OXIDATION AND REDUCTION
Definitions
In originally Oxidation means the 'combination with oxygen' and reduction means 'removal of oxygen'. These definitions have been very much expanded. Oxidation implies combination with a more electronegative element the removal of a less electronegative one, or just the removal of electrons. Reduction is the opposite of the oxidation and generally implies addition of electrons. In any reaction in which one species is oxidized, other must be reduced: the term redox reaction is employed to express this.
Two instances are: the reaction of zinc in aqueous acid,
Zn(s) + 2H+ (aq) → zn2+ (aq) + H2 (g)
In which the zinc metal is oxidized to Zn2+ and hydrogen reduced from H+ to H2; and the reduction of zinc oxide by carbon,
ZnO(s) + C(s) → Zn(s) + CO(g)
In which the zinc is reduced from ZnO to the metal, elemental carbon is oxidized to CO, and oxygen, combined with a less electronegative element on both sides is not oxidized or reduced.
A strong oxidizing agent is a substance able of oxidizing many others and so it is itself easily reduced; alternatively, a strong reducing agent is itself straightforwardly oxidized; these terms generally imply thermodynamic reaction tendency although kinetics may also be important. Atmospheric dioxygen is a good oxidizing agent, but several substances (example organic compounds) are kinetically stable in air. The Strong reducing agents include electropositive metals, particularly those of group 1.