Modern inorganic chemistry

GROUP VII: THE HALOGENS 327

Table 11.3

PROPERTIES OF THE HYDROGEN MAUDES

HF HC1 HBr HI

m-P-(K)

b.p.(K)

Enthalpy of formation

(kJmoP^1 )

Bond dissociation energy

(kJmor^1 )

Dielectric constant of liquid

190

293

-269

566

66

159

188

-92.3

431

9

186

206

-36.2

366

6

t.4.4*•vn

238

+ 26.0

299

3

page 52. The abnormal behaviour is attributed to hydrogen bond-
ing which causes association of hydrogen fluoride molecules.
In the solid state hydrogen fluoride exists as an infinite zig-zag
chain of molecules. Association also occurs in the liquid and
gaseous phases and in the latter phase, investigations indicate the
presence of (HF) 2 molecules and also more highly associated forms
existing not only as chains but also as rings, for example (HF) 6.
The ability to form hydrogen bonds explains the formation of
complex ions such as HF^ and H 2 p3 when a fluoride salt, for
example potassium fluoride, is dissolved in aqueous hydrofluoric
acid:

KF + HF ^ KHF 2

This reaction can be reversed by heating and is a convenient method

of obtaining anhydrous hydrogen fluoride from an aqueous solution.

The dipole moments of the hydrogen halides decrease with

increasing atomic number of the hydrogen, the largest difference

occurring between HF and HC1, and association of molecules is not

an important factor in the properties of HC1, HBr and HI. This

change in dipole moment is reflected in the diminishing permittivity

(dielectric constant) values from HF to HI.

THERMAL STABILITY OF HYDROGEN HALIDES

The enthalpies of formation and hydrogen-halogen bond strengths

are given in Table 113. The formation of hydrogen fluoride from

its elements occurs with explosive violence; the hydrogen-fluorine

bond produced is extremely strong (H—F = 566 kJ mol"^1 , cf.

C—C in diamond 356 kJ mol~ *) and stable to heat up to very high

temperatures. Both chlorine and bromine undergo a photochemical

chain reaction with hydrogen. The hydrogen-halide bond strength