The standard cell potentials we discussed in a previous section refer
to cells in which all dissolved substances are at unit activity, which
essentially means an "effective concentration" of 1M. Similarly, any
gases that take part in an electrode reaction are at an effective
pressure (known as the fugacity) of 1 atm. If these concentrations or
pressures have other values, the cell potential will change in a
manner that can be predicted from the principles you already know.
Suppose, for example, that we reduce the concentration of Zn2+ in
the Zn/Cu cell from its standard effective value of 1M to an to a much
smaller value: