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When you have to find which metal is the stronger reducing agent, how do you know which reduction potential to use? For example, if the question just asks about Fe, how would you know whether to use the reduction potential using Fe2+ or Fe3+?
for the ones with different oxidation states, i think you would have to think about the context of the rxn that you're using. if you write out the rxn you can calculate the oxidation state of Fe and go from there.
First of all, remember the reducing agent is the one being oxidized, so you have to flip the given standard reduction potential equation to get the standard reduction potential of the oxidation half-reaction. When calculating the reduction potential of the cell, we want the value to be positive because that's how it will be spontaneous. So when you have to choose, for the reduction half-reaction pick the one with the biggest (most positive) reduction potential, and for the oxidation half-reaction pick the one with the smallest (most negative/least positive) reduction potential (because the sign will change when you flip the equation. For your example specifically, Fe2+ would be the stronger reducing agent because the reduction potential is smaller than that of Fe3+ (-0.45 vs +0.77).
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