How Is Chlorine Produced? Methods and Applications Explored
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
FAQs on Chlorine Production and Its Uses in Chemistry
1. What are the primary industrial methods for producing chlorine gas?
The main industrial method for chlorine production is the electrolysis of a sodium chloride solution (brine). This is known as the Chlor-alkali process. Two other methods, though less common, are Deacon's process and the electrolysis of molten NaCl. In the Chlor-alkali process, electrolysis of brine produces chlorine gas at the anode, hydrogen gas at the cathode, and an aqueous solution of sodium hydroxide (NaOH) as a co-product.
2. What are the key physical and chemical properties of chlorine?
Chlorine is a highly reactive element with distinct properties.
Physical Properties:
It is a greenish-yellow gas at room temperature.
It has a sharp, pungent, and suffocating odour.
It is about 2.5 times heavier than air and is moderately soluble in water.
Chemical Properties:
It is a powerful oxidising agent.
It reacts with most metals and non-metals to form chlorides.
With water, it forms hydrochloric acid (HCl) and hypochlorous acid (HOCl).
It exhibits a bleaching action due to oxidation.
3. What are the most important uses of chlorine in daily life and industry?
Chlorine has a wide range of applications due to its high reactivity and disinfectant properties. Key uses include:
Water Purification: It is used to disinfect drinking water and swimming pools by killing harmful bacteria and viruses.
Manufacturing: It is essential for producing polymers like Polyvinyl Chloride (PVC), as well as solvents such as chloroform (CHCl₃) and carbon tetrachloride (CCl₄).
Bleaching Agent: Used in the paper and textile industries to bleach wood pulp and fabrics.
Chemical Synthesis: It is used to manufacture dyes, drugs, and other important organic and inorganic compounds like bleaching powder (CaOCl₂).
Extraction of Metals: It plays a role in the extraction of metals like gold and platinum.
4. How does chlorine's bleaching action work, and is it permanent?
Chlorine's bleaching action is due to its oxidation property. When chlorine dissolves in water, it forms hypochlorous acid (HOCl). This acid is unstable and readily decomposes to release nascent oxygen [O].
Cl₂ + H₂O → HCl + HOCl
HOCl → HCl + [O]
This nascent oxygen is a very powerful oxidising agent that oxidises coloured substances into colourless ones. Since the bleaching is achieved through oxidation, the effect is permanent.
5. Why is the Chlor-alkali process the preferred method for industrial chlorine production?
The Chlor-alkali process is preferred for several reasons:
Raw Material Availability: The primary raw material, brine (concentrated NaCl solution), is abundant and inexpensive.
Valuable Co-products: The process yields two other highly valuable industrial chemicals: sodium hydroxide (caustic soda) and hydrogen gas. The sale of these co-products makes the overall process economically very efficient.
High Purity: Modern methods, especially the membrane cell process, produce very pure chlorine and sodium hydroxide, which are required for many industrial applications.
6. How can you test for the presence of chloride ions in a given salt sample?
The presence of chloride ions (Cl⁻) is confirmed by the silver nitrate test. The procedure is as follows:
Prepare an aqueous solution of the salt.
Acidify the solution with a few drops of dilute nitric acid (HNO₃). This is done to remove any interfering ions like carbonate.
Add a few drops of silver nitrate (AgNO₃) solution.
The formation of a curdy white precipitate of silver chloride (AgCl), which is insoluble in dilute nitric acid but soluble in ammonium hydroxide, confirms the presence of chloride ions.
7. What is the difference in the reaction of chlorine with cold, dilute NaOH versus hot, concentrated NaOH?
The reaction of chlorine with sodium hydroxide (NaOH) is a classic example of a disproportionation reaction where the products depend on the temperature and concentration.
- Cold and Dilute NaOH: Chlorine reacts to form sodium chloride (NaCl) and sodium hypochlorite (NaOCl).
2NaOH (cold, dilute) + Cl₂ → NaCl + NaOCl + H₂O - Hot and Concentrated NaOH: Chlorine reacts to form sodium chloride (NaCl) and sodium chlorate (NaClO₃).
6NaOH (hot, conc.) + 3Cl₂ → 5NaCl + NaClO₃ + 3H₂O
The key difference is the oxidation state of chlorine in the oxyanion formed: +1 in hypochlorite and +5 in chlorate.
8. What are some examples of poisonous gases that can be prepared using chlorine?
Chlorine gas itself is toxic, but it is also a key reactant in the synthesis of several highly poisonous gases. Important examples include:
Phosgene (COCl₂): A highly poisonous gas formed by the reaction of carbon monoxide and chlorine in the presence of sunlight.
Tear Gas (C₁₀H₅ClN₂): Prepared using chlorine in its synthesis process.
Mustard Gas (Cl-CH₂-CH₂-S-CH₂-CH₂-Cl): A chemical warfare agent produced from the reaction of ethene with sulphur monochloride, which is synthesised using chlorine.
9. Since chlorine is toxic and reactive, how is it safely handled and transported?
Handling and transporting chlorine requires strict safety protocols due to its hazardous nature. It is typically cooled and pressurised to be transported as a liquid in specially designed steel cylinders or tankers. Key safety measures include:
Using corrosion-resistant materials for containers and pipes.
Ensuring all fittings and valves are leak-proof.
Implementing advanced sensor systems to detect any leaks immediately.
Having emergency neutralisation systems (e.g., caustic soda scrubbers) in place at production and storage facilities.
10. What are the environmental concerns associated with chlorine and its compounds?
While chlorine is essential, its use raises environmental concerns. The primary issue is the formation of stable and often toxic organochlorine compounds. For example:
DDT (Dichlorodiphenyltrichloroethane): A powerful insecticide that is persistent in the environment and undergoes biomagnification in food chains.
CFCs (Chlorofluorocarbons): Once used as refrigerants, these compounds were found to deplete the stratospheric ozone layer.
Dioxins: Toxic byproducts formed during the manufacturing of certain chlorinated organic compounds and during the bleaching of paper pulp with chlorine.
These concerns have led to stricter regulations and the development of alternative, greener chemical processes.
