Ace the AP Biology Challenge 2026 – Unleash Your Inner Scientist and Conquer the Course!

A process is considered spontaneous if it occurs without requiring an input of external energy. This is closely tied to the concept of free energy, particularly Gibbs free energy, which is a thermodynamic potential that reflects the capacity of a system to do work at constant temperature and pressure.

When the free energy change (ΔG) of a process is negative, it indicates that the process can proceed spontaneously. This negative value implies that the energy available in the system decreases as the reaction moves toward equilibrium. Therefore, spontaneous processes tend to release energy, which can be harnessed to perform work.

In contrast, a positive free energy change would mean that the process requires energy input to occur, meaning it is non-spontaneous. A free energy change that is zero indicates that the system is at equilibrium, meaning there is no net change occurring in the system, and thus, it is neither spontaneous nor non-spontaneous. Lastly, the concept of consistent free energy does not specifically define the spontaneity of a process.

In summary, a negative free energy change clearly indicates that a process can occur spontaneously, which is the key reason for selecting this option.