Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
hydrogen peroxide : H2O2
sodium sulfite : Na2SO3
zinc : Zn
hydrochloric acid : HCl
nitric acid : HNO3
rhenium : Re
Class:
------
elem=Re
redox
Summary:
--------
Rhenium is oxidized easily by nitric acid. The oxidation product is a color-
less compound, soluble in water (according to literature it is perrhenic
acid, HReO4). Perrhenic acid is not a really strong oxidizer. It can be
reduced by zinc, but addition of sodium sulfite does not result in formation
of the same compound.
Hydrogen peroxide is capable of oxidizing back to perrhenic acid, but some
light yellow compound remains. What is it?
Description:
------------
Preparation of perrhenic acid:
------------------------------
Add a very small amount (at most 0.5 ml) of HNO3, 52% by weight, to 50 mg
of powdered rhenium metal: The metal quickly dissolves in the acid. A fairly
large amount of brown NO2 is formed, the liquid becomes green. The green
color is not due to formation of a colored Re-compound, but it is due to
formation of NO2 and N2O3, which remain in solution. This green color also
is observed when other metals with colorless ions are dissolved in HNO3.
Dilute the liquid to approximately 2.5 ml: The liquid becomes colorless and
totally clear. This is a solution, containing perrhenic acid and nitric acid.
This solution is called PERRHEN.
Sequence 1:
-----------
Take some PERRHEN and mix with an equal volume of 10% HCl: The liquid remains
colorless.
Add some metallic zinc and constantly shake the test tube: The zinc dissolves,
while forming hydrogen gas. The liquid becomes luke-warm. While the zinc
dissolves, the liquid becomes more and more brown. When all zinc has dissolved,
the liquid is clear and brown with a sepia tinge.
Add some of this brown liquid to a concentrated solution of sodium hydroxide:
A light yellow/brown precipitate is produced, which redissolves on shaking.
Finally the liquid becomes clear and very light yellow. The precipitate mainly
is zinc hydroxide, which redissolves to zincate in the excess alkali.
Add a few drops of 10% H2O2 to the light yellow liquid: A white precipite is
formed again, which redissolves on shaking. The liquid, however, remains very
slightly turbid.
Add a few drops of 10% H2O2 to the brown acidic liquid directly: The brown
color disappears and the liquid becomes very light yellow. It certainly is
not colorless after adding the H2O2.
Sequuence 2:
------------
Add some zinc metal to PERRHEN (without adding HCl first): The observation is
quite similar to the observation of sequence 1. The liquid again becomes brown.
The reaction is much slower, and some fine black particles are produced as
well. This difference probably is due to the much lower concentration of acid.
Add a few drops of dilute HNO3 (appr. 2 mol/l): The zinc now dissolves much
faster and the liquid becomes clear and brown. The small particles, however,
do not dissolve anymore. These particles probably are metallic rhenium, formed
by adding the zinc in the only lightly acidic environment at the start of
this second sequence of experiments.
Add a few drops of H2O2, 10 % by weight: The brown color disappears and the
small black particles also quickly dissolve. The resulting liquid is clear,
but a faint yellow color remains.
Sequence 3:
-----------
Add some solid Na2SO3 to PERRHEN: The Na2SO3 dissolves and the liquid becomes
very light yellow. The liquid remains clear.
Add a few drops of dilute HNO3 (2 mol/l): The liquid slowly becomes colorless
again and a strong smell of SO2 can be observed. This indicates that there is
excess sulfite.
Remarks:
--------
A faint light yellow color can be observed in all the experiments. What is the
cause of the faint color? Is this a perrhenic acid compound, or is this Re2O7,
which remains unchanged in solution?
What is the brown compound, formed on reduction with zinc? Is this Re3+(aq)?