[Avigail]

Hello!

I think I lack a basic understanding of the Nernst equation. Say I have a cell with two electrodes: one is a standard hydrogen electrode (E=0) and the other is a Metal electrode in a solution of the metal cations.
According to the Nernst equation- the cell potential will depend on the concentration of the metal cations:
E=E⁰+RT*ln[M+]

Here's what I don't understand: the moment we connect the electrodes through an external circuit (like in a battery)- a reaction will occur where the metal M will dissolve and become M+ - this means the concentration of metal ions will change over time! how then is the cell voltage constant?

I basically don't understand what are the conditions in which I can use the Nernst equation to calculate cell voltage?

I also don't understand how the voltage of a battery remain constant until the battery dies because one of its components has completely reacted? If the concentration of species in the solution constantly changes because of the chemical reaction- doesn't that mean the potential has to change over time?

Please help.
Thank you..

[forrestgreen]

For your case, cell voltage is not constant. It changes with metal ion concentration and hydrogen pressure.
The voltage of a battery does not remain constant. It declines with use. It is dead when its voltage is too low to power any devices.