Final answer:
A spontaneous reaction is one where the formation of products is favored, free energy is released, and there's an increase in entropy, without needing continuous external energy; this differs from instantaneous reactions which occur quickly.
Step-by-step explanation:
Spontaneous (favorable) reactions are those in which product formation is favored under the given conditions, and they occur without continuous energy input from an external source. These reactions release free energy and typically lead to a more stable state with increased entropy. For example, combustion is a spontaneous reaction because it releases energy in the form of heat and increases the entropy by producing gases like carbon dioxide and water vapor.
It is important not to confuse 'spontaneous' with 'instantaneous'. While spontaneous reactions are thermodynamically favorable, they do not necessarily occur rapidly. An example is the combustion of a piece of paper: it will not ignite spontaneously at room temperature despite the reaction being favorable, because the activation energy required to start the combustion is not present. Only when heated to a high enough temperature will the paper burn, hence proceeding with the spontaneous reaction until it is complete.