Explain the trend in boiling points for the group 6 hydrides (O,S,Se,Te). Diagram would be included.
There are 2 factors that affect this problem.
(1) The hydrogen bonding ability of H2O which increases the boiling point.The partial charges, induced by the higher electronegativity of oxygen, mean that the hydrogens and oxygens will be attracted to one another by coloumbic forces. This in turn will increase the energy needed to pull the molecules apart, thus an increase in the boiling point is observed. Hydrogen bonding forces are stronger than van der Waals forces and thus, this effect is dominant. The other group 6 elements do not have a high enough electronegativity to induce hydrogen bonding.
(2) The increasing molecular weight of the other group 6 elements.With higher atomic numbers going down the group, there is an increase in valence electrons which in turn increases the probability that there are instantaneous imbalances in the electron sphere. This will induce van der Waals forces between molecules, increasing the energy needed to pull the molecules apart and thus an increase in the boiling point is observed.
Hence the Trend is; H2O > H2Te > H2Se > H2S
