Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
silver nitrate : AgNO3
Class:
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elem=Ag
precipitation
redox
Summary:
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The peroxosulfate ion ([S2O8]2-) is capable of oxidizing silver (I) to
silver (II) or silver (III). A higher oxide of silver is precipitated.
This oxide is easily decomposed under the liberation of oxygen.
Description:
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Add some solid Na2S2O8 to a solution of AgNO3 (2.5% by weight): The liquid
slowly turns grey, after a few minutes it is dark grey. A dark precipitate
slowly sinks to the bottom of the test tube.
Heat the liquid until it gently boils: The liquid turns even darker (almost
black). Suddenly the liquid starts foaming vigorously. A lot of a gas is
evolved (oxygen). After a few minutes the dark grey precipitate disappears
again and the evolution of gas ceases. Finally the liquid is almost clear
and colorless.
Add a little more Na2S2O8 to the still warm clear liquid: The liquid
immediately turns dark grey (almost black) again and it starts bubbling
again. On cooling down, the bubbling ceases and after a few hours the
liquid is clear and a dark grey precipitate is on the bottom of the
test tube. When the test tube is moved carefully, part of the precipitate
moves to the surface of the liquid. It has many bubbles attached to it.
It appears that the precipitate slowly generates oxygen, even at room
temperatures.
Remark:
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This reaction shows that peroxosulfate is capable of oxidizing silver (I)
to an higher oxidation state. The dark grey solid, which is formed, seems
to be Ag2O2, having the constitution [Ag+ Ag+++ O-- O--]. The latter
compound has average oxidation state 2 for the silver it contains, but
research shows strong evidence that silver (I) and silver (III) are mixed
in the solid at a 1:1 ratio.