When is a redox reaction considered spontaneous?

A redox reaction is considered spontaneous when the cell potential, also known as the electromotive force (EMF), is positive. The cell potential is a measure of the driving force behind the reaction; a positive value indicates that the reaction can occur under standard conditions without the need for external energy input. In practical terms, this means that the products are more stable than the reactants, leading to a net release of energy during the reaction, which manifests as a positive cell potential.

This principle is rooted in thermodynamics, where a positive cell potential correlates with a decrease in Gibbs free energy (ΔG). A reaction with a positive cell potential will have ΔG values that are negative, indicating that it occurs spontaneously. Therefore, when assessing whether a given redox reaction is spontaneous, the first step is to look at the sign of the cell potential.

In contrast, a negative cell potential indicates that the reaction is non-spontaneous under standard conditions, meaning that energy would need to be added to drive the reaction forward. The status of reactants being in excess or whether products are favored can influence the reaction dynamics but do not fundamentally define spontaneity. The critical factor for spontaneity is the cell potential, thus affirming that a positive cell potential