What characterizes a spontaneous process in thermodynamics?

A spontaneous process in thermodynamics is characterized by its ability to occur without the need for continuous external energy input. This means that such processes typically lead to a decrease in free energy of the system. When a chemical reaction occurs and releases free energy, it indicates that the reaction can proceed on its own, under standard conditions, without additional energy requirements.

In the broader context, spontaneous processes tend to move systems towards equilibrium and increase the overall entropy of the universe. The release of free energy is a driving factor contributing to the spontaneity of the process, resulting in the system reaching a more stable state.

The other options describe scenarios that do not align with the definition of spontaneity: processes that absorb energy or require constant energy input do not occur naturally without a source of energy, and the need for a catalyst while it may speed up certain reactions, does not characterize all spontaneous processes. Thus, the release of free energy distinctly marks a spontaneous reaction as it confirms the inherent tendency of the system to evolve toward a more energetically favorable state.