Reducing agent

In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include the alkali metals, formic acid, oxalic acid, and sulfite compounds. 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). This is commonly expressed in terms of their oxidation states. An agent's oxidation state describes its degree of loss of electrons, where the higher the oxidation state then the fewer electrons it has. So initially, prior to the reaction, a reducing agent is typically in one of its lower possible oxidation states; its oxidation state increases during the reaction while that of the oxidizer decreases. Thus in a redox reaction, the agent whose oxidation state increases, that "loses/donates electrons", that "is oxidized", and that "reduces" is called the or , while the agent whose oxidation state decreases, that "gains/accepts/receives electrons", that "is reduced", and that "oxidizes" is called the or . For example, consider the overall reaction for aerobic cellular respiration: The oxygen () is being reduced, so it is the oxidizing agent. The glucose () is being oxidized, so it is the reducing agent. Consider the following reaction: 2 [Fe(CN)_6]^4− + ChlorineCl2 → 2 [Fe(CN)_6]^3− + 2 ChlorideCl- The reducing agent in this reaction is ferrocyanide ([Fe(CN)_6]^4−). It donates an electron, becoming oxidized to ferricyanide ([Fe(CN)_6]^3−). Simultaneously, that electron is received by the oxidizer chlorine (Cl2), which is reduced to chloride (Cl-). 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.