A spontaneous reaction is a chemical process that occurs naturally without the need for external energy input. These reactions happen because they lead to a decrease in free energy, making the products more stable than the reactants. Common examples include the rusting of iron and the combustion of fuels. In thermodynamics, spontaneous reactions are often analyzed using the concept of Gibbs free energy. If the change in Gibbs free energy is negative, the reaction is spontaneous. Factors such as temperature and concentration can influence the spontaneity of a reaction, making it essential to consider these variables in chemical studies.