It is both inefficient and unnecessary to tabulate the heat or free energy for every single chemical reaction that could occur. Since heats and free energies of reactions are additive, it is sufficient to tabulate only the heat and free energy of reaction of each compound as made from its elements.

The heats and free energies of all reactions between these compounds then can be found by combining their heats of formation, with the contributions from the elements canceling out.

In a similar way, it is not necessary to measure and tabulate the free energies and cell potentials of all conceivable oxidation-reduction cells.

If there were a hundred different substances capable of oxidation and reduction, then these might be combined into as many as 100 x 99 = 9900 different electrochemical cells, each with its own free energy drop and voltage.