Explaining 14.4 (Cell potential and gibbs)

[Adriana Rangel 1A]

Hello can someone please explain to me what I just read in simpler terms please (page 572):

"There is a hidden assumption in the derivation of Eq. 1. Maximum nonexpansion work is obtained when a cell is operating reversibly. Therefore, Eq. 1 applies only when the pushing power of the cell is balanced against an external matching source of potential. In practice, that means using a voltmeter with such a high resistance that the potential difference is measured without drawing any current. The potential difference under these conditions is the maximum potential that can be produced. It is called the cell potential (which is still widely called the electromotive force, emf, of the cell). From now on, Ecell will always be taken to represent this potential difference. A working cell, one actually producing current, such as the cell in a media player, will produce a potential difference smaller than that predicted by Eq. 1."

Thank you :)

[Christy Lee 2H]

I think that Ecell when we calculate it represents the maximum potential difference. SInce this potential difference can be used like a battery, we are talking about the maximum possible difference, but real-world batteries would not actually produce this much voltage.

[zanekoch1A]

I agree with the first response, the statement is saying first that to use a voltmeter it must have very high resistance (be very non-conductive) because otherwise some of the electricity that would normally be going from one end of the cell to the other would be drawn away. Second it says how because it is hard to measure this current we will always use max cell potential which is never measured, only calculated, and thus will always be an over-estimate except under perfectly ideal conditions which essentially do not exist.