Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium formiate : HCOONa
sodium sulfite hepta hydrate : Na2SO3 . 7H2O
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
formic acid : HCOOH
sulphuric acid : H2SO4
vanadyl sulfate : VOSO4.5H2O
vanadium pentoxide : V2O5
Class:
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elem=V
redox
Summary:
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Without the help of a strong acid, vanadium pentoxide is not capable of
oxidizing formic acid, not even when heated. When some sulfite is added,
then incomplete reduction of the V2O5 can be observed.
With the help of a strong acid and when in solution, vanadium (V) is
capable of oxidizing formic acid, but only very slowly.
With formic acid and vanadium (IV), apparently a coordination complex
is formed. Another explanation is given below at the end of the
description.
Description:
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Sequence 1:
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Add some solid V2O5 to HCOOH (85% by weight): No reaction.
Heat liquid and gently boil for a few minutes: No visible reaction. The solid
V2O5 remains at the bottom, with a colorless liquid on top of it.
Add some solid Na2SO3.7H2O to the still warm liquid: Liquid starts foaming
a little and it becomes yellow/green. Part of the vanadium pentoxide is
reduced now. The yellow/green color is caused by a mixture of blue [VO]2+
and yellow [VO2]+ or one of its condensation products.
Sequence 2:
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Dissolve some V2O5 in a solution of NaOH (some heating is required to do so):
A colorless and clear liquid is obtained.
Add some H2O2 (10% by weight): Liquid becomes yellow. It looks like a strongly
acidic pervanadyl solution, but this is a strongly alkaline peroxo-vanadium (V)
solution.
Add some dilute H2SO4 (2 mol/l): Liquid becomes dark brown, some gas is
evolved.
Add a large excess amount of HCOOH: No visible change. After one hour, the
liquid still is dark brown and clear.
Wait one day: Liquid has changed color, it now has a hard to describe color,
something best described as a mixture between mossgreen/grey/brown. The liquid
is clear.
Two days later: Liquid is moss green, with a bluish hue.
Heat part of the liquid and boil gently: Liquid becomes green/blue, more green
than blue. The color is not really brilliant, it is a somewhat dull color,
certainly not to be compared with a bright cyan color.
Keep both the cold part and the boiled part of the liquid for two weeks:
Both liquids shift more towards blue, but the bright blue color of vanadyl
([VO]2+) is not obtained, a dull green compound remains in the color of the
liquid and the intensity of the color is somewhat higher than the intensity
of a plain vanadyl solution.
Sequence 3:
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Prepare an acidic solution of vanadyl sulfate and sulphuric acid and add
some sodium formiate to this: The solid partly dissolves, liquid remains
bright blue, also when heated for a while.
Add some H2O2 (10%) to the still hot liquid: Liquid at once becomes yellow
and foams fairly strongly for a short while. The solid now completely dissolves
and the liquid becomes clear.
Further heating of the liquid: Liquid remains yellow and more gas is evolved.
Add some sulphuric acid (appr. 2 mol/l): Liquid becomes brown/red and remains
clear. Some gas is evolved at a low rate. This evolution of gas remains for
all 20 minutes, during which an eye was kept on the liquid. Every time, when
the liquid is shaken, the evolution of gas becomes faster, but it slows down
again, when the liquid is put to rest again. The bubbling can be heard clearly.
Wait for two days: The liquid has become much lighter brown.
Wait another two days: The liquid has become green.
Wait another 10 days: The liquid has become blue, but not the original bright
blue of plain vanadyl.
Possible explanation:
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When formic acid is involved in reduction of vanadium (V), then there
apparently is a need for H2O2. Without H2O2 no reduction can be observed.
The reduction product, however, appears to be not pure vanadyl, put a blue
compound or mixture. A possible explanation may be the following:
- vanadium (V) is not capable of oxidizing HCOOH.
- vanadium (V) forms peroxo compounds with H2O2
- peroxo compounds are capable of oxidizing HCOOH, forming vanadyl
very slowly
- part of the peroxo compound decomposes again to plain vanadium (V),
which together with the blue of vanadyl gives a blue/green liquid.
- the remaining part of the strange color is due to traces of peroxo
compound.