Synthetic Inorganic Chemistry

EXPERIMENTS 349

In (a) the oxygen of the air oxidizes MnO 2 to MnO 3 , an acidic
oxide, in the presence of a base which will form a stable salt with
the acidic oxide.

MnO 2 + |O 2 -> MnO 3

MnO 3 + Na 2 CO 3 -»• Na 2 Mn0 4 + CO 2

In (6) the Cr 2 O 3 is oxidized to the acidic CrO 3 which gives the

yellow salt Na 2 CrC>4.

7. Permanganate, (a) Add 6N HNO 3 , drop by drop, to
   the solution of Na 2 Mn0 4 reserved from Experiment 6 (c).
   The intense green color changes to the equally intense purplish
   red of permanganate. Let the solution settle and carefully
   pour off the red liquid; a very small dark brown precipitate is
   found in the bottom. Reserve the solution for (6).
   (6) To the red permanganic acid solution reserved in (a),
   add 6 N NaOH until it is strongly alkaline. The color changes
   back to the intense green of the manganate.
   (c) To 1 drop of Mn(NO 3 ) 2 solution and 10 cc. 6N HN0 3
   in a test tube add 1 gram of lead dioxide and boil. Let the
   surplus of lead dioxide settle, and note that the clear solution
   has the intense permanganate color.

In (a), nitric acid displaces manganic acid from its salt:

2HNO 3 + K 2 Mn0 4 -» H 2 Mn0 4 + 2KNO 3

The free manganic acid is very unstable and decomposes

3H 2 Mn0 4 -> 2HMnO 4 + MnO 2 + 2H 2 O

or

3Mn^6 -»• 2Mn^7 + Mn*

In (6) the instability of permanganate ion in presence of OH~

ions is shown

2MnO 4 ~ + 2OH" -»• 2MnO 4 ~~ + H 2 O + §O 2

In (c) in an acid solution the very strong oxidizing agent PbO 2

oxidizes the manganous ion Mn++ to the permanganate ion MnC>4~

2Mn(NO 3 ) 2 + 5PbO 2 + 6HNO3 -»• 2HMnO 4

+ 5Pb(NO 3 ) 2 + 2H 2 O