Reducing agents
What is a reducing agent?
Reducing agents are also called reducers and are substances that donate electrons and in this way reduce other substances. In this process, the reducing agents themselves become oxidized.[1]
Thus, reducing agents are the opposite of so-called oxidizing agents or oxidizers. While oxidizing agents are substances that can accept electrons, reducing agents donate electrons. Therefore, oxidizing agents are also called electron acceptors and reducing agents are called electron donors. Thus, in contrast to reducing agents, oxidizing agents oxidize other substances and are themselves reduced.[2]
How can oxidizing and reducing agents be used economically?
Reducing agents and oxidizing agents play an important role in the so-called redox reaction, also called reduction-oxidation reaction. In this chemical reaction, one of the reactants transfers electrons to the other. In every redox reaction, therefore, there is a release of electrons (oxidation) as well as an acceptance of electrons: substance A (reducing agent) releases electrons and substance B accepts the electrons (oxidizing agent). This is why this chemical reaction is also called an electron transfer reaction.
As a rule, oxidations are exothermic, i.e. light or heat energy is released. One speaks of a galvanic system when oxidation and reduction take place spatially separated from each other. In this case, the flow of electrons can be used to generate energy. Examples of applications are batteries or accumulators, as used in the hydrogen fuel cell and the direct methanol fuel cell.[3]
Reducing agent examples: Which reducing agents do exist?
Which substances act as reducing agents? A reducer, for example, is the substance that removes oxygen from another substance – for example, an oxide – and oxidizes itself in the process. Coke, for example, reduces iron ore under extreme heat, oxidizing itself and becoming carbon dioxide. Just like the reduction of iron oxide to iron, this reaction takes place in the blast furnace.[1]
The combustion of magnesium to magnesium oxide is also a good example of the interplay between oxidizing agent and reducing agent. Here, the oxygen acts as the oxidizing agent, while the magnesium represents the reducing agent. The combustion is strongly exothermic.
An exothermic redox reaction, which serves as the basis for the fuel cell, is the combustion of hydrogen to H2O. In this process, the hydrogen gives off electrons, thus acting as a reducing agent, while the oxygen accepts electrons and thus acts as an oxidizing agent.[4] The fuel cell functions here as a galvanic element, as an electrochemical energy converter. A redox reaction takes place continuously, in which energy is generated by the flow of electrons. Suitable reducing agents include hydrogen and the fuels methanol or natural gas. Oxygen is one of the oxidizing agents.[6]
What is the best reducing agent?
The question of whether a substance is suitable as a reducing agent or as an oxidizing agent is primarily determined by two things: the oxidation state of the substance and the nature of the respective reaction partner. It is therefore important which of the substances reacting with each other accepts electrons and which donates them.
Thus, reducing agents are very easily able to give up their electrons. Oxidizing agents, on the other hand, very easily accept electrons. The reason: By accepting further electrons, they rise in the coveted noble gas configuration. Examples of reducing agents are base metals. The less noble the metal, the better its performance as a reducing agent.
Substances with very low oxidation numbers are also strong reducing agents. These include hydrides such as lithium aluminum hydride (LiAlH4) and sodium borohydride (NaH), but also sulfides such as hydrogen sulfide (H2S) and sodium sulfide (Na2S). Other examples include thiosulfates (S2o32) and the sulfites sodium sulfite (Na2SO3), sulfurous acid (H2SO3), and sulfur dioxide (SO2 5 Lithium is listed as the strongest reducing agent, and fluorine as the strongest oxidizing agent.[7]
Conclusion
The definition of reducing agents is easy: A reducing agent is a substance that is oxidized by donating electrons. This is why reducing agents are also called electron donors. They help reduce a substance in a reduction reaction. The opposite of the reducing agent is the oxidizing agent or electron acceptor. It ensures that a substance is oxidized during a reduction. In a redox reaction, the oxidizing and reducing agents react with each other, releasing heat. This principle is used in fuel cell technology, which generates heat and thus energy through the flow of electrons.
