1. Dissolution and Hydrolysis:
* Chlorine gas dissolves in water to a limited extent.
* It then reacts with water molecules in a reversible reaction, undergoing hydrolysis to form hypochlorous acid (HOCl) and hydrochloric acid (HCl):
```
Cl2 + H2O ⇌ HOCl + HCl
```
2. Equilibrium:
* The reaction above establishes an equilibrium.
* The position of this equilibrium is influenced by factors such as pH, temperature, and the presence of other chemicals.
* At typical water treatment conditions, hypochlorous acid (HOCl) is the primary disinfection agent.
3. Further Reactions:
* Hypochlorous acid can further react with water to form hypochlorite ions (OCl-):
```
HOCl + H2O ⇌ H3O+ + OCl-
```
* The relative amounts of HOCl and OCl- are determined by the pH of the water. At lower pH values (more acidic), HOCl predominates, while at higher pH values (more basic), OCl- becomes more prevalent.
4. Disinfection:
* Both HOCl and OCl- are powerful disinfectants.
* They work by oxidizing and disrupting the cell walls and enzymes of microorganisms, leading to their death.
5. Other Reactions:
* Chlorine can also react with organic matter in water, forming various byproducts such as trihalomethanes (THMs), which are potential health concerns.
* It can also react with dissolved minerals, leading to the formation of salts.
In summary:
Adding chlorine gas to water creates a complex mixture containing dissolved chlorine, hypochlorous acid, hydrochloric acid, and other species. This mixture is used for disinfection, but it's important to manage the process carefully to minimize the formation of harmful byproducts.
