Key Chemical Reactions and Properties of Chlorine (Cl)

Chemical Properties Overview

Chlorine (Cl, Atomic Number 17) is a non-metallic element belonging to Group 17 (Halogens).

• Reactivity Series Position: Highly reactive non-metal, second most reactive halogen after fluorine. Its reactivity is due to its strong tendency to gain an electron to achieve a stable octet configuration.
• Electronegativity: High (Pauling scale: 3.16), leading to its strong oxidizing power.
• General Reactivity: A powerful oxidizing agent. It readily reacts with metals, non-metals, and organic compounds.

Action of Air and Oxygen

Chlorine does not react directly with oxygen (O₂) or nitrogen (N₂) under normal conditions. Various oxides of chlorine (e.g., Cl₂O, ClO₂, Cl₂O₆, Cl₂O₇) can be formed indirectly. These are generally unstable and explosive.

Action of Water and Steam

Chlorine reacts with water, forming hydrochloric acid (HCl) and hypochlorous acid (HOCl). The nature of the products depends on the temperature.

1. With Cold Water (Reversible Reaction): Chlorine dissolves in cold water to form chlorine water, which is a mixture of hydrochloric acid and hypochlorous acid. This is the basis of its bleaching action. $$ \text{Cl}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{HCl}(\text{aq}) + \text{HOCl}(\text{aq}) $$ (Hypochlorous acid, HOCl, is an unstable and powerful oxidizing agent.)

2. With Hot Water (Irreversible Reaction): At higher temperatures, chlorine reacts with water to form hydrochloric acid and chloric acid. $$ 3\text{Cl}_2(\text{g}) + 3\text{H}_2\text{O}(\text{l}) \rightarrow 5\text{HCl}(\text{aq}) + \text{HClO}_3(\text{aq}) $$

Action of Acids and Bases

Action with Acids

Chlorine typically acts as an oxidizing agent when reacting with certain acids. It generally does not react with non-oxidizing acids.

1. Oxidation of Hydrogen Sulfide (H₂S): Chlorine oxidizes hydrogen sulfide to sulfur. $$ \text{Cl}_2(\text{g}) + \text{H}_2\text{S}(\text{g}) \rightarrow 2\text{HCl}(\text{aq}) + \text{S}(\text{s}) $$

2. Oxidation of Sulfur Dioxide (SO₂): Chlorine oxidizes sulfur dioxide in the presence of moisture to sulfuric acid. $$ \text{Cl}_2(\text{g}) + \text{SO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{HCl}(\text{aq}) + \text{H}_2\text{SO}_4(\text{aq}) $$

3. Reaction with Ammonia (NH₃): The products depend on the relative amounts of ammonia and chlorine.

   • With excess Ammonia: Nitrogen gas is formed. $$ 3\text{Cl}_2(\text{g}) + 8\text{NH}_3(\text{g}) \rightarrow \text{N}_2(\text{g}) + 6\text{NH}_4\text{Cl}(\text{s}) $$
   • With excess Chlorine: Explosive nitrogen trichloride (NCl₃) is formed. $$ \text{NH}_3(\text{g}) + 3\text{Cl}_2(\text{g}) \rightarrow \text{NCl}_3(\text{l}) + 3\text{HCl}(\text{g}) $$

Action with Bases (Alkalis)

Chlorine reacts with alkalis to form chlorides and oxoanions, with products depending on concentration and temperature.

1. With Cold, Dilute NaOH (or KOH): Forms sodium chloride (NaCl) and sodium hypochlorite (NaClO), used as a bleaching agent. $$ \text{Cl}_2(\text{g}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{NaClO}(\text{aq}) + \text{H}_2\text{O}(\text{l}) $$

2. With Hot, Concentrated NaOH (or KOH): Forms sodium chloride (NaCl) and sodium chlorate (NaClO₃). $$ 3\text{Cl}_2(\text{g}) + 6\text{NaOH}(\text{aq}) \rightarrow 5\text{NaCl}(\text{aq}) + \text{NaClO}_3(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) $$

3. With Calcium Hydroxide (Slaked Lime): Forms bleaching powder. $$ \text{Ca(OH)}_2(\text{s}) + \text{Cl}_2(\text{g}) \rightarrow \text{CaOCl}_2(\text{s}) + \text{H}_2\text{O}(\text{l}) $$

Key Laboratory Test/Identification Reactions

Identification of Chlorine Gas (Cl₂)

1. Physical Properties: Greenish-yellow gas with a pungent, suffocating odor.
2. Litmus Paper Test: It bleaches moist litmus paper. First, moist blue litmus paper turns red (due to HCl formed) and then gets decolorized (bleached) due to the oxidizing action of HOCl. $$ \text{Cl}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{HCl}(\text{aq}) + \text{HOCl}(\text{aq}) $$ $$ \text{Dye} + \text{HOCl} \rightarrow \text{Colorless product} + \text{HCl} $$

Identification of Chloride Ions (Cl⁻)

1. Silver Nitrate Test: Add dilute nitric acid (HNO₃) to the solution containing chloride ions, followed by silver nitrate (AgNO₃) solution. A white precipitate of silver chloride (AgCl) is formed. $$ \text{Cl}^-(\text{aq}) + \text{AgNO}_3(\text{aq}) \rightarrow \text{AgCl}(\text{s}) + \text{NO}_3^-(\text{aq}) $$
2. Ammonia Test: The white precipitate of AgCl is soluble in aqueous ammonia (NH₄OH), forming a diamminesilver(I) complex. $$ \text{AgCl}(\text{s}) + 2\text{NH}_4\text{OH}(\text{aq}) \rightarrow [\text{Ag}(\text{NH}_3)_2]\text{Cl}(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) $$