When do you multiply by the coefficients?

[dbeast]

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One of the practice tests (AAMC #7 PS #19) is asking to calculate the total electrode potential of a cell, and we are given values for oxidation and reduction potentials. You get the correct answer when you just add the two values of the half reactions, although one requires twice as many moles. The explanation says for cells, you never multiply Eo potential values by the coefficient.

My question is that I definitely remember certain half reactions where you do multiply by the mole ratio and reverse reactions (reversing the signs) until they add up to the desired product... I just don't remember what it applies to. Is it oxidation/reduction? Heat of formation? Driving me crazy... Thanks!

 

[addo]

For potentials, you reverse the signs if you reverse the equation, but you never multiply it by the coefficients, it is a intensive property, independent of stoichiometric coefficients.
On the other hand, enthalpy and entropy are extensive properties, so you must not only reverse signs if you reverse the equation, but you must also multiply by stoichiometric coefficients.