Many oxidations and other useful chemical reactions are spontaneous (accompanied by a decrease in free energy) and exothermic (accompanied by a decrease in enthalpy) and hence are useful as energy sources; yet the reactions often are extremely slow.

As was pointed out in Chapters 13 through 15, we must distinguish between spontaneity and rapidity in chemical reactions. Spontaneous reactions eventually will take place without outside help, but they may take from a microsecond to a billion years to occur.

At room temperature hydrocarbons are spontaneously oxidizable with 02, but are inert. Heat is required to trigger a reaction.

If an initial heat supply is provided to start the process, then the heat given off by oxidation is enough to keep the reaction going.

Once ignited, combustion is self-sustaining thereafter. A high temperature is needed to overcome the high activation energy (Ea) of the reaction (right).

Alkanes are relatively unreactive; the term "paraffins" often applied to them means "little affinity." Why should reactions of saturated hydrocarbons have such high activation energies? What is the barrier to reaction?