Redox Reactions

What are Redox Reactions?

The term redox is made up of two words reduction and oxidation. A type of chemical reaction that involves a transfer of electrons between two species is called redox reaction. In these types of reactions oxidation and reduction both take place together. 


Examples of Redox Reactions 

  • Cellular Respiration – In respiration reaction glucose reacts with oxygen and forms carbon dioxide and water and releases energy which is stored in the cells. Glucose gets oxidized into carbon dioxide by losing hydrogens while oxygen gets reduced into water by gaining hydrogens. reaction is given below –

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  • Combustion – Combustion is an exothermic redox chemical reaction which occurs at high temperature and in presence of an oxidant. Methane burns in presence of atmospheric oxygen and gives carbon dioxide and water with energy. A well explained combustion reaction of methane is given below –

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  • Photosynthesis – Photosynthesis is a process by which plants prepare their food. In this reaction carbon dioxide reacts with water in presence of sunlight and chlorophyll and gives carbohydrate and oxygen. A well explained reaction of photosynthesis is given below –

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  • Corrosion – Rusting process is an example of corrosion. In this reaction iron reacts with atmospheric oxygen in presence of moisture and forms iron oxide which is also called rust. A well explained reaction of rusting is given below –

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  • Formation of Sodium Chloride – In this reaction sodium reacts with chlorine and forms sodium chloride. In the reaction oxidation number of sodium increases from 0 to +1 while oxidation number of chlorine atom decreases from 0 to -1. It can also be explained in terms of gain and loss of electrons. A well explained reaction is given below –

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What is Oxidation? 

Oxidation in Terms of Oxygen Transfer – Oxidation is gain of oxygen. 

Example – 2Mg(s) + O2(g) 🡪 2MgO(s) 

Oxidation in Terms of Electron Transfer – Oxidation is loss of electrons. In the above example magnesium is losing two electrons and getting oxidized to form magnesium oxide. 

Oxidation in Terms of Hydrogen Transfer – Oxidation can be defined as loss of hydrogen.

Example - (image will be uploaded soon)

Oxidation in Terms of Oxidation Number – Oxidation is an increase in the oxidation state or oxidation number of an atom in a reaction. Oxidation number defines the degree of oxidation of an atom in a chemical compound. 

Example -  (image will be uploaded soon)

In the above example oxidation state of sodium is increasing from 0 to +1. Thus, oxidation is taking place and sodium is getting oxidized. 


What is Reduction?

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Reduction in Terms of Oxygen Transfer – Reduction is loss of oxygen. 

Example - (image will be uploaded soon)

Reduction in Terms of Electron Transfer – Reduction is gain of electrons. 

Example - 2Mg(s) + O2(g) 🡪 2MgO(s)

In the above example, each atom of oxygen gains two electrons and forms two O-2 anions. Thus, reduction takes place and oxygen get reduced.  

Reduction in terms of hydrogen transfer – Reduction can be defined as gain of hydrogen.

Example - (image will be uploaded soon)

Reduction in Terms of Oxidation Number – Reduction is decrease in the oxidation state or oxidation number of an atom in a reaction. 

Example - (image will be uploaded soon)

In the above example oxidation state of chlorine is decreasing from 0 to -1. Thus, reduction is taking place and chlorine is getting reduced. 


What is Oxidizing Agent and Reducing Agent?

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A substance which can oxidize another substance is called an oxidizing agent. It is also called oxidant. It oxidizes another substance by accepting their electrons. Thus, we can say an oxidizing agent is an electron acceptor group. An oxidizing agent always gets reduced itself and oxidizes another substance. Oxygen, hydrogen peroxide and halogens are examples of some common oxidizing agents. 


The oxidizing agent can be defined as those groups which transfer oxygen atom to the substrate although its not true in every case as in many redox reactions, oxidation – reduction takes place in absence of oxygen atoms as well. These oxidizing agents are also called oxygenation reagents or oxygen – atom transfer (OAT) agents. MnO4-, CrO4-2 etc. are examples of these types of oxidizing agents. You can notice here that these all are oxides. 

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A substance which can reduce another substance is called reducing agent. It is also called reductant or reducer. It reduces another substance by donating its electrons. Thus, we can say the reducing agent is an electron donor group. A reducing agent always gets oxidized itself and reduces another substance. Lithium aluminium hydride (LiAlH4), Hydrogen, sodium amalgam [Na(Hg)]are examples of some common reducing agents.


Reducing agent can be defined as those groups which gain oxygen atom from the substrate (or oxidizing agent) although it is not true in every case as in many redox reactions, oxidation – reduction takes place in absence of oxygen atoms. Examples of these reducing agents include formic acid, oxalic acid, sulfites etc.


Oxidizing Agent and Reducing Agent in a Redox Reaction 

Let’s understand oxidizing and reducing agents by taking an example of a redox reaction. When aluminum reacts with iron(III) oxide in presence of heat, it gives aluminum oxide and molten iron metal. It is a redox reaction. Reaction is given below-

2Al(s) + Fe2O3(s) 🡪 Al2O3(s) + 2Fe(l)

If you calculate the oxidation number for Al then you see that it's increasing from 0 to +3, it means oxidation is taking place. Now if you calculate it for iron then you see that its decreasing from +3 to 0, it means reduction is taking place. It is represented below in the reaction –

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You can see as Al is reducing Fe2O3 by removing its oxygen atoms, so it means aluminum is reducing agent. While Fe2O3 is oxidizing Al by giving oxygen to it so it means Fe2O3 is acting as an oxidizing agent. In redox reactions reducing agent always convert into its conjugating oxidizing agent in an oxidation – reducing reaction. Thus, the products of this reaction will include a new oxidizing agent and a new reducing agent. It is represented below in the reaction –

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As we know the above reaction proceeds in the forward direction, so it means Al is a stronger reducing agent and Fe2O3 is a stronger oxidizing agent than iron and Al2O3 respectively.

Another example of a redox reaction in which oxidizing agent and reducing agent have been indicated clearly –

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What is Oxidation Number? 

In order to keep track of electron shifts in chemical reactions involving formation of covalent compounds, a more practical method of using oxidation number has been developed. Oxidation number can be defined as an imaginary or apparent charge developed over an atom of an element when it goes from its elemental free state to combined state in molecules. 


Oxidation number denoted the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that the electron pair in a covalent bond belongs entirely to more electronegative element. 


Rules for Calculating Oxidation Number

The following rules must be kept in mind while assigning the oxidation number to the atom in a molecule or in ionic state - 

  • Different elements in their elementary state or in allotropic form bears 0 as their oxidation number. For example, Nitrogen(N2) and hydrogen(H2) in their elemental state will have zero oxidation state. Another example is, in diamond, graphite and buckminsterfullerene carbon has oxidation number ‘0’.

  • The charge present on any monoatomic ion is its oxidation number. For example, the oxidation number of Mg2+ is 2, whereas the oxidation number of Al3+ is 3.

  • Hydrogen shows two oxidation states. When it combines with non-metals, it shows the oxidation state of +1 and when combined with metals and metal hydrides shows the oxidation state of -1. For example, in H2S, the oxidation state of hydrogen is +1 because S is non-metal, whereas in MgH2 the oxidation state of hydrogen is -1 as Mg is a metal.

  • The most common oxidation state of oxygen is -2. When it combines with metals it shows the oxidation state of -2, although it shows other oxidation states as well such as in peroxide it shows -1 oxidation state. With fluorine in OF2, it shows the oxidation state of +2 and in O2F2 it shows the oxidation state of +1.

  • When different elements unite to form compounds, then more electronegative element will show negative oxidation state, whereas less electronegative element will show positive oxidation state. For example, in NaCl, Na shows +1 oxidation state, whereas Cl shows -1 oxidation state. Also, the oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is +1 in HOCl. 

  • The sum of oxidation numbers of all the atoms in a compound is zero. For example, in NaCl, the sum of oxidation state of Na and Cl is 0, which is +1-1=0.

  • In complex ions, the sum of the oxidation numbers of all the atoms in the ionic state is equal to the charge on the ion. For example, in hexacyano ferrate (II) ion [Fe (CN)6]-4, the sum of oxidation state of Fe and CN is -4. In other words, in Radicals/Polyatomic ions, the oxidation state is the charge on the ion, which in turn is equal to the algebraic sum of the charge on all the atoms. For example, in NH4+, the sum of oxidation state of N and H is +1.

Applying all the above rules on the compound (NH4)2SO4 to calculate the oxidation number of Nitrogen.

Let us suppose oxidation state of nitrogen is x in (NH4)2SO4

As we know, Oxidation state of hydrogen is +1 and that of sulphate is -2 and the given compound carries zero charge. 

Therefore, the sum of all the oxidation states of elements will be zero.

2x + 2(+1×4) + (-2) = 2x + 6

2x + 6 = 0

x = -3

Hence the oxidation number of Nitrogen in (NH4)2SO4 is -3. 


Types of Redox Reactions 

Redox reactions are of following four types – 

  • Combination reactions 

  • Decomposition reactions 

  • Displacement reactions 

  • Disproportionation reactions 

Combination Reactions - In combination reaction, two or more molecules are combined together chemically to form a new substance (compound). For example, when we burn magnesium ribbon (or magnesium) it gives grey – black ash of magnesium oxide. Another example is reaction between magnesium and nitrogen. 

Example - (image will be uploaded soon)


Decomposition Reaction - In decomposition reaction, molecules or compounds break down into two or more than two simpler chemically new substances. Combination and decomposition reactions are opposite of each other. For example, electrolysis of water. In electrolysis of water, water breaks down into hydrogen and oxygen which show completely different properties than water. 

Examples  - 2H2O electricity⟶ 2H2 + O2

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Displacement Reaction – In these reactions more reactive metal displaces less reactive metal from its salt. In these reactions, products can be determined through reactivity series. Reactivity series is a series in which elements are arranged in decreasing order of their reactivity. It means the elements present at the top of this reactivity series are more reactive than the elements present at the bottom. 


Reaction of potassium with magnesium chloride is an example of single displacement reaction. In this reaction, potassium displaces magnesium from its salt because potassium is more reactive than magnesium. Potassium is present at the top of the reactivity series and is the most reactive element.

Reaction – 2K + MgCl2 🡪 2KCl + Mg


Disproportionation Reactions – These are special type of redox reactions. In these reactions an element in one oxidation state is simultaneously oxidized and reduced. In these reactions one of the reacting substances always contains an element that can exist in at least three oxidation states. Decomposition of hydrogen peroxide is a common example of this type of reactions. Equation is given below –

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Balancing of Redox Reactions - For balancing the redox reactions oxidation number method and half reaction method are used. 


This ends our coverage on the topic “Redox reactions''. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various sub topics such as balancing of redox reactions, oxidation number, oxidation and reduction etc. on Vedantu website. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.