Modern inorganic chemistry

GROUPVI 269

COMPOUNDS OF GROUP VI ELEMENTS

HYDRIDES

All Group VI elements form a hydride H 2 X. With the notable excep-

tion of water, they are all poisonous gases with very unpleasant

smells. Table 10.2 gives some of their important physical properties.

Table 10.2

HYDRIDES OF GROUP IV ELEMENTS

Property H 2 O H 2 S H 2 Se H 2 Te

Formula weight

m.p. (K)

b.p. (K)

Enthalpy of formation

(AHf.kJmor^1 )

Enthalpy of vaporisation

(Aff f , kJ moP >)

Mean thermochemical

bond energy for M — H

bond(kJmorl)

18.0

273

373

-285.9

40.7

467

34.0

188

213

-20.6

18.7

347

80.0

207

232

+ 77.5

19.3

276

129.6

225

271

+ 143

23.2

The properties of water are seen to differ greatly from the other

hydrides; the deviations can be largely explained by the formation

of hydrogen bonds between water molecules.

In addition to the hydrides of formula H 2 X, oxygen forms the

hydride H 2 O 2 , hydrogen peroxide, and sulphur forms a whole

series of hydrides called sulphanes. These are yellow liquids which are

thermodynamically unstable with respect to hydrogen sulphide and

sulphur.

Water, H 2 O

PHYSICAL PROPERTIES

The fact that water is a liquid at room temperature with high
enthalpies of fusion and vaporisation can be attributed to hydrogen
bond formation. The water molecule is shown in Figure 10.3.
Because of the presence of the lone pairs of electrons, the molecule
has a dipole moment (and the liquid a high permittivity or dielectric
constant).
In ice, there is an infinite three-dimensional structure in which the