Identify the oxygen atoms that undergo reduction in the reaction below:

$$\ce{O2 + Cr^3+ -> H2O2 + Cr2O7^2-}$$

I know that the oxygen atoms in $\ce{O2}$ have an oxidation state of $0$.

In the product $\ce{H2O2}$, the oxygen atoms have an oxidation state of $-1$; and in $\ce{Cr2O7^2-}$, oxygen atoms have an oxidation state of $-2$.

Based on how the problem was solved on the answer key it was assumed that all the oxygen atoms from $\ce{O2}$ are reduced to peroxide atoms.

How do we know that there are no oxygen atoms that are oxidized to become chromate oxygens with a $-2$ oxidation state?

I understand that there are other $\ce{H2O}$ oxygen atoms available to provide the chromate oxygens that would change the oxidation state.

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    $\begingroup$ It would be nice if we started with a reaction that at least listed all of the component participating. Even nicer would be if that reaction were balanced. $\endgroup$
    – Zhe
    Commented Jan 24, 2017 at 15:28

1 Answer 1


The oxygens in the dichromate ion on the right have -2 oxidation number, the chromium being +6. http://ch302.cm.utexas.edu/worksheets/basic-redox-wkst-KEY.pdf https://socratic.org/questions/what-is-the-oxidation-state-of-chromium-in-the-dichromate-ion-cr2o7-2

As you seem to have deduced, the peroxide ions are -1 so all oxygens, as shown, have been reduced.

  • $\begingroup$ This does not answer the question. How do you know that the chromate oxygens do not come from dioxygen? $\endgroup$
    – DHMO
    Commented Jan 25, 2017 at 8:42

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