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If you are trying to calculate E°cell using the formula E°cell=E°cathode-E°anode, and one or both of the half reactions provided are in oxidation form with their respective voltage, would you need to flip the sign of the voltage so that the half-reaction would be in reduction form? For example, if a cathode half reaction was A --> B + e- and the voltage was -0.5 V, would the voltage we use in the formula be +0.5 V? I'm asking because this type of problem confused me a bit in the 2013 practice final.
Usually when we are given the E values, they are in the reduction form. However, for a galvanic cell, we need one reduction half and one oxidation. This is why we flip one of the reactions. Usually we flip the one that would give us an overall positive and favorable E value. However, sometimes this isn't the case because the two half reactions also have to be consistent with the chemical equation if one is given.
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