What best defines a spontaneous reaction in terms of Gibbs free energy?

A spontaneous reaction is characterized by a negative change in Gibbs free energy. This implies that the total energy of the system decreases as the reaction proceeds, which indicates that the reaction can occur without needing an input of external energy. In thermodynamics, a reaction is considered spontaneous if it can proceed forward under specific conditions, often at constant temperature and pressure.

The negative value of the change in Gibbs free energy corresponds to the driving force that makes the reaction favorable; it indicates that the products are at a lower energy state compared to the reactants. Therefore, the reaction can occur naturally until equilibrium is reached.

In terms of entropy, spontaneity also tends to be associated with an increase in disorder or randomness in the system, although this is not the direct definition in relation to Gibbs free energy. Overall, the clear marker of spontaneity in this context is the negative change in free energy.