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Oxidizing and Reducing Agents

  • Last Updated : 21 Sep, 2021

A process in which one or more reactants combine together to form one or more different substances, called products. The forming substances are called chemical elements or compounds. A chemical reaction results in the rearrangement of the constituent atoms of the reactants to result in the formation of products. The physical appearance of reactants are different from the products. Chemical reactions happen continuously in general surroundings, for instance, rusting of iron or fermentation of wine etc.

What is a Chemical Reaction?

Chemical reactions result in the conversion of chemicals or substances to one or more other substances. The groups of atoms rearrange themselves leading to the breakage and formation of chemical bonds in a chemical reaction. 

These reactions are accompanied by the appearance of gas, precipitate, and color, light. For instance, if we consider the reaction of oxidation of propane, it results in the release of heat and light, denoted by the following reaction, 

C3H8 + 5O2 → 3CO2 + 4H2O

Terminology associated with Chemical Reactions:



  • Two or more molecules associate with each other to form a new product(s) in a chemical reaction.
  • The compounds reacting together are called reactants whereas the newly formed compounds are called products.
  • The reaction rate is moderated by factors like pressure, temperature, the concentration of reactants.
  • A chemical reaction is accompanied by physical changes too, observable by the human eye, for instance, precipitation, heat production, colour change etc.

Types of Chemical Reactions

There are various types of chemical reactions based on the way the reactants combine together, the by products etc, for instance, 

  • Combination Reaction: The chemical reaction occurs when two elements, or one element and one compound or two compounds aggregate together to form a single product is called combination reaction.
  • Decomposition Reaction: The chemical reaction occurs when splitting of a compound into two or more simpler by products occur is called decomposition reaction.
  • Displacement Reaction: This type of chemical reaction happens when a more reactive metal displaces a less reactive metal.
  • Double displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
  • Precipitation Reaction: This type of chemical reaction is accompanied by the formation of an insoluble compound called precipitate forms in these types of reactions.
  • Exothermic Reaction: The chemical reactions which produce energy are called exothermic reactions. Mostly, the decomposition reactions are exothermic.
  • Endothermic Reaction: The chemical reactions which absorb energy are called endothermic reactions. Mostly, the combination reactions are endothermic. These reactions are reverse of exothermic reactions.
  • Oxidation Reaction: The oxidation reactions are accompanied by the gain of oxygen or removal of hydrogen or metallic element from the chemical compound.
  • Reduction Reaction: The reduction reactions are accompanied by the loss of oxygen or gain of hydrogen or metallic element from the chemical compound. These reactions are the reverse of oxidation reactions.
  • Redox Reaction: The chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction, formed by the words, reduction and oxidation.

Chemical Equations

The chemical equations are denoted using the chemical formulae and symbols to represent the changes occurring between the reactants and products. 

A chemical equation is represented by reactants written on the left-hand side and the products on the right-hand side. A symbol, possibly an arrow also denotes the direction of the reaction. Also, each of the reacting entities is assigned its corresponding stoichiometric coefficient.

Examples of Chemical Equations are:

  • PCl5 + 4H2O → H3PO4 + 5HCl
  • SnO2 + 2H2 → 2H2O + Sn
  • TiCl4 + 2H2O → TiO2 + 4HCl
  • Na2S + 2AgI → 2NaI + Ag2S

Oxidising Agents

The substances which can oxidize other substances are called an oxidizing agents. Therefore, oxidizing agent acts as an electron acceptor group, since they accept electrons. These agents hold a property of always getting reduced itself and oxidising another substance. 

Oxidising agents can also be termed as oxidants or oxygenation reagents or oxygen – atom transfer (OAT) agents. Some of the common examples of oxidising agents are oxygen, hydrogen peroxide and halogens. All the oxidising agents are oxides in nature. 

These groups are responsible for transferring oxygen atoms to the substrate. However, some of the redox reactions, oxidation – reduction takes place in absence of oxygen atoms. Some of the examples of the oxidising agents are MnO4, CrO4-2 etc. 

Common oxidizing agents are,



  • Oxygen compounds, such as, Oxygen (O2) and Ozone (O3)
  • Nitric acid (HNO3) and nitrate compounds
  • Hypochlorite, Chlorite, chlorate, perchlorate, and other analogous halogen compounds like household bleach (NaClO)
  • Permanganate compounds such as potassium permanganate (KMnO4)
  • Sodium perborate
  • Halogens, such as Fluorine (F2), chlorine (Cl2)
  • Hydrogen peroxide (H2O2) and other inorganic peroxides, Fenton’s reagent
  • Peroxymonosulfuric acid
  • Sodium dichromate
  • Nitrogen compounds, such as, Nitrous oxide , Nitrogen dioxide/Dinitrogen tetroxide (NO2 / N2O4)
  • Potassium nitrate (KNO3), the oxidizer in black powder
  • Sodium bismuthate (NaBiO3)
  • Lead dioxide (PbO2)

Applications of Oxidizing Agents

Oxidizing agents find their usage in a large number of both commercial and industrial applications, some of which are,

  1. Purification of water.
  2. Storage of energy in batteries.
  3. Fuel Combustion.
  4. Bleaching of fabrics.
  5. Biological processes such as metabolism and photosynthesis.
  6. Vulcanization of rubber (increasing the strength and the elasticity of rubber).

Reducing Agents

A reducing agent is an element or a compound which loses or “donates” an electron to an electron recipient. The recipient is the oxidising agent, as described earlier. Therefore, the reducing agents are also called a reductant, reducer, or electron donor. 

Therefore, both the chemical compounds therefore find their usage in the redox reactions. The reducing agent’s oxidation state increases whereas on the contrary the oxidising agent decreases. Therefore, the reducing agents “reduce” oxidizers by decreasing their oxidation state whereas, the oxidizing agents “oxidise” reducers by increasing their oxidation state. Also, the metals of the s-block in the periodic table are known to act as good reducing agents.

Any good reducing agent consists of the atoms which have low electronegativity and a good tendency to attract the bonding electrons having negligibly low ionization energies.

Characteristics of Reducing Agent

  1. Reducing agents tend to give away electrons and get oxidised. This also causes the other reactant to get reduced by supplying electrons to it.
  2. Since, they give electrons, they become less negatively charged, that is move towards the positively charged.
  3. Greater the tendency to lose electrons, stronger is the reducing agent. This is because the reducing agent will reduce the other substances by donating electrons powerfully.
  4. The oxidation and reduction reactions involve the transfer of electrons during the reactions.

Following are the common reducing agents,

  • Sodium borohydride
  • Lithium aluminum hydride (LiAlH4)
  • Acids, such as Oxalic acid and Formic acid
  • Amalgams, such as Zinc amalgam and Sodium amalgam
  • Diborane
  • Sodium lead alloy
  • Nascent hydrogen
  • Thiosulfates

Identifying Oxidizing and Reducing Agents

An oxidation reaction is a type of chemical reaction in which there is a loss of an electron from one substance. A reduction reaction, to the contrary is a reaction that supplies an electron to a substance. A redox reaction is a combination of oxidation and reduction reaction in which one of the substance gives up an electron and another substance takes that electron. 

In order to identify an oxidizing agent, the information can be grasped from the oxidation number of an atom before and after the reaction. An increase in the oxidation number of the compound towards the product side is because the substance lost electrons and was oxidized. A decrease in the oxidation number of the compound towards the product side is because the substance gained electrons and was reduced. The substance that is reduced in a reaction is the oxidizing agent because it gains electrons. The substance that is oxidized in a reaction is the reducing agent because it lost electrons. The following table summarises the above discussion :

Name 



Oxidation Number  

Change in Electrons

Oxidation 

Increases 

Electrons are lost

Reduction 

Decreases 

Electrons are gained

Oxidizing agent 

Decreases 

Electrons are gained

Reducing agent 

Increases 

Electrons are lost

Sample Questions

Question 1: Differentiate between Oxidising Agent and Reducing Agent.

Answer:

Following are the differences between Oxidising Agent and Reducing Agent:

Reducing Agent 

Oxidising Agent



A reducing agent can be oxidized by losing some of its electrons.An oxidizing agent can be reduced by obtaining electrons.
The oxidation state of reducing agent increases.The oxidation state of oxidizing agent decreases.
Electron donor.Electron receiver.
Reducing agent is oxidized during the reaction.Reduced during the reaction.
Responsible for causing the reduction of another reactant.Responsible for causing the oxidation of another reactant.

Question 2: Describe an oxidation reaction where the oxidation of a positive oxidation state increases over the reaction.

Answer:

Since, we know, H2C2O4 is known to be a good reducing agent. In this chemical composition, the oxidation state of the C atom is +3. The highest oxidation state that can be attained by the C atom is +4.  Therefore, we can carry out its oxidation into CO2. The half reaction is, therefore, given by, 

H2C2O4 → 2CO2 + 2H+ + 2e

Question 3: Sulphuric acid and hydrochloric acid are not oxidising agents in comparison to nitric acid. Explain why. 

Answer:

Hydrochloric acid acts as a reducing agent, since, it loses a hydrogen atom. Thereby, it carries out the reduction of other substances and gets oxidised to chlorine. Similarly, sulphuric acid acts as an oxidising agent which on carrying out thermal decomposition, yields nascent oxygen. Nitric acid (HNO3) is considered to be a powerful oxidising agent and the oxidising properties are due to the production of nascent oxygen which it gives on decomposition.

Question 4: Identify the reducing agent in the following reactions : 

4NH3 + 5O2  ⇢  4NO +6H2O

H2O + F2  ⇢ HF + HOF



Answer:

In the first reaction, ammonia (NH3) is reducing agent, since the oxidation state of oxygen is reduced from 0 to -1. The halogen, Fluorine (F2), undergoes the reduction reaction, therefore, acting as an oxidising agent. Hence, water (H2O) is a reducing agent here.

Question 5: Why does aqua regia dissolve noble metals? 

Answer:

We know, the formation of aqua regia can be depicted by the following chemical reaction:

HNO3 + 3HCl → NOCl + 2H2O +2Cl2

that is, Nitric acid reacts with the hydrochloric acid to form a mixture called aqua regia as the final product. The right hand side contains a product called NOCl nitrosyl chloride which is a stronger oxidizing agent than nitric acid. Therefore, pure nitric acid does not dissolve noble metals but aqua regia dissolves them easily.

Question 6: What are redox reactions?

Answer:

Redox reactions are formed by the combination of the terms oxidation-reduction chemical reactions in which the reactants, that is the constituents atoms and molecules undergo a change in their oxidation states. All the redox reactions can be further broken down into two different processes – a reduction process and an oxidation process which occur simultaneously.

Question 7: Why is oxygen considered to be oxidising agent? Explain using a reaction with a metal. 

Answer:

Oxygen in elemental form is a strong oxidizing agent. Oxygen shows good reactivity and easily combines with most other elements and many compounds. When a compound like magnesium reacts with oxygen, the magnesium atoms give or donate electrons to oxygen (O2) molecules. This results in the reduction of oxygen. Magnesium metal therefore acts as a reducing agent. Magnesium is therefore oxidised. Oxygen, thereby is an oxidizing agent in this case.




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