EXPERIMENTS 2634Zn + 8HNO3 -»4Zn(NO 3 ) 2 + 8H
HNO 3 + 8H -»• 3H 2 O + NH 3
NH 3 + HNO3 -> NH4NO3
4Zn + IOHNO3 -»4Zn(NO 3 ) 2 + 3H 2 O + NH 4 NO 3
4Zn 0 to +2 4 X (+2) = +8
IN +5 to-3 = -8
Total change = 0
Explain how the final step in the experiment is an example of the
displacement of a weak base from its salt by a stronger base and
that it involves no change in the primary valence of any of the
elements.
- Nitrous Acid, (a) Dissolve 0.5 gram of sodium nitrite
in 5 cc. of ice water and add 1 cc. of cold 6 N H 2 SC>4. A blue
solution results which at 0° effervesces very slowly. Add a
few drops of this solution to a half test tube of water con-
taining a few drops of iodide-starch solution. An intense
blue color is produced. Add a few drops of sodium nitrite
solution alone, of sulphuric acid alone, and of nitric acid alone
to tubes made up with a similar amount of iodide-starch, and
note that in none of these cases is any color produced. (To
produce no effect the nitric acid must be free from nitrous
acid and it should be taken from a special bottle prepared for
this experiment.)
Allow the rest of the potassium nitrite-sulphuric acid mix-
ture to warm up to room temperature. Note that it effer-
vesces rather strongly, giving off a reddish brown gas, and that
the blue color quickly disappears.
Nitrous acid is of about the same strength as acetic acid, and it
is blue in color. It is formed when its ions are brought together.
It is very unstable, decomposing mainly according to the equation
3HNO 2 -»• HN0 3 + 2NO + H 2 O
the NO giving the reddish brown color when it comes in contact
with air. Nitrous acid is an oxidizing agent, liberating iodine
from hydriodic acid,
2HNO 2 + 2HI -> 2H 2 O + 2NO + I 2
Since nitric acid does not liberate iodine under similar circum-
stances, the nitrous acid is shown to be the more vigorous oxidizing