There is a chemical reaction in which the oxidation states of the atoms vary. These reactions involve a reduction process and a complementary oxidation process. In these reactions, one or more electrons from one molecule, atom, or ion are transferred to another molecule, atom, or ion. This involves the production of an oxide-reduction reaction.
During the oxide-reduction process, that element or compound that loses (or donates) its electron (or electrons) is called a reducing agent, in contrast to that oxidizing agent that is the electron receptor. The reducing agents are then said to reduce the oxidizing agent, and the oxidizing agent to oxidize the reducing agent.
The best or strongest reducing agents are those with the highest atomic radius; that is, they have a greater distance from their nucleus to the electrons that surround it.
What are reducing agents?
As already mentioned, reducing agents are responsible for reducing an oxidizing agent when an oxide-reduction reaction occurs.
A simple and typical reaction of the oxidation-reduction reaction is that of aerobic cellular respiration :
C 6 H 12 O 6 (s) + 6O 2 (g) → 6CO 2 (g) + 6H 2 O (l)
In this case, where glucose (C 6 H 12 O 6 ) is reacting with oxygen (O 2 ), glucose is behaving as the reducing agent to give electrons to oxygen – that is, it is being oxidized – and oxygen is becomes an oxidizing agent.
In organic chemistry, the best reducing agents are considered to be those reagents that provide hydrogen (H 2 ) to the reaction. In this field of chemistry, the reduction reaction refers to the addition of hydrogen to a molecule, although the above definition (oxide-reduction reactions) also applies.
For a substance to be considered “strong” it is expected that they are molecules, atoms or ions that more or less easily shed their electrons.
For this, there are a series of factors that must be taken into account to recognize the force that a reducing agent may have: electronegativity, atomic radius, ionization energy and reduction potential.
Electronegativity is the property that describes the tendency of an atom to attract a pair of bonded electrons towards itself. The higher the electronegativity, the greater the attractive force that the atom exerts on the electrons that surround it.
In the periodic table, electronegativity increases from left to right, so the alkali metals are the least electronegative elements.
It is the property that measures the number of atoms. It refers to the typical or average distance from the center of an atomic nucleus to the boundary of the surrounding electron cloud.
This property is not precise – and in addition, several electromagnetic forces are involved in its definition – but it is known that this value decreases from left to right in the periodic table, and increases from top to bottom. This is why the alkali metals, especially cesium, are considered to have a higher atomic radius.
This property is defined as the energy required to remove the least bound electron from an atom (the valence electron) to form a cation.
It is said that the closer the electrons are to the nucleus of the atom they surround, the greater the ionization energy of the atom.
Ionization energy increases from left to right and from bottom to top in the periodic table. Again, metals (especially alkaline ones) have lower ionization energy.
It is the measure of the tendency of a chemical species to obtain electrons and, therefore, to be reduced. Each species has an intrinsic reduction potential: the higher the potential, the greater its affinity for electrons and also its ability to be reduced.
Reducing agents are those substances with the lowest reduction potential, due to their low affinity with electrons.
Stronger reducing agents
With the factors described above, it can be concluded that to find a “strong” reducing agent, an atom or molecule with low electronegativity, high atomic radius and low ionization energy is desired.
As already mentioned, alkali metals have these characteristics and are considered the strongest reducing agents.
On the other hand, lithium (Li) is considered the strongest reducing agent because it has the lowest reduction potential, while the LiAlH 4 molecule is considered the strongest reducing agent of all, because it contains this and the other desired characteristics.
Examples of reactions with reducing agents
There are many cases of rust reduction in everyday life. Some of the most representative ones will be detailed below:
The combustion reaction of octane (the main component of gasoline):
2C 8 H 18 (l) + 25O 2 → 16CO 2 (g) + 18H 2 O (g)
It can be seen how octane (reducing agent) donates electrons to oxygen (oxidizing agent), forming carbon dioxide and water in large quantities.
Hydrolysis of glucose is another useful example of a common reduction:
C 6 H 12 O 6 + 2ADP + 2P + 2NAD + → 2CH 3 COCO 2 H + 2ATP + 2NADH
In this reaction the molecules of NAD (an electron receptor and oxidizing agent in this reaction) take electrons from glucose (reducing agent).
Lastly, in the ferric oxide reaction
Fe 2 O 3 (s) + 2Al (s) → Al 2 O 3 (s) + 2Fe (l)
The reducing agent is aluminum, while the oxidizing agent is iron.