What is a spontaneous reaction?

A spontaneous reaction is characterized by its ability to occur without an external input of energy under specific conditions. When a reaction releases more energy than it consumes, it typically indicates that the process is thermodynamically favorable. This phenomenon is often quantified in terms of Gibbs free energy; a spontaneous reaction will have a negative change in Gibbs free energy, aligning with the understanding that the system moves toward a lower energy state.

In spontaneous reactions, energy is released to the surroundings, often in the forms of heat or light, making it easier for the reaction to proceed without requiring additional energy. This contrasts with reactions that require an input of energy to occur, which are not considered spontaneous.

Understanding spontaneous reactions also involves recognizing that they can occur despite being endothermic or absorbing energy in some cases, provided that the total change in energy for the reaction leads to lower free energy overall. This is why the other options do not accurately capture the essence of spontaneity.