Reversing Half-Reactions

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Reversing Half-Reactions

Postby nelms6678 » Mon Feb 26, 2018 3:49 pm

When reversing a Half-Rxn for example
Cu2+(aq) + 2e- ---> Cu(s) E=+0.34V
to Cu(s) ---> Cu2+(aq) + 2e-

why doesn't the value of E switch from positive to negative?

Victoria Draper 1G
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Re: Reversing Half-Reactions

Postby Victoria Draper 1G » Mon Feb 26, 2018 4:11 pm

My TA told me that the reason we do not switch the signs of the potential cell energy is because the equation Ecell=E(reduction)-E(oxidation) includes a negative sign next to the half reaction for the oxidation portion of the cell. The negative sign makes the value of the whole oxidation half reaction positive if the half reaction is flipped and turns out to be a negative value. This therefore makes the rule Ecell=E(reduction)+E(oxidation) (the equation commonly found on other online sources) true.

Ishita Monga 1B
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Re: Reversing Half-Reactions

Postby Ishita Monga 1B » Mon Feb 26, 2018 4:11 pm

If you are going from reduction potentials to oxidation potentials, the sign should be reversed.
If you're referring to solving for cell potential, whether or not you reverse the E value depends on what method you are using to calculate the cell potential. If you do cathode-anode, you don't reverse the E value, but if you simply add the E values of the two half reactions then you do reverse the E value of the anode.

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