What term describes a reaction that can occur without the addition of outside energy?

A spontaneous reaction is a chemical reaction that occurs naturally without the need for external energy inputs. This term is crucial in understanding thermodynamic principles, as such reactions typically lead to an increase in entropy, indicating a tendency toward a more disordered state. They are characterized by having a negative change in Gibbs free energy, meaning the products are more stable than the reactants. This stability allows the reaction to proceed on its own under the given conditions.

In contrast, a non-spontaneous reaction requires an input of energy to proceed, making it dependent on external factors for its completion. A catalytic reaction involves the use of a catalyst to speed up a reaction without changing the overall energy requirements, while a thermodynamic reaction may not specifically refer to the spontaneity of the reaction but rather focuses on energy changes during the reaction.

Thus, the defining feature of a spontaneous reaction is its ability to occur autonomously, underscoring its importance in both biological processes and chemical systems.