Final answer:
A catabolic reaction, or decomposition reaction, typically occurs on its own, involving the breakdown of compounds into simpler substances with changes in oxidation numbers, also known as a redox reaction. Combustion reactions also happen on their own but require ignition to start.
Step-by-step explanation:
The type of chemical reaction that generally occurs on its own is a catabolic reaction, which is a type of decomposition reaction. A decomposition reaction involves the breakdown of a compound into simpler substances or elements. A common example is the decomposition of hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2), which can occur spontaneously. This process is also considered a redox reaction because it involves changes in the oxidation numbers of the reactants and products.
A combustion reaction is another type of chemical reaction that can appear to occur on its own once initiated, but it requires a source of ignition to begin. Combustion is an exothermic oxidation-reduction reaction where a substance combines with oxygen, releasing energy in the form of heat and light. The combustion of hydrogen gas, for example, produces water vapor as a product.
Therefore, while both decomposition and combustion reactions can proceed on their own once started, it is the catabolic, or decomposition reaction, that is generally considered to occur spontaneously without the need for an ignition source or continuous external energy.