Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium acetate : CH3COONa . 3H2O
sulphuric acid : H2SO4
sodium borohydride : NaBH4
chrome alum : KCr(SO4)2 . 12H2O
potassium chromate : K2CrO4
Class:
------
elem=Cr
redox
Summary:
--------
Chromium (III) is not reduced to chromium (II) by borohydride. Alkaline
chromium (VI) is only reduced slowly by borohydrode. On acidification the
reduction goes at once, but no further than the +3 oxidation state.
Description:
------------
Sequence 1:
-----------
Add a solution of NaBH4 to a solution of chrome alum: Formation of a greenish
grey precipitate. Also formation of quite some gas (H2).
Add some dilute sulphuric acid: The liquid becomes violet blue, the well
known color of aqueous Cr3+. A lot of gas is produced. Part of the liquid is
thrown out of the test tube.
Prepare a concentrated solution of sodium acetate and add part of the violet
liquid to the concentrated solution of sodium acetate: The liquid becomes
greyish green. No a red color can be observed.
Sequence 2:
-----------
Add a solution of NaBH4 to a solution of K2CrO4: The liquid remains yellow
and clear. Only slight evolution of gas.
Heat the liquid for a while at 60 C: The liquid slowly becomes turbid and
a little greenish. Reduction of the chromate occurs, but only very slowly.
Add the turbid yellow/green liquid to some excess dilute H2SO4 (appr. 5%
by weight): Violent production of a gas, the liquid becomes green and clear
at once.