For a chemical reaction to occur spontaneously, it generally requires a decrease in free energy, often characterized by a negative Gibbs free energy change (∆G < 0). This indicates that the reaction can proceed without the need for external energy input. Several factors influence spontaneity, including enthalpy and entropy changes. A spontaneous reaction may still proceed at a slow rate absent a catalyst to speed up the process. Understanding spontaneity is key in thermodynamics and reaction kinetics.