Final answer:
A spontaneous reaction proceeds without continuous external energy input, releasing free energy and generally moving towards stability, although it may not be rapid.
Step-by-step explanation:
A spontaneous reaction is one that favors the formation of products under the given conditions and does not require continual external energy input to proceed. Such reactions lead to the release of free energy and often result in the system moving to a more stable state.
It's important to note that spontaneous does not necessarily mean rapid; some spontaneous reactions, like the oxidation of plastics, can be very slow (kinetically stable).
For instance, a piece of paper's combustion is spontaneous, but it will not burn without the required activation energy to initiate the process. Once the activation energy is provided, for example by heating the paper, the reaction will then proceed spontaneously.