Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
potassium iodide : KI
hydrogen peroxide : H2O2
sodium thiocyanate : NaSCN
ferrous sulfate : FeSO4 . 7H2O
hydrochloric acid : HCl
sodium nitrite : NaNO2
Class:
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elem=N
redox
Summary:
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Thiocyanate reacts with nitrogen dioxide to form a red/brown compound. It
does not react with nitrogen monoxide. When a reaction occurs with NO2,
a white fume is produced.
Description:
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Create some NO-gas as follows:
- Put some solid NaNO2 in a syringe of 50 ml and leave a little bit
of air in the syringe (appr. 5 ml).
- Dissolve some ferrous sulfate in dilute HCl (appr. 5 % by weight).
- Suck some of the ferrous sulfate solution in the syringe and quickly
turn the syringe with the tip upwards and cap the tip with a finger.
When the syringe is shaken, a gas is evolved and the pressure of the
gas increases the volume inside the syringe.
- When evolution of gas ceases, put syringe with tip pointing downwards
under water and press out the green solution from the syringe, without
pressing out any of the gas. Suck in some clean water and shake. Repeat
the process of pressing out the liquid and replacing with clean water.
Finally we have appr. 30 ml of a colorless gas mixture, rich in NO
and with clean water in the syringe.
Create some O2-gas as follows:
- Put some solid KI in a second syringe of 50 ml and leave a little bit
of air in the syringe (appr. 5 ml).
- Suck up a few ml of H2O2 (appr. 5% by weight) and quickly turn the syringe
with the tip upwards and cap the tip with a finger. When the syringe is
shaken, some oxygen is evolved. In appr. 2 minutes appr. 20 ml of almost
pure oxygen was obtained this way. The liquid becomes light yellow/brown.
- Cleaning the liquid in the syringe is done in the same way as for the
NO-syringe.
Both syringes are stored with appr. 20 ml clean water in them in a beaker
of water and the tip pointing downwards. The gas is above the water in the
syringes.
Sequence 1:
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Dissolve some sodium thiocyanate in some water and add a few ml of dilute
HCl (appr. 10% by weight) and add water, until the test tube is completely
filled. Stopper this test tube (without any air inside it) with a rubber
cap and keep upside down in a large tray, filled with water. When under
water, open up the test tube, such that the tip of the syringe with NO-gas
is under it and quickly bubble appr. 10 ml of NO-gas in the test tube and
then quickly cap the test tube again in order to avoid excessive loss of
the thiocyanate/HCl solution: The gas does not dissolve in the liquid, the
liquid remains almost colorless (just a very faint reddish/pink color can
be observed).
Again open the test tube under water (with the remaining liquid and the
NO-gas above it) and quickly bubble the oxygen in the test tube: The gas
mixture above the liquid inside the test tube becomes brown. The volume
of the gas mixture quickly decreases and water is sucked into the test
tube. When the test tube is capped quickly, then the pressure inside the
test tube drops. At the surface of the liquid, a dark brown/red compound is
formed, which is much more dense than the rest of the liquid. This dark
compound sinks to the bottom and diffuses with the liquid. When shaken, the
liquid becomes reddish/brown.
The gas mixture becomes turbid, a white fume is formed inside the gas mixture.
When the test tube is shaken, then the fume disappears again and the gas
mixture becomes colorless. When the cap of the test tube is removed under
water, then again a fairly large amount of water is sucked into the test
tube.
Sequence 2:
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Dissolve some NaSCN in some water and put this water in an erlenmeyer of
appr. 100 ml volume. Total volume of liquid is appr. 10 ml. Put the syringe
with remaining NO-gas with the tip inside the erlenmeyer and slowly press
the gas into the erlenmeyer: The gas mixture becomes brown and after a few
seconds a white fume can be observed.
Cap the erlenmeyer with a rubber stopper and gently swirl the liquid inside
the erlenmeyer (with the brown turbid gas mixture above it): It looks as if
at the surface of the liquid the fume becomes much more dense. Just above
the surface of the liquid the fume is so dense, that a few cm. of the gas
mixture is almost opaque. At the top of the erlenmeyer (just below the rubber
stopper), the gas mixture still is fairly clear.
The brown color of the gas mixture completely disappears.
The liquid becomes red/brown.
After a while, the fume becomes less dense and after a few minutes, the
liquid still is red/brown and a colorless gas mixture is above the liquid,
with a faint turbidity left over from the white fume.
After a few hours the liquid has become slightly lighter and it is yellow/
brown. After a few days, the liquid has become colorless and a bright yellow
precipitate is on the bottom of the erlenmeyer. The gas mixture and the
liquid have become odorless after a few days.
Decant the liquid and add a lot of water: The precipitate does not change,
not does it dissolve.
Let precipitate settle and decant most of the water. To the remaining liquid
with the yellow precipitate some solid NaOH is added: The NaOH dissolves and
the precipitate becomes a little bit less bright. It does not dissolve.
Let the liquid with dissolved NaOH stand for a few weeks, with air-contact:
The liquid becomes very light yellow and a little amount of precipitate
still does not dissolve. The liquid was shaken every few days, but this
did not help in dissolving the slight amount of precipitate.
Add some dilute HCl (appr. 10% by weight) after a few weeks: When the acid
mixes with the liquid, a fairly large amount of a colorless and odourless
gas is evolved. The resulting liquid is almost colorless (very light yellow)
and a little bit turbid (due to the small amount of precipitate).
The gas probably is CO2, due to standing of the NaOH-solution in air for a
few weeks.
Conclusion:
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Thiocyanate reacts with NO2 (formed from NO and O2) building a red/brown
compound in water and forming a white fume in the gas mixture.
With NO, no reaction occurs. This is shown by sequence 1.
The red/brown compound is not stable and decomposes, forming a yellow compound,
which precipitates.