What is defined as a spontaneous process?

A spontaneous process is characterized as one that occurs naturally under specific conditions without the need for external intervention. This definition hinges on the concept of thermodynamics, where a spontaneous change is associated with a decrease in Gibbs free energy (ΔG < 0). Such processes often lead to an increase in entropy, reinforcing the tendency of systems to evolve towards a state of greater disorder over time.

In contrast, the other definitions do not accurately capture the essence of spontaneity. For instance, a process requiring continuous external input contradicts the notion of spontaneity, as it implies that external energy is needed for the process to occur. Similarly, a reversible process suggests that the system can return to its original state without any net change, which although it may describe some spontaneous processes, does not represent spontaneity as a whole. Lastly, processes that require a catalyst typically still require some form of energy input or conditions to proceed, making them non-spontaneous by definition. Thus, the correct understanding of a spontaneous process is one that happens naturally without the need for external influences.