The Foundations of Chemistry

Hydrogen halides can be prepared by combination of the elements.

H 2 
X 2 88n2HX(g) XF, Cl, Br, I

The reaction with F 2 to produce HF is explosive and very dangerous. The reaction
producing HCl does not occur significantly in the dark but occurs rapidly by a photo-
chemical chain reactionwhen the mixture is exposed to light. Light energy is absorbed
by Cl 2 molecules, which break apart into very reactive chlorine atoms, which have unpaired
electrons (radicals). These subsequently attack H 2 molecules and produce HCl molecules,
leaving hydrogen atoms (also radicals). The hydrogen radicals, in turn, attack Cl 2 mole-
cules to form HCl molecules and chlorine radicals, and the process continues.

This chain reaction continues as long as there is a significant concentration of radicals.
Termination stepseliminate two radicals and can eventually terminate the reaction.

The reaction of H 2 with Br 2 is also a photochemical reaction. That of H 2 with I 2 is
very slow, even at high temperatures and with illumination.
All hydrogen halides react with H 2 O to produce hydrohalic acidsthat ionize.

The reaction is essentially complete for dilute aqueous solutions of HCl, HBr, and HI.
In dilute aqueous solutions; HF is a weak acid (Ka7.2 10 ^4 ). In concentrated solu-
tions more acidic dimeric (HF) 2 units are present (Figure 24-1). They ionize as follows.

HHHOO

H 


HH

XX

Cl

H 2

Cl 2 termination steps

HCl

Cl Cl

H

H H

Cl 2

H 2

Cl 2

2Cl initiation step

chain propagation steps

HCl

HCl

 
H

Cl

Cl

H

h

24-6 The Hydrogen Halides and Hydrohalic Acids 949

A photochemical reaction is one in

which a species (usually a molecule)

interacts with radiant energy to

produce very reactive species. These

then undergo further reaction.

Concentrated solution (HF) 2

Figure 24-1 Hydrogen bonding

(dashed lines) in dilute and

concentrated aqueous solutions

of hydrofluoric acid, HF.

Dilute solution