Recall that the determining factors for spontaneity of a reaction are the enthalpy and entropy changes that occur for the system. The free energy change of a reaction is a mathematical combination of the enthalpy change and the entropy change.
When both cases are satisfied, i.e. if the change in enthalpy is negative and the change in entropy is positive, the reaction is said to be spontaneous, and thus, enthalpy and entropy are the two driving thermodynamic forces of chemical reactions.
Combustion reactions, such as this fire, are spontaneous reactions. Once the reaction begins, it continues on its own until one of the reactants (fuel or oxygen) is gone. A nonspontaneous reaction is a reaction that does not favor the formation of products at the given set of conditions.
A spontaneous reaction is a reaction that favors the formation of products at the conditions under which the reaction is occurring. ... The entropy of the system increases during a combustion reaction. The combination of energy decrease and entropy increase dictates that combustion reactions are spontaneous reactions.