Halogens are Group 17 elements consisting of F, Cl, Br, I, and At. They tend to become monovalent anions and act as strong oxidizing agents.
Basic Knowledge
All elemental halogens are diatomic molecules (F2, Cl2, Br2, I2), and their oxidizing power decreases in the order F2 > Cl2 > Br2 > I2. At room temperature, F2 is a pale-yellow gas, Cl2 is a yellow-green gas, Br2 is a reddish-brown liquid, and I2 is a blackish-purple solid. They are generated by downward displacement and are not collected by wet water displacement.
Laboratory preparation of chlorine: MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O (heating). Industrially, it is produced by electrolysis of salt water (the ion-exchange membrane process). Chlorine water undergoes Cl2 + H2O ⇄ HCl + HClO and shows bleaching and disinfecting actions due to hypochlorous acid, HClO.
Halogens (Group 17, represented by Cl2)
Oxidizing power (F2>Cl2>Br2>I2, stronger toward the top)
Hypochlorous acid HClO (bleaching and disinfecting, unstable weak acid)
Silver halides (AgF soluble, AgCl white, AgBr pale yellow, AgI yellow precipitates, photosensitive)
Hydrogen fluoride HF (a weak acid that dissolves glass, SiO2+6HF→H2SiF6+2H2O)
Iodine-starch reaction (blue-violet color, detection of I2)
Further Study (Principles and Applications)
The order of oxidizing power of halogens is determined by the combined effects of electron affinity and hydration energy. Chlorine is produced on a scale of tens of millions of tons per year for disinfecting tap water, bleach (sodium hypochlorite), and vinyl chloride production. Fluorine is essential for Teflon (PTFE), etching gases in semiconductor production, and pharmaceuticals (SSRIs and anticancer drugs).
The photosensitivity of silver halides was the principle of traditional silver photography. AgBr is reduced to Ag by light and amplified during development. It is still used in X-ray film and special measurements.
In toxicology, Cl2 (poison gas in World War I), F- (accumulation in bones), and I2 (an essential element in thyroid hormones, but toxic in excess) are important.
- Oxidizing power F2>Cl2>Br2>I2 (stronger toward the top)
- Only HF is a weak acid; HCl, HBr, and HI are strong acids
- HF dissolves glass (SiO2)
- AgCl white, AgBr pale yellow, AgI yellow (only AgCl and AgBr dissolve in aqueous NH3)
- Chlorine water = HCl + HClO; HClO causes bleaching and disinfection
- I2 dissolves in KI solution through
I2 + I- → I3- - Iodine-starch reaction = blue-violet (disappears on heating and reappears on cooling)
Points to Note (Common Confusions and Frequent Mistakes)
① Only HF is a weak acid (because of hydrogen bonding); the other HX compounds are strong acids. ② AgF is soluble in water (the only exception). ③ Reactions between halogens and hydrogen: F2 (explodes in the dark), Cl2 (explodes in light), Br2 (requires heating), I2 (high temperature and equilibrium). ④ Oxoacids of chlorine: HClO (+1) < HClO2 (+3) < HClO3 (+5) < HClO4 (+7), with acidity and oxidizing power increasing in this order.
Practice
- Write the reaction equation for the laboratory preparation of chlorine.
- Describe how to distinguish AgCl from AgBr among silver halides (using aqueous NH3).
- Write the reaction equation for HF dissolving glass.