Trends in boiling points, melting points and enthalpies of vaporization for p-block binary hydrides

It is generally expected that the melting and boiling points of members of a series of related molecular compounds increase with increasing molecular size, owing to an increase in intermolecular dispersion forces. This is seen, for example, along a homologous series of alkanes. However, a comparison of the melting and boiling points of p-block hydrides, EHn, provides evidence for hydrogen bonding. Figure 1.1 shows that, for E = group 14 element, melting and boiling points follow the expected trends, but for E = group 15, 16 or 17 element, the first member of the group shows anomalous behaviour, i.e. the melting and boiling points of NH₃, H₂O and HF are higher than expected when compared with their heavier congeners. Figure 1.2 illustrates that values of Δ_vapH show a similar pattern. It is tempting to think that Figures 1.1 and 1.2 indicate that the hydrogen bonding in H₂O is stronger than in HF; certainly, the values for H2O appear to be particularly high. However, this is not a sound conclusion. Boiling points and values of Δ_vapH relate to differences between the liquid and gaseous states, and there is independent evidence that while H2O is mhydrogen-bonded in the liquid but not in the vapour state, HF is strongly hydrogen-bonded in both. Deviations from Trouton’s empirical ruleare another way of expressing the data in Figures 1.1 and 1.2. For HF, H₂O and NH₃, Δ_vapS = 116, 109 and 97 JK^-1 mol^-1 respectively. Hydrogen bonding in each liquid lowers its entropy, and makes the change in the entropy on going from liquid to vapour larger than it would have been had hydrogen bonding not played an important role