Although the zinc rod in copper sulfate and the two-solution cell just described are physically different, the same chemical reaction takes place in both:

In the cell, however, the 51 kcal of free energy released by the reaction can be used for other purposes.

Every time a mole of the above reaction occurs in the cell, two moles of electrons flow from the zinc anode, through the external circuit, to the copper cathode.

As we saw previously, in chemical reactions and phase changes it is useful to define a chemical potential as the free energy change per mole of a specific reactant, as the equation is written.

This chemical potential is the intrinsic capacity of the reaction to do work; and the actual work done, or free energy released, is the product of this potential times the number of moles of a substance undergoing chemical change.