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HYDROHALIC ACIDS

Hydrohalic acids are volatile, covalent compounds which may be prepared by the direct reaction of the elements or, more conveniently, by the reaction of the respective halide ion with hydrogen ion. All of these compounds are Bronsted acids, readily donating protons to Bronsted bases.

NOTE: With the exception of HF, all hydrohalic acids are strong acids in aqueous solution. In liquid HF, the molecules are strongly hydrogen bonded, and HF has a relatively high boiling point. Vapour density determinations indicate that strong hydrogen bonding persists in gaseous HF.

The compounds HBr and HI are strong acids. Most other nonvolatile, strong acids in concentrated solution, H₂SO₄, HClO₄ etc. are also strong oxidizing agents, capable of oxidizing bromide and iodide ions. Therefore HBr is usually prepared from covalent halides.

They are characterised by

 

1. except hydrofluoric acid, which is a liquid, all other ate gases.

 

2. They fume in air and have a pungent odour

 

3. Their melting and boiling points are as follows

	HF	HCl	HBr	HI
MELTING POINT	-83	-111	-86	-50.8
BOILING POINT	19.5	-85	-67	-35.5

PREPARATION

1. HCl, HBr and HI can be obtained by the direct combination of elements

H₂ + X₂ è 2HX

2. Hydrochloric  acid can be prepared by treating a chloride with an acid

2NaCl + H₂SO₄ è Na₂SO₄ + 2HCl

3.HBr and HI are prepared by the hydrolysis of corresponding phosphorous halides

3KI + H₃PO₄ è K₃PO₄ + 3HI

3KBr + H₃PO₄ è K₃PO₄ + 3HBr

4.hydrogen fluoride can be prepared by the action of heat on potassium hydrogen fluoride.

2KHF₂ è K₂F₂ + H₂F₂

CHEMICAL PROPERTIES

1. Reducing nature

The reducing nature of halogen acids decreases from HI to HF. HF does not show any reducing properties.

4HI + O₂ è 2H₂O + 2I₂

H₂SO₄ + 2HI è SO₂ + I₂ + 2H₂O

4HBr + O₂ è 2H₂O + 2Br₂

H₂SO₄ + 2HBr è SO₂ + Br₂ + 2H₂O

2. Reaction with ammonia

HCl + NH₃ è NH₄Cl

HI + NH₃ è NH₄I

HBr + NH₃ è NH₄Br

3. Precipitation reactions

AgNO₃ + HCl è HNO₃ + AgCl

White

AgNO₃ + HBr è HNO₃ + AgBr

pale yellow

AgNO₃ + HI è HNO₃ + AgI

Yellow

USES

Hydrochloric acid	Hydrobromic acid	Hydroiodic acid	Hydrofluoric acid
Preparation of chlorine, chlorides and aqua-regia	In the manufacture of AgBr, NaBr and KBr	N the manufacture of KI, AgI	In the manufacture of fluorine
In medicines		As a reducing agent	In the etching of glass
Laboratory reagent	Laboratory reagent		As an antiseptic in the brewing industries.