Why is hydrogen’s reduction potential assigned a value of zero?

The reduction potential of hydrogen is assigned a value of zero because it serves as the universal standard reference point for comparing the relative tendencies of other chemical species to gain or lose electrons.123

By arbitrarily setting the potential of the hydrogen electrode—where hydrogen ions in a one-mole-per-litre solution are in equilibrium with hydrogen gas at one atmosphere of pressure—to 0.00 volts, scientists created a consistent baseline. This allows for the construction of the electromotive series, where all other electrode potentials are measured relative to this hydrogen scale.342

Consequently, half-reactions with positive reduction potentials involve reactants that are more easily reduced than hydrogen ions, while those with negative potentials are more difficult to reduce. This convention provides a standardized, practical framework for predicting the spontaneity of oxidation-reduction reactions without requiring complex thermodynamic calculations for every individual system.45