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For simple reactions involving elements like Zn/Zn2+ or Cu/Cu2+, it is easier to see which one is oxidized or reduced. However, for complex compounds or more complex reactions, it is better to identify the couple and then calculate the oxidation state for that half-reaction. Although it takes time, it will get you to the right answer.
If you do the half reactions, notice where the electrons end up. If they end up as a product, that means the compound lost electrons and was therefore oxidized. If the electrons are a reactant, that means the compound gained electrons and was reduced.
An fast way to quickly identify what elements in a reaction could be being oxidized is to pinpoint anything that is not oxygen or hydrogen, since it is really rare for these two to have their oxidation numbers change. This usually leaves only two elements you have to calculate the oxidation numbers for.
For very easy problems typically involving just one element turning into an ion of itself where you can instantly tell if it was reduced or oxidized by balancing the charge by adding e- to one of the sides. Other than that you have to calculate the oxidation states of the molecules using the oxidation charges on the periodic table below and based off the change in charge from one molecule to what it changes into you can tell if it is reduced or oxidized.
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