# Determine if the following change is an oxidation, a reduction, or neither

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.

• If you are familiar with oxidation numbers, what are the oxidation numbers of the chromium atoms in each compound? – jerepierre Feb 26 '15 at 1:20
• @jerepierre in the first compound +6, in the second compound +6 as well. – McB Feb 26 '15 at 1:22
• @jerepierre Does this mean that since there was no change in oxidation numbers, it is neither? – McB Feb 26 '15 at 1:27

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)

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.