The activity series combines both thermodynamics and kinetics. Our small list, with further development, is mainly concerned with thermodynamics. In most textbooks today the two lists are generally identical but it was not always so.

The voltage generated by the reactions is a direct result of electron flow from one electrode to the other. The tendency of the electrons to flow is represented in some respects by the magnitude of the voltage. This allows us to investigate the nature of the reactions, specifically, whether they are equilibrium systems. Changing the electrolyte concentrations to 0.0001 M instead of 1 M with the silver and copper seems to indicate--by the change in voltage--that the reactions are reversible.

Among other things that means a standard of some sort is required if voltages for reactions are to be compared in a meaningful way. The standard voltages are known as E^o values and are measured relative to the reduction of hydrogen ions in 1 M acid at 298 K and 1 atm pressure. It is now common practice to represent these voltages as reduction potentials.