Final answer:
Spontaneous combustion is when a substance ignites without external heat due to spontaneous reaction conditions. Combustion reactions are exothermic and increase entropy,
Step-by-step explanation:
Spontaneous combustion is a type of combustion reaction where a substance ignites without the application of an external heat source. This phenomenon is part of a broader category of reactions known as spontaneous reactions, which occur naturally under a given set of conditions and favor the production of products.
Not to be mistaken with rapid occurrence, spontaneous reactions can happen very slowly and they require an initial supply of activation energy to begin. For example, the combustion of paper is spontaneous but will not occur without heating it to a high enough temperature to ignite.
The process of combustion involves the rapid combination of oxygen with a fuel source, releasing energy in the form of heat and light. Classic examples include the burning of methane on a kitchen stove or a bonfire. In these reactions, reactants such as carbon and hydrogen combine with oxygen, producing carbon dioxide, water vapor, and releasing a significant amount of heat. This release of heat and the formation of gaseous products contribute to an increase in the system's entropy, making combustion reactions both exothermic and favorable in terms of an increase in entropy, thereby qualifying them as spontaneous.