A reagent in a chemical reaction is known to be a ‘reducing agent’, if the reagent removes oxygen, contributes hydrogen, or contributes electrons. Reducing agents, reductants, or reducers are the substances that have the capacity to reduce other substances. If reduction of some substance is the preffered ending, then it is useful to find an agent which will readily achieve the reduction. Substance that brings about reduction (depletion of oxygen or addition of hydrogen) in other substances by being itself oxidized in a chemical reaction and is known as “Reductant” or “reducing agent”. Hydrogen gas is a very useful reducing agent, used widely in the separation of pure metals by reduction. The reducing agents play a vital role in photosynthesis which is crucial to our continued existence. On the other hand, substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents. In simple words, reduction is the gain of electrons and oxidation is the loss of electrons.
Generally, the short form for Reduction-Oxidation is known as “Redox” reactions. In order to understand the concept of reducing agents in chemistry, it is important to get a thorough knowledge of Redox reactions. Redox reactions explains each and every one the chemical reactions in which atoms have change in their oxidation number (oxidation state), as redox reaction leads to the loss or gain of electrons which, in turn, changes the oxidation number. This can be either a simple redox process, such as the oxidation of carbon to give carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a multifaceted phenomenon like the oxidation of sugar (C6H12O6) in the human body through a series of complex electron transfer processes. Oxidation number is the number allocated to an element in chemical combination that depicts the number of electrons lost (or gained, if the number is negative) by an atom of that element in the compound.
The name “Redox” arises from the two perceptions of reduction and oxidation which can be described simply as:
Oxidation and reduction appropriately refer to a change in oxidation number — the actual transfer of electrons may never take place. As a result, oxidation is defined better as an increase in oxidation number, and reduction as a decrease in oxidation number. In practice, the transfer of electrons will always cause a change in oxidation number, but there are many reactions that are classed as "redox" even though no electron transfer takes place like those which encompasses covalent bonds, which leads to the sharing of electrons between the atoms. A simple example of a redox reaction which can be portrayed in terms of oxygen transfer is given as: CuO + Mg → Cu + MgO. If this equation is rewritten as ionic equation (in terms of transfer of ions in the metals), then the equation becomes: Cu2+ + Mg → Cu + Mg2+. In this ionic equation, the magnesium is reducing the copper (II) ions by donating electrons to neutralize the charge. So, here Magnesium is a ‘reducing agent’. On the other hand, the copper (II) ions are removing electrons from the magnesium to create the magnesium ions. The copper (II) ion here acts as an ‘oxidizing agent’.
A reducing agent, or reductant, loses electrons and is oxidized in a chemical reaction. Classically, a reducing agent is in one of its lower possible oxidation states and is known as the ‘electron donor’. A reducing agent is oxidized because it loses electrons in the redox reaction. Examples of reducing agents include the earth metals, formic acid, and sulfite compounds. A Reducing Agent may be defined in different ways, depending upon the circumstance in which the term is used. Generally, it is often taken to mean a chemical which can act as an electron donor. Consequently, in the following reaction,
Zn2+ (aq) + 2Fe2+ (aq) → Zn(s) +2Fe3+ (aq), where ‘aq’ stands for aqueous solution and‘s’ stands for solid. In this example, the zinc is being reduced (gaining electrons) by reaction with the iron cations; the Fe2+ in this example is acting as a reducing agent. This explanation is more general than the one which follows if we regard "reduction" as being just the removal of oxygen from a substance.