Note on Hydrogen Halide

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Hydrogen Halide

Preparation

  • Hydrogen fluoride: It can be prepared by treating calcium fluoride (CaF2) with concentrated sulphuric acid (H2SO4).

$$CaF_2+H_2SO_4\longrightarrow CaSO_4+2HF$$

The vapors are dissolved in water to get hydrofluoric acid.

  • Hydrogen chloride: Hydrogen chloride is prepared by heating sodium chloride (NaCl) crystal with concentrated sulphuric acid (H2SO4).

$$NaCl+H_2SO_4\longrightarrow NaHSO_4+HCl$$

$$NaCl+ NaHSO_4\longrightarrow Na_2SO_4+HCl$$

Procedure

The crystals of sodium chloride are taken in a round bottom flask fitted with the delivery tube and thistle funnel. The concentrated sulphuric acid (H2SO4) is added to it when the mixture is heated, HCl gas is collected in the gas jar by upward displacement of air.

Preparation of HCl acid

Hydrogen chloride is highly soluble in water. In order to prepare HCl acid, care must be taken so that water should not be sucked back to the generated flask, otherwise that can cause an explosion. To avoid this anti-suction device is made by connecting inverted funnel to delivery tube by using rubber tube and the funnel is placed just over the surface of the water.

Preparation of hydrogen bromide [HBr]

Hydrogen bromide can be prepared by the action of red phosphorus and bromine. In this process, hydrogen bromide along with bromine in produced.

$$P_4+6Br_2\longrightarrow 4PBr_3$$

$$PBr_3+3H_2O\longrightarrow 3HBr+H_3PO_3] X\space 4$$

i.e $$P_4+6Br_2+12H_2O\longrightarrow 4H_3PO_3+12HBr$$

The mixture of gasses [HBr + Br2] is passed through ‘U’ shaped tube packed with glass beads and moist red phosphorus which absorbs excess bromine. It is collected by upward displacement of air.

HBr acid can be obtained by dissolving HBr gas in water using the anti-suction device.

Preparation of HI acid: same as HBr

Note:

HBr and HI can’t be prepared like HCl by using concentrated Sulphuric acid [H2SO4] because HBr and HI being strong reducing agent, reduces concentrated Sulphuric acid [H2SO4] to sulphur dioxide [SO2]

$$2HBr+H_2SO_4\longrightarrow 2H_2O+SO_2+Br_2$$

$$2HI+H_2SO_4\longrightarrow2H_2O+SO_2+I_2$$

Hence phosphoric acid [H3PO4] is used instead of concentrated Sulphuric acid [H2SO4] to prepare HBr and HI. Since phosphoric acid is non – oxidizing agent.

$$NaBr+H_3PO_4\longrightarrow NaH_2PO_4+HBr$$

$$NaI+H_3PO_4\longrightarrow NaH_2PO_4+HI$$

Properties of halogen

  • Acidic property: The acidic property decrease in order.

HI > HBr > HCl > HF

  • Action with halogens: Chlorine (Cl2) replaces bromine (Br2) and iodine (I2) from aqueous HBr and HI respectively. Similarly, bromine (Br2) replaces iodine (I2) from aqueous HI.

$$Cl_2+2HBr\longrightarrow 2HCl+Br_2$$

$$Cl_2+2HI\longrightarrow 2HCl+I_2$$

$$Br_2+2HI\longrightarrow 2HBr+I_2$$

  • Action with air:

$$4HCl+O_2\longrightarrow 2H_2O+Cl_2$$

$$4Hl+O_2\longrightarrow 2H_2O+2I_2$$

Test for halides [AgNO3 test]

  1. A curdy white ppt. soluble in NH4OH and reappears on adding HNO3 indicate the presence of chloride.

$$Cl^- +AgNO_3\longrightarrow AgCl+NO^-_3$$

$$AgCl+2NH_4OH\longrightarrow Ag(NH_3)_2Cl+2H_2O (Formation\space of\space diamine\space silver\space chloride)$$

$$Ag(NH_3)_2Cl+2HNO_3\longrightarrow Agcl+2NH_4NO_3$$

  1. A pale yellow ppt. difficultly soluble in NH4OH and reappears on adding HNO3 indicates the presence of bromide.

$$Br^-+AgNO_3\longrightarrow AgBr+NO_3^-$$

$$AgBr+2NH_4OH\longrightarrow Ag(NH_3)_2Br+2H_2O (Formation\space of\space diamine\space silver\space bromide$$

$$Ag(NH_3)_2Br+2HNO_3\longrightarrow AgBr+2NH_4NO_3$$

  1. A yellow ppt. indicates presence of iodide.

$$I^- +AgNO_3\longrightarrow AgI+NO_3^-$$

Comparison between HBr, HCl, and HI

HCl

HBr

HI

Colorless gas

Colorless gas

Colorless gas

Pungent suffocating smell

Pungent suffocating smell

Pungent suffocating smell

Soluble in water

More soluble in water

Most soluble in water

It is very stable

It is less stable

It is least stable

Acidic in aqueous solution

More acidic in aqueous solution

Most acidic in aqueous solution

React with metal oxide to give salt and water

React with metal oxide to give salt and water

React with metal oxide to give salt and water

Decompose carbonate and bicarbonate to give CO2 gas

Decompose carbonate and bicarbonate to give CO2 gas

Decompose carbonate and bicarbonate to give CO2 gas

Contrast properties

HCl

HBr

HI

Gives white ppt. with AgNO3

Gives pale yellow ppt. with AgNO3

Gives yellow ppt. with AgNO3

Does not give Cl2 gas with concentrated sulphuric acid (H2SO4)

Gives red vapor of Br2 with concentrated sulphuric acid (H2SO4)

Gives violet vapor of I2 with concentrated sulphuric acid (H2SO4)

It is not affected by sunlight and heat

Oxidized to bromine (Br2) n presence of sunlight

Decompose to Hydrogen (H2) and Iodine (I2) in presence of sunlight

It has no action on chlorine water and starch

Aqueous solution of HBr become orange-yellow with chlorine water and starch

Gives blue colored solution with chlorine water and starch

Uses of halogen acid

  • Hydrochloric acid (HCl) is used to prepare aquaregia.
  • Hydrobromic acid (HBr) is used to prepare bromides and bromine.
  • HI is used to prepare iodide salt and iodine.
  • HBr and HI are used as reducing agent.

Reference

poulse, tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

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

  • HCl is used to prepare aqua regia.
  • HBr is used to prepare bromides and iodides.
  • HI is used to prepare iodide salt and iodine.
  • HBr and HI are used as reducing agent.
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