FORMATION AND PROPERTIES OF HYDROGEN HALIDES 169The primary reaction in this experiment,Nal + H2SO4 -» NaHSO* + HIis of the same nature as that in the preceding two experiments.
That hydrogen iodide escapes abundantly from the reaction mix-
ture is attested by the fogging of the breath, by the reddening of
litmus, and by smoke with ammonia. But that hydrogen iodide
enters more extensively into a secondary reaction with sulphuric
acid is shown by the abundance of the secondary products. These
consist of free iodine, shown by the purple vapor and the black
crystalline deposit; hydrogen sulphide, shown by the darkening
of lead acetate paper (PbAc 2 + H 2 S -> PbS + 2HAc), and the
odor; sulphur, shown by the light yellow deposit. Their forma-
tion can be accounted for by the equations:
H2SO4 + 8HI -» H 2 S + 4H 2 O + 4Ij
H 2 S + I, -» 2HI + SHydrogen sulphide and iodine do not react in the gaseous condi-
tion, but where they dissolve in the film of moisture condensed on
the cooler walls of the tube they react and this accounts for the
deposit of sulphur.
This experiment, taken in conjunction with the preceding two,
shows the gradation in properties of the hydrogen halides. The
activity of the halogens decreases in the order: fluorine, chlorine,
bromine, iodine. Hydrogen fluoride is so stable therefore that no
chemical substance can displace fluorine, and hydrogen fluoride
is unable to act as a reducing agent in any circumstance. Hy-
drogen chloride can act as a reducing agent upon the most powerful
oxidizing agents. Hydrogen bromide can reduce the compara-
tively weak oxidizing agent sulphuric acid, but it can only carry
the valence of the sulphur down to +4 in sulphur dioxide. Hy-
drogen iodide can reduce sulphuric acid much more completely,
carrying the valence all the way from +6 down to the lowest
possible, namely —2 in hydrogen sulphide.
- Hydrogen bromide and hydrogen iodide might be
obtained pure from their salts, provided the latter were
treated with a non-volatile acid which would not at the same
time behave as an oxidizing agent.