Characteristics of Reducing Agents
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 oxidize or lose electrons is known as its oxidation potential". The table below shows a few reduction potentials that could easily be changed to oxidation potential by simply reversing the sign. Reducing agents can be ranked by increasing strength by ranking their oxidation potentials. The reducing agent is stronger when it has a more positive oxidation potential and weaker when it has a negative oxidation potential. The following table provides the reduction potentials of the indicated reducing agent at 25 °C.
| Oxidizing agent | Reducing agent | Reduction potential (V) |
|---|---|---|
| Li+ + e− = | Li | −3.04 |
| Na+ + e− = | Na | −2.71 |
| Mg2+ + 2e− = | Mg | −2.38 |
| Al3+ + 3e− = | Al | −1.66 |
| 2H2O(l) + 2e− = | H2(g) + 2OH − | −0.83 |
| Cr3+ + 3e− = | Cr | −0.74 |
| Fe2+ + 2e− = | Fe | −0.44 |
| 2H+ + e− = | H2 | 0.00 |
| Sn4+ + 2e− = | Sn2+ | +0.15 |
| Cu2+ + e− = | Cu+ | +0.16 |
| Ag+ + e− = | Ag | +0.80 |
| Br2 + 2e− = | 2Br− | +1.07 |
| Cl2 + 2e− = | 2Cl− | +1.36 |
| MnO4− + 8H+ + 5e− = | Mn2+ + 4H2O | +1.49 |
To tell which is the strongest reducing agent, one can change the sign of its respective reduction potential to make it oxidation potential. The bigger the number, the stronger the reducing agent. For example, among Na, Cr, Cu and Cl−, Na is the strongest reducing agent and Cl− is the weakest one.
Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain the H− ion, those being NaH, LiH, LiAlH4 and CaH2.
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.
- 2 Li(s) + H2(g) → 2 LiH(s)
Hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.
Half reactions: 2 Li(s)0 → 2 Li(s)+ + 2 e−::::: H20(g) + 2 e− → 2 H−(g)
- H2(g) + F2(g) → 2 HF(g)
Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.
Half reactions: H20(g) → 2 H+(g) + 2 e−::::: F20(g) + 2 e− → 2 F−(g)
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