Final answer:
Chemical reactions that release free energy and move to a more stable state are the ones that happen spontaneously. Usually, these reactions are exothermic, like the combustion of gasoline.
Step-by-step explanation:
Reactions that occur spontaneously are those that release free energy and move to a more stable state. Spontaneity in a chemical reaction refers to its natural tendency to progress without the continuous input of external energy. Typically, a spontaneous reaction is exothermic, meaning it releases energy, often in the form of heat, as it proceeds. This release of energy contributes to the overall stability of the system. By contrast, a reaction that absorbs energy from its surroundings is called endothermic and usually does not occur spontaneously because it requires a continuous input of energy to proceed.
An example of a spontaneous reaction is the combustion of gasoline, where chemical bonds are broken and new ones are formed, releasing energy in the process. The spontaneity of a process also affects the distribution of energy and matter within a system, as seen when a helium-filled balloon spontaneously deflates because the helium atoms diffuse through the wall, redistributing matter and leading to the deflation without added energy.