Final answer:
A spontaneous reaction is a process that proceeds without added energy input and tends toward a more stable state, releasing free energy. It can be slow, often requiring activation energy, with the rusting of iron being a good example.
Step-by-step explanation:
A spontaneous reaction in chemistry is one that occurs without the requirement of additional energy input and moves the system to a more stable state. This means that during a spontaneous process, free energy is released and the overall energy of the system decreases. It's important to understand that spontaneity does not necessarily mean the reaction occurs quickly. Spontaneous reactions can be quite slow, as they may require a certain amount of activation energy to begin. A classic example of a spontaneous reaction would be rusting of iron in a moist atmosphere. Though the process of oxidation is slow, it is spontaneous because it results in a lower energy, more stable product.