In the first example, electrons are physically removed from copper atoms to produce positively charged copper ions. In the second example, electrons on hydrogen that originally were shared equally with another hydrogen atom are partially lost by being shared unequally with oxygen atoms. Since electrons are never created or destroyed in chemical reactions, whenever one atom is oxidized, another atom must be reduced

.When hydrogen is oxidized by the process above, oxygen is reduced. The copper reaction is incomplete, since some unspecified substance must become reduced by taking up the two electrons indicated on the right side of the equation.

In the water reaction, hydrogen is oxidized and oxygen is reduced. Free energy is given off because oxygen is a strong oxidizing agent (meaning that it has a strong attraction for electrons) and hydrogen is a good reducing agent (meaning that it lets go of its electrons easily to something else).

The standard free energy change during this reaction is D G° = -54.6 kcal mol-l of water vapor. In general, the oxidation of a substance with a tenuous hold on its electrons, by a strong oxidant with a powerful pull for electrons, is accompanied by the release of free energy. It is a spontaneous process.