Final answer:
The redox potential of hydrogen is compared to other compounds to determine the electron donor. Measured redox potentials depend on several factors including the potential energy of valence electrons, concentrations of the species, and the temperature.
Step-by-step explanation:
The redox potential of hydrogen is taken as zero at pH 0 (-0.417), 25° C, in a solution of 1 molar concentration (1.0 atom of hydrogen).
A compound having a negative value of E° is a better electron donor than hydrogen. A compound having a positive value of E° is a poor electron donor than hydrogen. The electrons flow from compounds with negative value of redox potential to those with positive values of redox potential, because there will be loss of energy and thus, the compound becomes stable.
The measured redox potentials depend on the potential energy of valence electrons, the concentrations of the species in the reaction, and the temperature of the system.