How do standard reduction potentials show whether a redox reaction can occur spontaneously?

Standard reduction potentials ($E^{\circ }$) provide a reliable method for predicting the spontaneity of a redox reaction by analyzing the relative tendencies of half-reactions to occur.1

To determine if a reaction is spontaneous, you must break the overall process into two half-reactions. By reversing one half-reaction to balance the electron transfer, you also reverse the sign of its corresponding reduction potential. When these two potentials are added together, the resulting $E^{\circ }$ value for the net reaction indicates its thermodynamic feasibility:1

• Positive $E^{\circ }$: If the calculated potential is positive, the reaction is expected to occur spontaneously under standard conditions (1M concentration, 25°C, 1 atm pressure).21
• Negative $E^{\circ }$: If the potential is negative, the reaction is not spontaneous in the direction written.1

This relationship is rooted in thermodynamics, where the standard Gibbs free energy change ($\Delta G^{\circ }$) is related to the potential by $\Delta G^{\circ }=-nF{E}^{\circ }$. A spontaneous reaction requires a negative $\Delta G^{\circ }$, which corresponds to a positive $E^{\circ }$ value. Deviations from standard conditions can be calculated using the Nernst equation.32