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When potassium dichromate is being used as an oxidising agent, it gets reduced from chromium(VI) to chromium(III). Is there any way to know that or calculate that other than being familiar with general oxidation states?

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    $\begingroup$ Long story short, no. $\endgroup$ – Ivan Neretin Aug 30 '19 at 5:45
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Similar to other redox reactions (e.g., permanganate [purple] to either manganate [greenish, at a basic pH] or $\ce{Mn^{2+}}$ [colorless, at an acidic pH]), the reduction of chromate to $\ce{Cr^{3+}}$ in aqueous solutions occurrs with a significant change of color of this solution. Even more in acidic solutions if you have dichromate being reduced to $\ce{Cr^{3+}}$. And you could infer «being there» since there are both stable compounds with chromate / dichromate, as well as $\ce{Cr^{3+}}$ you can dissolve in water as a visual reference to check with.

But:

  • it is not so easy to see if your other reagent equally dyes your solution

  • the color change does not tell you the oxidation state as a number. UV-Vis spectroscopy might hint you to which electronic transitions now become more probable to access, though.

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  • $\begingroup$ I would add that magnetic properties of the solution (if you know the concentration and all other ions are diamagnetic) can also give an indication about the oxidation state. $\endgroup$ – TAR86 May 19 at 5:32
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    $\begingroup$ In my question understanding, the OP is not interested in how to experimentally determine it, but rather if there is possibility to predict it without being familiar with chromium chemistry, just from basic principles. Unless one is a quantrum chemistry magician, I would refer to the Ivan's comment. I guess it is a Holy Graal of chemistry students - How to avoid a lot of empirical knowledge ? $\endgroup$ – Poutnik May 19 at 6:19
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That is the colour change. There are several reactions of colour change as you are probably more familiar with the orange dichromate(VI) ion ($\ce{Cr2O7^{2-}}$) than the yellow chromate(VI) ion ($\ce{CrO4^{2-}}$) . Changing between them is easy. If dilute sulfuric acid is added to the yellow solution, it turns orange. If you add sodium hydroxide solution to the orange solution it turns yellow. There are more reaction of this you can find on internet and this reaction itself is elaborated in the following manner: \begin{align} \ce{H2SO4 + 2Na2CrO4 &-> Na2Cr2O7 + Na2SO4 + H2O}\\ \ce{Na2Cr2O7 + 2NaOH &-> 2Na2CrO4 + H2O} \end{align}

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