The literature values listed for the unknowns in this assignment are from either the Merck Index or the CRC Handbook, the two most used reference handbooks. However, the values tend to vary slightly across literature sources and sometimes the temperatures are given as ranges. Give at least one reason for the variations in these reported temperatures. In general, the boiling points of compounds increase down a group in the periodic table. The melting points and boiling points for the hydrogen compounds of group 6A elements are in the table below.
Melting point (0C) Boiling point (oC)
H2O 0.0 100.0
H2S -82.0 -60.0
H2Se -65.7 -41.2
H2Te -49.0 -2.2

See explanation

Explanation:

One of the important trends in the periodic table is electronegativity. Electro negativity decreases down the group and increases across the period. This trend has important consequences on the observed properties of the compounds of elements in a particular group in the periodic table.

As we move down in group 6A, the electro negativity of the elements elements the group decrease and as such, the magnitude of intermolecular hydrogen bonding between the molecules also decrease accordingly. Hydrogen bonds occur between

molecules of a substance when hydrogen is covalently bonded to an electronegative element. Hydrogen bonding is responsible for the high melting and boiling points of small molecules such as water which contain the highly electronegative oxygen atom.

So, as we move down the group there is lesser intermolecular hydrogen bonding between the hydride molecules of group 6A elements resulting in the observed trend in melting and boiling points of the hydrides.

The weaker hydrogen bonds that occurbetween molecules of group 6A hydrides lead to a steady decrease in melting and boiling points of the hydrides of group 6A elements as we move down the group.