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should the standard potential at the cathode and anode of a galvanic cell always be positive (which results in negative delta G--because of delta G=-nFE), because the 1/2 reactions should be spontaneous? If not, then can someone explain why/how the reaction could be spontaneous if the standard potential isn't positive for a certain half reaction?
For a galvanic cell to work and provide a spontaneous current, you do NOT need both (or any) half-reactions to have a positive standard reduction potential. Remember that for these problems you want to find the E cell of the OVERALL reaction. If the cathode has a higher reduction potential than the anode, then you will get a positive E cell. This means when you have , the delta G will be negative for the overall reaction. Even if both the reduction and oxidation half-reactions have negative E0 values, the difference is what matters for the overall cell. If there is positive difference between cathode and anode, it should be spontaneous.
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