Halogen

Halogens

Laboratory preparation of chlorine, bromine, and iodines

• By application of heat: chlorine, bromine, and iodine can be prepared by heating halide salt with MnO₂ and concentrated H₂SO₄.

$$MnO_2+conc. H_2SO_4\longrightarrow MnSO_4+H_2O+[O]$$

$$[NaX+H_2SO_4\longrightarrow NaHSO_4+HX]\space X\space 2$$

$$2HX+[O]\longrightarrow H_2O+X_2$$

These equatios results

$$2NaX+3H_2SO_4+MnO_2\longrightarrow MnSO_4+2NaHSO_4+2H_2O+X_2$$

Where X = Cl₂, Br₂, I₂

• Preparation of chlorine: Sodium chloride with little MnO₂ and concentrated H₂SO₄ is taken in a round bottom flask. On heating flask, chlorine gas is liberated and collected in a gas jar by upward displacement of air. The impurity like HCl from Cl₂ is removed by passing through water in wash bottle and then another wash bottle containing concentrated H₂SO₄ to remove moisture present in Cl₂.

• Preparation of bromine: A mixture of sodium or potassium bromide with little MnO₂ and concentrated H₂SO₄ is taken in a retort. When retort is heated various vapor of bromine evolve out which are passed into the receiver, cooled under ice water to get liquid bromine.

• Preparation of iodine: In the lab, iodine is prepared by heating potassium iodide, MnO₂ and concentrated H₂SO₄.The vapors of iodine liberated are condensed as solid in the inner wall of the cold dish.

https://www.youtube.com/watch?v=JZTpelNikP0

Manufacturer of bromine by carnallite process

Carnallite [KCl.MgCl₂.6H₂O] contains the small amount of KBr and MgBr₂ as impurities. It is first dissolved in water and resulting solution is concentrated where less soluble chloride salt separates out leaving behind more soluble bromide in the mother liquor. The hot mother liquor is about to trickle down to tower packed with earthen balls with a current of air up which current of chlorine is passed. Chlorine liberates bromine from bromide.

$$2KBr+Cl_2\longrightarrow 2KCl+Br_2$$

$$Mg+Br_2\longrightarrow MgCl_2+Br_2$$

Bromine thus liberated is converted to vapor. Bromine vapor travels up and then condenses in a spiral condenser to form liquid bromine. The vapor which is not condensed are absorbed by iron filings by forming ferrous - ferric bromide (Fe₃Br₈).

$$3Fe+4Br_2\longrightarrow Fe_3Br_8$$

Purification

Bromine thus obtained contains chlorine and iodine as impurities which are removed by distillation with KBr and ZnO respectively.

$$2ZnO+I_2\longrightarrow 2ZnI+O_2$$

$$2KBr+Cl_2\longrightarrow 2KCl+Br_2$$

Manufacturer of iodine

From seaweeds

Iodine is manufactured from deep seaweeds especially laminaria variety which contains iodine in the form of alkali iodide (NaI & KI). First of all, seaweeds are collected dried and burnt carefully so that no iodine is lost by decomposition and sublimation and ash called kelp is obtained. It contains 0.4 % to 1.3 % iodine in the form of iodide along with chloride and sulphate. The kelp is then treated with water and the aqueous solution is crystallized out where chloride and sulphate are crystallized whereas more soluble NaI and KI remains I mother liquor. It is mixed with MnO₂ and concentrated H₂SO₄ and heated in the iron retort. Iodine vapors are liberated in a series of earthenwares condenser called aludels.

$$2KI+3H_2SO_4+MnO_2\longrightarrow 2K_2SO_4+MnSO_4+2H_2O+I_2$$

From caliche [crude chile salt pitre]

Crude chile salt pitre is NaNO₃. It contains 0.3 % to 0.4 % iodine in form of iodide salt. The mineral is treated with water and then crystallized where crystal of NaNO₃ separate out and NaIO₃ remains in the mother liquor. The mother liquor is treated with sodium bisulphate which reduces sodium iodide to free iodine.

$$3NaIO_3+5NaHSO_3\longrightarrow 3NaHSO_4+2Na_2SO_4+H_2O+I_2$$

Properties of halogen

• Action with water: fluorine reacts with vigorously with water.

$$H_2O+F_2\longrightarrow 2HF+O_2$$

$$3H_2O+3F_2\longrightarrow 6HF+O_3$$

Chlorine and bromine react with water forming hydrohalide acid and hypohalous acid.

$$X_2+H_2O\longrightarrow HX+HOX$$

Where X₂= Cl₂ and Br₂

HOX is unstable so it decomposes to give oxygen and hydrohalic acid in sunlight.

$$2HOX\longrightarrow 2HX+O_2$$

$$KI+I_2\longrightarrow 2KI_3 (Formation\space of\space potassium\space tri-iodide)$$

• Action with alkali: cold and dilute alkali forms oxy difluoride [OF₂] when reacted with fluorine.

$$NaOH+2F_2\longrightarrow OF_2+2NaF+H_2O$$

• Bleaching action: fluorine being most reactive destroys the substance to be bleached. Chlorine is good bleaching agent, bromine is mild, and iodine has no bleaching property.

$$Cl_2+H_2O\longrightarrow 2HCl+HClO$$

$$HClO\longrightarrow HCl+[o]$$

Colored matter + [O] → Colorless matter

• Reaction with chlorine and slaked lime: This reaction occurs to form bleaching powder (CaOCl₂).

$$Cl_2+Ca(OH)_2\longrightarrow CaOCl_2+H_2O$$

• Reaction with iodine with hypo (sodium thiphosphate): This reaction occurs to form sodium tetrathimate(Na₂S₄O₆).

$$I_2+Na_2S_2O_3\longrightarrow Na_2S_4O_6+2NaI$$

Uses

• Fluorine compound such as sodium fluoride and rhyolite are used as the insecticide.
• NF₃ and OF_2­ are used as rocket fuel.
• Chlorine is used as disinfectant and germicide.
• Chlorine is used as the bleaching agent.
• Silver bromide is used in photographic film.
• Bromine is used to manufacture dyes, drugs, and tear gas.
• Iodine is used as antiseptic in the form of tincture.

Reference

pulse, Tracy. Introduction to chemistry. u.s.a: flex book, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.