Many chemical reactions occur spontaneously at normal ambient temperatures and pressures, but some chemical reactions require energy from an external source, called activation energy, to initiate the reaction. Once the reaction is initiated, it may proceed spontaneously to completion, or it may cease if external energy is not continuously supplied. (Technically, all reactions have an activation energy; however, for a spontaneous reaction at ambient temperature, that amount of energy is present in the system already.)
For example, the gasoline used to fuel an automobile does not react spontaneously with the oxygen ...
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