Chlorine Gas and Water Reaction | Cl2 + H2O

Chlorine gas dissolves to some extent in water and form hydrochloric acid and hypochlorous acid as products. Therefore, an acidic solution is given when products are formed. This reaction is a dangerous one because chlorine gas is a toxic gas.

Written by: Heshan Nipuna, updated:2020/05/18

chlorine gas and water reaction

Cl2(aq) + H2O(l) ⇌ HCl(aq) + HOCl(aq)

Products of chlorine and water reaction

Reaction between chlorine gas and water is a reversible reaction. Therefore, all dissolved chlorine gas does not react with water. Hence, unreacted chlorine will remain in the aqueous solution. After treating chlorine gas to water, , check the pH value of aqueous solution. You may see, pH value will be lower than seven. If you send more chlorine gas, pH value will be reduce more due to formation of more HCl and HOCl.

Solubility of chlorine depends on the temperature. Therefore, amount of given products also depend on the temperature too. At lower temperature, solubility of chlorine is great and gives higher yield.

Hydrochloric acid and hypochlorous acid

Hydrochloric acid (HCl) is a strong acid and hypochlorous acid (HOCl) is a weak acid. Due to existance of hydrochloric acid, dissociation of hypochlorous acid is reduced. Both acids are soluble in water.

Explanation of equilibrium of chlorine and water

Get a partially filled container by distilled water (to have a free air space). Container should be completely sealed and closed on top to avoid gases are leaked to outside from the container. When you treat chlorine gas to the distilled water in that container, there will be several equilibriums in several phases.

  • equilibrium between gaseous chlorine and dissolved aqueous chlorine: Because chlorine gas is poorly soluble in water, little amount of chlorine gas is remaining in aqueous medium.
  • Cl2(g) ⇌ Cl2(aq)

  • Equilibrium of aqueous chlorine, water, hydrochloric acid, and hypochlorous acid: Dissilved chlorine in water reacts with to produce HCl and HOCl.
  • Cl2(aq) + H2O(l) ⇌ HCl(aq) + HOCl(aq)

  • Formed HOCl is a weak acid and dissociate to OCl- or H3O+ ions.
  • HOCl(aq) + H2O(l) ⇌ H3O+(aq) + OCl-(aq)

Chemical balanced equation of chlorine and water reaction

Cl2(aq) + H2O(l) ⇌ HCl(aq) + HOCl(aq)

One chlorine mole reacts with one water mole and produce one mole of HCl and one mole of HOCl respectively.

Reaction of chlorine and water is a redox reaction

Oxidation number of chlorine is changed during the reaction. Therefore, this reaction is a redox reaction. Chlorine is at 0 oxidation state in chlorine gas.

  • Oxidation number of chlorine in HCl is -1.
  • In HOCl, oxidation number of chlorine is +1.

Comparison of reaction of chlorine with water and other halogens with water

Here we are going to see how other halogens will react with water.

  • Fluorine violently reacts with water.
  • Bromine dissolves in water less than chlorine.
  • Iodine's solubility in water is poor. Therefore, we cannot expect a reaction between iodine and water.

How to conduct this reaction in the laboratory?

  • If you have chlorine gas cylinders, you can use them to feed to the distilled water.
  • Otherwise, you can prepare chlorine gas in the laboratory. Mixing dilute HCl acid and potassium permanganate will give chlorine gas.

Safety during the experiment

You should extremely careful about safety of you can other when you are doing experiment with chlorine gas. If you inhale chlorine gas, it will react with water in the mucous/linings of your lungs and form hydrochloric acid. This will destroy your lung's ability to take in oxygen and will be very painful and may cause to the death too.

Uses of chlorine and water reaction

This reaction produces hypochlorous acid as a product. Hypochlorous acid (HOCl) is very good disinfectant. But, presence of HCl will make difficulties to separate HOCl from HCl.


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