172 NON-METALLIC ELEMENTS IN BINARY COMPOUNDS
to free iodine which is quite soluble in an iodide solution giving a
brown solution
2H+ + 21" + £O 2 -» H 2 O + I,
In the course of this reaction hydrogen ions are used up; it is
therefore obvious that the presence of hydrogen ions will aid in
the displacement of iodine ions by oxygen. In a neutral potas-
sium iodide solution no iodine is observed and it is thus apparent
that the help of hydrogen ions is necessary to accomplish the
liberation of iodine.
Chlorine and bromine are not liberated by the action of air on
hydrochloric acid and hydrobromic acid, or on neutral solutions
of chlorides and bromides.
- Add a few drops of iodine solution to a little hydrogen
sulphide water. The brown iodine solution is immediately
decolorized and a white cloudiness (precipitate of sulphur)
appears in the solution.
Observe the precipitate which slowly forms in the bottle
of hydrogen sulphide water to which air has some access.
Explain the reaction in each case, and place sulphur,
oxygen, chlorine, bromine, and iodine in the order of their
chemical activity in acid solutions.
From these experiments it is seen that both iodine and oxygen
are more active than sulphur. The reactions are quite certainly
ionic displacements although it is rather complicated to represent
them in intersecting ionic equations.
H 2 S + I 2 = S 1 + 2HI
H 2 S + K> 2 = S 1 + H 2 O
From this experiment and the preceding one we should conclude
that the non-metallic elements fall in the order F, Cl, Br, 0, I, S
(fluorine being strongest) with respect to their activity in aqueous
solutions containing free acid. This is approximately the order of
the electromotive series for these non-metals. If the solution is
made neutral the electromotive potential of oxygen is lowered so
that the oxygen is no longer able to displace iodine.
In dry gaseous mixtures the order of activity of the non-metals
is somewhat altered, oxygen moving up ahead of bromine and
chlorine, and sulphur and iodine changing places: F, 0, Cl, Br,
S, I.