Description of experiment
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Needed compounds:
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sodium acetate : CH3COONa . 3H2O
cobalt sulfate : CoSO4.7H2O
Class:
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elem=C,Co
coordination
Summary:
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A solution of cobaltous acetate is hydrolysed when the solution is heated. When
the liquid cools down again, then the effect is reversed again. The visual
result is formation of a precipitate on heating and the slow redissolving of
the precipitate on cooling down.
Description:
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Dissolve some cobaltous sulfate in water: A pink/rose solution is obtained.
Dissolve quite some sodium acetate in water and add this to the solution of
cobaltous sulfate: The color changes. It becomes more intense and the color
also shifts more towards purple, it can best be described as red/purple.
Slowly heat the liquid until it boils: The color becomes more intense while the
liquid becomes hotter. It also shifts more towards purple. At a certain moment,
the liquid has a strong purple color. At that point it also becomes somewhat
turbid. If heating is continued further, then the liquid becomes quite dark
purple with a bluish hue and a blue flocculent precipitate is formed.
Allow the liquid to stand: A blue precipitate settles at the bottom and a clear
purple liquid is above the precipitate.
Allow the liquid to cool down completely: The liquid becomes lighter again and
its color again becomes red/purple, like the originally prepared cobaltous
sulfate/sodium acetate solution.
Shake the liquid and allow to stand for an hour or so: The liquid has become
completely clear again, all blue precipitate has redissolved.
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Apparently the acetato-complex of cobalt(II) hydrolyses on heating, leading to
formation of basic cobalt(II) acetate and free acetic acid. On cooling down
this process is reversed again. A possible explanation is that the
acetato-complex is more favorable at low temperatures and this favored complex
may prevent formation of a basic precipitate of cobalt(II).