284 GROUP VI
solution, making the reactions valuable in the detection of metal
cations in aqueous solution.TESTS FOR HYDROGEN SULPHIDE- Its smell.
- The blackening of filter paper, moistened with a soluble lead(II)
salt (e.g. the ethanoate or nitrate), by the formation of lead(II)
sulphide.
Hydrogen polysulphides or sulphanesCompounds of hydrogen and sulphur, with a higher proportion of
sulphur than in hydrogen sulphide, have been obtained as yellow
oils by adding acids to the polysulphides of metals. They are un-
stable, decomposing into sulphur and hydrogen sulphide and thus
making analysis difficult; however, sulphanes H^ (x = 3 to 6) have
been obtained in a pure state.Hydrogen selenide (selenium hydride), H 2 Se, and hydrogen telluride
(tellurium hydride), H 2 TeThese two gases can readily be prepared by the action of acids on
selenides and tellurides respectively, the reactions being analogous
to that for the preparation of hydrogen sulphide.
These gases have lower thermal stabilities than hydrogen sulphide
as expected from their enthalpies of formation (Table 10.2) and they
are consequently more powerful reducing agents than hydrogen
sulphide.
Since the hydrogen-element bond energy decreases from sulphur
to tellurium they are stronger acids than hydrogen sulphide in
aqueous solution but are still classified as weak acids—similar
change in acid strength is observed for Group VII hydrides.
Many of the reactions of these acids, however, closely resemble
those of hydrogen sulphide, the main difference being one of degree.
Polonium hydride, H 2 PoThis has been made in trace quantities by the action of dilute
hydrochloric acid on magnesium plated with polonium. As expected,
it is extremely unstable and decomposes even at 100K.