The term spontaneous is used by chemists in a way that is not consistent with evryday usage of that term. To a chemist, spontaneous means that the free energy relationships for a reaction are favorable, and that no outside energy source would be needed for a reaction to take place.

Normally, we use the term spontaneous to mean "goes by itself." The reaction between hydrogen and oxygen to form water is, to a chemist, spontaneous. However, chemists also know that mixtures of hydrogen and oxygen, while dangerous, are stable until something starts the reaction -- a flash of light, a spark, a piece of platinum metal.

This chart shows the four possible combinations for ΔH and ΔS.

When ΔS is negative, and ΔH is positive, the reaction is never spontaneous. When ΔS is positive, and ΔH is negative, the reaction is always spontaneous. The other cases depend upon the temperature.

Finally, there is one very important point. Nearly any chemical reaction can be made to go, no matter how unfavorable the reaction is. To do this, we need to find some way of adding free energy. Sometimes we do this by coupling an unfavorable reaction to a favorable one. Sometimes we use electricity. For example, few reactions are as spontaneous as that between oxygen and hydrogen producing water. However, we can use electric current to break water back into hydrogen and oxygen. (We store some of the energy as chemical potential energy.)