Uses & Reactions of Chlorine (AQA A-Level Chemistry): Revision Notes

2.3.4 Uses & Reactions of Chlorine

Chlorine (Cl_2) is a highly reactive element with important applications, particularly in water treatment and the production of bleach. It undergoes key reactions with water and sodium hydroxide, both of which involve disproportionation, where chlorine is both oxidised and reduced.

Reaction of Chlorine with Water

When chlorine gas is dissolved in water, it undergoes a disproportionation reaction, producing both chloride ions and chlorate(I) ions. This reaction is crucial for water purification processes, as chlorate(I) ions (ClO^-) are effective at killing bacteria.

Equation

$$\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HClO} + \text{HCl}$$

• Oxidation: Cl in Cl_2 (0) is oxidised to ClO^- (+1) in hypochlorous acid (HClO).
• Reduction: Cl in Cl_2 (0) is reduced to Cl^- (-1) in hydrochloric acid (HCl).

Advantages and Disadvantages

• Advantages: Chlorate(I) ions produced during this reaction are effective in killing bacteria, making chlorine essential for treating drinking water and swimming pools. Chlorination helps prevent diseases such as cholera.
• Disadvantages: Despite its effectiveness, chlorine is toxic and can be carcinogenic to humans in high concentrations. However, the health benefits of water chlorination typically outweigh these risks.

Decomposition in Sunlight:

In the presence of sunlight (UV light), chlorine decomposes water, releasing oxygen and forming hydrochloric acid.

Equation:

$$2\text{Cl}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HCl} + \text{O}_2$$

Reaction of Chlorine with Sodium Hydroxide

Chlorine reacts with cold, dilute sodium hydroxide (NaOH) to form sodium chloride (NaCl) and sodium chlorate(I) (NaClO). Sodium chlorate(I) is a powerful disinfectant and the main ingredient in household bleach.

Equation:

$$\text{Cl}_2 + 2\text{NaOH} \rightarrow \text{NaCl} + \text{NaClO} + \text{H}_2\text{O}$$

• Oxidation: Cl in Cl_2 (0) is oxidised to NaClO (+1).
• Reduction: Cl in Cl_2 (0) is reduced to NaCl (-1).

Observation

Green chlorine gas forms a colourless solution as it reacts.

Product

The NaClO produced is the active ingredient in bleach, which is used for its antibacterial properties in cleaning and disinfection.