What is a spontaneous reaction?

A spontaneous reaction is a chemical or physical process that occurs naturally without the need for external intervention. In other words, it proceeds on its own accord under certain conditions, typically without requiring an input of energy. Spontaneous reactions tend to occur in the direction that leads to a decrease in the system's free energy or an increase in its entropy.

In thermodynamics, spontaneous reactions are characterized by a negative change in Gibbs free energy (∆G < 0) under constant temperature and pressure conditions. This means that the reaction will proceed forward without the need for added energy. However, it's essential to note that spontaneous reactions can occur at different rates; some may be fast, while others may be slow.

Examples of spontaneous reactions include the rusting of iron, the combustion of fuels, and the dissolution of salt in water.