| Element | Formula | State (RT) | Color | Oxidizing power |
|---|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow | Strongest |
| Chlorine | Cl₂ | Gas | Yellow-green | Strong |
| Bromine | Br₂ | Liquid | Red-brown | Moderate |
| Iodine | I₂ | Solid | Purple-black | Weakest |
Comparison of halogen elements
Comparing oxidizing power: Passing Cl₂ into KBr solution liberates Br₂ → solution turns red-brown.
Cl₂ is a stronger oxidizer than Br₂, so it oxidizes Br⁻ to Br₂.
Hydrogen Halides
| Compound | Boiling point | Acid strength | Key feature |
|---|---|---|---|
| HF | 19.5°C | Weak acid | High b.p. from H-bonding; etches glass |
| HCl | −85°C | Strong acid | Used widely as hydrochloric acid |
| HBr | −67°C | Strong acid | Stronger reducing agent than HCl |
| HI | −35°C | Strong acid | Strongest reducing agent |
Comparison of hydrogen halides
HF is unique:
- Hydrogen bonding gives an anomalously high boiling point
- It is a weak acid (all other HX are strong acids)
- Reacts with → etches glass
Chemistry of Chlorine
Lab preparation of Cl₂:
Chlorine reacting with water:
The hypochlorous acid (HClO) produced has bleaching and disinfecting power.
Bleaching powder: — made by absorbing Cl₂ into Ca(OH)₂.
Silver Halides
| Compound | Color | Solubility | Photosensitivity |
|---|---|---|---|
| AgF | Colorless | Soluble | Low |
| AgCl | White | Insoluble | Moderate |
| AgBr | Pale yellow | Insoluble | High (photography) |
| AgI | Yellow | Insoluble | High |
Check Your Understanding
Q1 Which halogen has the strongest oxidizing power?
Q2 Why is HF a weak acid?
Q3 What causes the bleaching action of chlorine water?
Exercises
Q1. Write the reaction when Cl₂ is passed into KI solution, and describe the color change.
Solution
Cl₂ oxidizes I⁻ to I₂. The solution turns brown (I₂ also gives a blue-purple color with the starch test).