Hydrogen Halide

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

Contrast properties

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.