Gen chem redox reaction question

[DocDoon]

When a question asks for the number of electrons needed to balance a redox reaction, can I just go by oxidation number or do I have to balance out the question with H2O and H+ and then electrons to balance? The problem was MnO4 => Mn2+. Basing my answer of oxidation numbers, I got 5 electrons which was correct but would this method work every time?

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[woobb]

Yes, you have to balance out each half reaction with H2O and H+ in order to find the number of electrons needed. If you rely only on the oxidation number, you could get the wrong answer. There are rules for oxidation states, and the oxidation state for an element might change depending on how it appears (e.g. O is -1 in a peroxide and -2 otherwise).