168 NON-METALLIC ELEMENTS IN BINARY COMPOUNDSThe failure to darken lead acetate indicates absence of hydrogen
sulphide.
The principal reaction in this experiment is similar to that in
the preceding experiment:
NaBr + H 2 SO 4 -> NaHSO 4 + HBr
The properties of hydrogen bromide are very similar to those of
hydrogen chloride but differ in that hydrogen bromide reduces
sulphuric acid to form sulphur dioxide and is itself oxidized to free
bromine.
H 2 SO 4 + 2HBr-> H 2 SO 3 + H 2 O + Br 2
H 2 SO 3 -» H 2 O + SO 2
The major part of the hydrogen bromide, it is true, escapes
from the reaction mixture unaffected, but on bubbling up through
the concentrated sulphuric acid a small part of it is oxidized ac-
cording to the secondary reaction which accounts for the red
color of free bromine.
It is recalled from Experiment 11 that bromine is a less active
element than chlorine; hence it is but natural that we should
find it here displaced from its hydrogen compound by a weaker
oxidizing agent. Chlorine is displaced from hydrogen chloride
by strong oxidizing agents, MnO 2 , PbO 2 , K 2 Cr 207 , KMnO 4 , but
bromine is displaced by the comparatively weak oxidizing agent
H 2 SO 4 which has no action on hydrogen chloride. The sulphur
of the sulphuric acid is reduced from valence +6 to valence
+4 in sulphur dioxide.
- Hydrogen Iodide. Add 1 cc. of 36 N sulphuric acid
to 0.5 gram of powdered potassium or sodium iodide, and
apply all the tests enumerated in Experiments 12 and 13;
also inspect the walls of the test tube carefully to see if any
solid sulphur condenses.
As in Experiments 12 and 13 the solid effervesces in the con-
centrated sulphuric acid and the gas evolved fogs the breath
(even more markedly in this case), turns litmus red, and gives
a dense smoke with ammonia. When the tube is warmed,
the beautiful purple iodine vapor is seen inside and nearly
black crystals collect on the cooler upper walls. Lead acetate
paper is colored dark brown. Sometimes a powdery light
yellow substance (sulphur) is seen collecting on the walls of
the tube.