Free energies of balanced chemical reactions always are additive. Using only the information that we have so far about Zn-Cu and Ni-Cu cells, we can predict what would happen if we built a Zn-Ni cell. The reaction of this Zn-Ni cell can be obtained by subtracting the Ni-Cu reaction from the Zn-Cu reaction. (Remember that subtracting a reaction is the same as adding the reverse reaction, with the opposite sign for the free energy.)

The negative free energy of the Zn-Ni reaction means that it is spontaneous in the direction written. The potential of the Zn-Ni cell can be calculated from D G⁰ =-nFE⁰

The positive cell potential means the same thing as the negative free energy change: The cell reaction in the direction written is spontaneous.