What does a negative E° value indicate about a half-cell reaction?

A negative standard electrode potential (E°) value for a half-cell reaction indicates that the reaction is not spontaneous under standard conditions. Instead, it suggests that the reaction would require an input of external energy to proceed in the forward direction.

In electrochemistry, a spontaneous reaction is one that has a positive E° value, signifying a natural tendency to occur without additional energy input. Conversely, when the E° value is negative, the driving force for the reaction to occur in the forward direction is lacking, which necessitates external energy, such as electrical energy, to make it happen.

Therefore, the interpretation of a negative E° is fundamentally about the energy dynamics involved in the reaction, highlighting that energy must be supplied for it to occur. Understanding this principle is crucial in applications such as electrolysis, where energy is applied to drive non-spontaneous reactions.