Balancing Redox Reactions: How to Determine Half Reactions and Oxidation Numbers

[MaxNumbers]

I'm not sure how to balance this equation with regards to the half reaction and electrons, as well as the elements:

Fe⁺² + Cr₂O^-2₇--->Fe⁺³ + Cr⁺³

Right now, I've gotten this far, though I don't know if it's the right track:

Fe⁺² + 2Cr⁺⁷₂O^-2₇--->Fe⁺³ + 4Cr⁺³ + 7O^-2₂

What should I do next?

 

[ShawnD]

My first post was totally wrong so here's a revamp.

Fe⁺² + Cr₂O^-2₇--->Fe⁺³ + Cr⁺³

Ok well here's one part..

$$Fe^{2+} \rightarrow Fe^{3+} + e^-$$

I can't figure out what happens with the Cr2O7. It usually reacts with acid, like this:

$$Cr_2O_7 ^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$$

But there's no acid so that ain't it.

Another thing is that I'm not sure your second equation works out.

Fe⁺² + 2Cr⁺⁷₂O^-2₇--->Fe⁺³ + 4Cr⁺³ + 7O^-2₂

The charges are not the same on both sides.

 

[thunderfvck]

Maybe it should be 2CrO4(2-)? I noticed that on a website...Still can't figure it out though. It's pissing me off.

 

[MaxNumbers]

ShawnD, that answered my question exactly. I wasn't sure how the notation worked, as far as just throwing the correct number of H+ and so forth.
Thanks.