A reducing agent is thus oxidized when it loses electrons in the redox reaction. Reducing agents "reduce" (or, are "oxidized" by) oxidizing agents. Oxidizers "oxidize" (that is, are reduced by) reducers.
Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'. The modern sense of donating electrons is a generalisation of this idea, acknowledging that other components can play a similar chemical role to oxygen.
In their pre-reaction states, reducers have extra electrons (that is, they are by themselves reduced) and oxidizers lack electrons (that is, they are by themselves oxidized). A reducing agent typically is in one of its lower possible oxidation states and is known as the electron donor. Examples of reducing agents include the earth metals, formic acid, oxalic acid, and sulfite compounds.
For example, consider the overall reaction for aerobic cellular respiration:
In organic chemistry, reduction usually refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene is reduced to cyclohexane in the presence of a platinum catalyst:
Consider the following reaction:
Strong reducing agents easily lose (or donate) electrons. An atom with a relatively large atomic radius tends to be a better reductant. In such species, the distance from the nucleus to the valence electrons is so long that these electrons are not strongly attracted. These elements tend to be strong reducing agents. Good reducing agents tend to consist of atoms with a low electronegativity, the ability of an atom or molecule to attract bonding electrons, and species with relatively small ionization energies serve as good reducing agents too. The measure of a material to reduce, or gain electrons, is known as its reduction potential. The table below shows a few reduction potentials (which can be changed to oxidation potentials by reversing the sign). Reducing agents can be ranked by increasing strength by ranking their reduction potentials. The reducing agent is stronger when it has a more negative reduction potential and weaker when it has a more positive reduction potential. The following table provides the reduction potentials of the indicated reducing agent at 25 °C. For example, among Na, Cr, Cu+ and Cl-, Na is the strongest reducing agent and Cl- is the weakest one.
Some elements and compounds can be both reducing or oxidizing agents. Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals.
Hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.
Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.
Reducing agents and oxidizing agents are the ones responsible for corrosion, which is the "degradation of metals as a result of electrochemical activity". Corrosion requires an anode and cathode to take place. The anode is an element that loses electrons (reducing agent), thus oxidation always occurs in the anode, and the cathode is an element that gains electrons (oxidizing agent), thus reduction always occurs in the cathode. Corrosion occurs whenever there's a difference in oxidation potential. When this is present, the anode metal begins deteriorating, given there is an electrical connection and the presence of an electrolyte.
The formation of iron(III) oxide;
In the above equation, the Iron (Fe) has an oxidation number of 0 before and 3+ after the reaction. For oxygen (O) the oxidation number began as 0 and decreased to 2-. These changes can be viewed as two "half-reactions" that occur concurrently:
Iron (Fe) has been oxidized because the oxidation number increased. Iron is the reducing agent because it gave electrons to the oxygen (O2). Oxygen (O2) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe).