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The following reaction is not a complete reaction. Determine if the change is an oxidation, a reduction, or neither: $$\ce{CrO4^{-2} -> Cr2O7^{-2}}$$

I know that the answer to this problem is neither, but I don't have any idea as to how that answer is reached. Could anyone give me a hint? I am familiar with oxidation, reduction, and oxidation numbers.

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    $\begingroup$ If you are familiar with oxidation numbers, what are the oxidation numbers of the chromium atoms in each compound? $\endgroup$ – jerepierre Feb 26 '15 at 1:20
  • $\begingroup$ @jerepierre in the first compound +6, in the second compound +6 as well. $\endgroup$ – McB Feb 26 '15 at 1:22
  • $\begingroup$ @jerepierre Does this mean that since there was no change in oxidation numbers, it is neither? $\endgroup$ – McB Feb 26 '15 at 1:27
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As pointed out in the comments and the other answer, it is neither an oxidation, nor a reduction.

Q: So, what is it?

A: It is a condensation: two molecules of an ortho acid combine under loss of one molecule of water to yield a diacid (pyro acid).

This is frequently found in oxoacids, see

  • $\ce{H3PO4}$ (phosphoric acid) and $\ce{H4P2O7}$ (diphosphoric acid)
  • $\ce{H3AsO4}$ (arsenic acid) and $\ce{H4AsO7}$ (diarsenic acid)
  • $\ce{H2SO4}$ (sulfuric acid) and $\ce{H2S2O7}$ (disulfuric acid)
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Find the oxidation numbers of the substances in each compound.

You should find the oxidation number of Cr in each compound to be +6, and the oxidation number of O to be -2. Since there is no change in the oxidation numbers, this reaction is neither an oxidation or reduction.

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Two ways you can go about this.

One, recall that the oxidation number of oxygen is always -2 beyond hydrogen peroxide and the elemental forms of oxygen (monoatomic, diatomic, and ozone). Then you can show that the oxidation number for chromium is the same.

If you want to be clever (though not as efficient or just want another way to check yourself), you can attempt to balance the half-reaction like a half-reaction occurring in acidic solution. Why does that attempt show that no oxidation/reduction exists?

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