Chemical Reactivity towards hydrogen

i. Group 16 elements : The elements of group 16 react with hydrogen to form hydrides of the type $\mathrm{H_2E}$ (where E = O, S, Se, Te, Po) — for example $\mathrm{H_2O}$, $\mathrm{H_2S}$, $\mathrm{H_2Se}$, $\mathrm{H_2Te}$ and $\mathrm{H_2Po}$. Some properties of the hydrides of group 16 are given in Table 7.5.

$\mathrm{H_2O}$ is a colourless, odourless liquid, while $\mathrm{H_2S}$, $\mathrm{H_2Se}$, $\mathrm{H_2Te}$ and $\mathrm{H_2Po}$ are colourless, bad-smelling, poisonous gases at ambient conditions. All the hydrides have angular structures which involve $\mathrm{sp^3}$ hybridisation of the central atom (E).

The hydrides of group 16 elements are weakly acidic. The acidic character of the hydrides increases, while the thermal stability decreases, from $\mathrm{H_2O}$ to $\mathrm{H_2Te}$. This is due to the decrease in the bond dissociation enthalpy of the H-E bond down the group (Table 7.5). All the hydrides except $\mathrm{H_2O}$ possess reducing property, which increases in the order $\mathrm{H_2S < H_2Se < H_2Te}$.

Table 7.5 Properties of hydrides of group 16 elements

ii. Group 17 elements : The elements of group 17 react with hydrogen to give hydrogen halides:

Some of the properties of hydrogen halides are given in Table 7.6. The acidic strength of the halogen acids increases in the order:

This is due to the decreasing bond dissociation enthalpy of the H-X bond in the order $\mathrm{HF > HCl > HBr > HI}$. The thermal stability of the hydrogen halides decreases in the same order, $\mathrm{HF > HCl > HBr > HI}$ — again a consequence of the decrease in bond dissociation enthalpy of the H-X bond down the group.

Table 7.6 Properties of hydrides of group 17 elements. …