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Oxidation #s

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qfruit:
I'm looking at an exam question and it says that

2SO42-  --> S2O82-

is an oxidation half reaction.


When I work out the charges, they come out equal on both sides. I know that the gain of oxygens may also be a clue, but my question is...do you include the balancing 2 in the first sulphate when you figure out oxidation numbers? I did when I tried to figure out this question, but in the answer explanation, they did not.

Thanks.

mike:
I think maybe you have miscalculated, try again.

qfruit:
I'm sure. If I include the 2 that balances the sulphate, both sulphurs come out as 7+. Without the 2, the sulphate comes out as 6+, like it says in the answers, so then it would be oxidizing...but I thought you do include the balancing 2 when you do these questions.

AWK:
2SO42-  --> S2O82- + 2e-
Persulfate is obtained by electrochemical oxidation

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