Halogens: Chlorine · 8 min read
Preparation and properties of chlorine
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Introduction to Chlorine
Chlorine is a member of the Halogen family in Group 7 of the periodic table. It is a very reactive gas and is never found alone in nature. You usually find it combined with other things, like in common table salt (sodium chloride) which we use to cook soup at home.
Simply put, chlorine is a poisonous gas that likes to react with almost everything it touches.
Laboratory Preparation of Chlorine
In the school lab, we prepare chlorine by removing hydrogen from concentrated hydrochloric acid. This process is called oxidation. We use a strong oxidizing agent like Manganese(IV) oxide (MnO2) or Potassium tetraoxomanganate(VII) (KMnO4).
Preparation from Manganese(IV) Oxide:
1. Place some black MnO2 powder in a round-bottom flask.
2. Add concentrated HCl through a thistle funnel.
3. Heat the mixture gently. You will see a greenish-yellow gas coming out.
4. Pass the gas through water to remove any HCl fumes.
5. Pass it through concentrated H2SO4 to dry it.
6. Collect the gas by downward delivery (upward displacement of air) because it is heavier than air.
Equation: MnO2 + 4HCl -> MnCl2 + 2H2O + Cl2
Industrial Preparation
In big factories, chlorine is made by the electrolysis of brine. Brine is just a very concentrated solution of common salt (NaCl) in water. When electricity passes through it, chlorine gas is released at the anode.
Physical Properties of Chlorine
It is a greenish-yellow gas with a sharp, choking smell.
It is about 2.5 times heavier than air.
It is moderately soluble in water, forming a pale yellow solution called chlorine water.
Chemical Properties and Bleaching
Chlorine is a powerful bleaching agent, but it only works if water is present. If you put a dry piece of red cloth in dry chlorine gas, the color stays. But if the cloth is wet, the chlorine reacts with water to form hypochlorous acid (HOCl), which kills the color.
Equation: H2O + Cl2 -> HCl + HOCl (then) HOCl + Dye -> HCl + Colorless dye
Key points
- •Prepared in the lab by oxidizing concentrated HCl with MnO2.
- •Collected by downward delivery because it is denser than air.
- •Industrially produced by the electrolysis of brine.
- •Requires moisture to act as a bleaching agent.
- •It is a very reactive greenish-yellow poisonous gas.