Chlorine is a dense green-yellow gas with a strong odour. It has twice the density of air. The symbol of Chlorine is 'Cl' and it belongs to the halogen group. Chlorine was discovered in the 1770s and became a commercial agent ever since. It is easily detected in its natural state. Since it is toxic at low concentrations, it should be treated with caution. Due to its highly toxic nature, it has been used as a chemical weapon in wars.
The molecular formula of Chlorine gas Cl2.
Uses of Chlorine gas
During the First World War, the Germans used chlorine gas as a chemical weapon against the allied forces.
Chlorine is most commonly used in wastewater treatment for disinfection.
In the activated sludge phase, it is used to monitor odours and filamentous species.
Despite this, it is most widely used in disinfection methods of preventing the spread of waterborne diseases.
Production and Use of Chlorine
Here is a brief on Chlorine production and use (some main methods and applications).
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Typically, rock salt deposits are mined; on rare occasions, water is pumped down, and brine containing around 25% sodium chloride is brought to the surface. Impurities separate first and can be absorbed as the brine evaporates. In warm climates, salt is made by the Sun evaporating shallow seawater, resulting in bay salt.
Chlorine is processed on a large scale using a variety of methods, including:
1. Electrolysis of Concentrated Sodium Chloride Solution in Water:
The cathode produces hydrogen, while the anode produces Chlorine. Since sodium hydroxide is formed in the electrolyte simultaneously, this process is known as chlorine-alkali electrolysis.
The following equations describe the chemical reactions that occur at each electrode as well as the overall cell process:
At Cathode (Iron Cathode) = 2H20 + 2e- → 2OH + H2
At Anode (Graphite Anode) = 2Cl- → Cl2 + 2e-
Cell Process = 2H20 + 2Cl- → 2OH- + H2 + Cl2
The symbol e- represents a single electron. Free Chlorine and hydroxide ions should not come into contact in the reaction tank; otherwise, Chlorine would be absorbed due to the reaction.
Cl2 + 2OH- → (ClO)- + Cl- + H2O
One can insert a porous wall between the electrodes to separate the chlorine gas. The hydroxide ion (diaphragm process), or the iron cathode, is substituted with a mercury cathode (mercury cathode process), which prevents the formation of hydroxide ions at the electrode. Instead, at the cathode, free sodium is discharged, and this metal readily dissolves in mercury, forming an amalgam, as shown below:
2Na+ + 2e- ⇔ 2Na (amalgam)
The amalgam can then react with the water outside the cell as:
2Na (amalgam) + 2H2O → 2Na+ + 2OH- + H2
This entire process is equivalent to the cell process.
2. Electrolysis of Fused Sodium Chloride:
It also contains metallic sodium, and at the anode, Chlorine is emitted once more.
3. Electrolysis of Fused Magnesium Chloride:
Chlorine is generated as a by-product of the Production of metallic magnesium in this process.
4. Hydrogen Chloride's Oxidation:
As seen in the following equation, gaseous hydrogen chloride mixed with air or oxygen is passed over pumice in contact with cupric chloride as a catalyst:
4HCl + O2 (in presence of catalyst) ⇔ 2H2O + 2Cl2
With increasing temperature, the equilibrium constant for this reaction decreases, implying that the reaction continues more slowly at higher temperatures. However, to achieve a fair conversion rate, a temperature of 400 °C (750 °F) is needed in practice.
5. The Reaction Between Solid Chloride and Manganese Dioxide:
The method of producing Chlorine from a mixture of almost any solid chloride and manganese dioxide (MnO2) when heated with concentrated sulfuric acid (H2SO4) is historically interesting. The following is how the reaction happens:
2NaCl + 3H2SO4 + MnO2 ⇔ MnSO2 + 2NaHSO4 + 2H2O + Cl2