Redox Reactions

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Combination of two words (reduction and oxidation), a redox reaction is a type of chemical reaction which involves transfer of electrons between two species. Oxidation and reduction takes place simultaneously in such reactions.


Cellular Respiration

In cellular respiration, while glucose gets oxidized into carbon dioxide by losing hydrogens, oxygen gets reduced to water by gaining hydrogens. 


Being an exothermic redox chemical reaction, it occurs at high temperatures, that too in the presence of an oxidant. Methane burns in the presence of atmospheric oxygen to give carbon dioxide and water energy.


In photosynthesis, carbon dioxide reacts with water in the presence of sunlight and chlorophyll to give carbohydrates and oxygen. 


Iron reacts with atmospheric oxygen in the presence of moisture to form iron oxide, also known as rust. 

Formation of Sodium Chloride

Sodium reacts with chlorine to form sodium chloride. While the oxidation number of sodium increases from 0 to +1, the oxidation number of chlorine atoms decreases from 0 to -1.


‘Oxidation’ involves gain of oxygen.

Example – 2Mg(s) + O2(g) 🡪 2MgO(s)

It results in loss of electrons, hydrogen as well.

‘Oxidation number’ of an atom increases in such reactions.


It results in loss of oxygen, gain of electrons.

Example – 2Mg(s) + O2(g) 🡪 2MgO(s) 

Here, each atom of oxygen gains two electrons forming O-2 anions.

‘Reduction’ can also be described as the gain of hydrogen.

Reduction results in the decrease of oxidation number of an atom involved in the reaction.

Oxidising Agent

  • A substance that is capable of oxidizing another substance is called an oxidizing agent.
  • Also known as an oxidant, it is an electron acceptor group.
  • Oxygen, hydrogen peroxide, and halogens are among the common examples of oxidizing agents.  
  • In certain situations, oxidation-reduction happens in absence of oxygen atoms as well. Such oxidizing agents are known as oxygenation reagents or oxygen-atom transfer (OAT) agents.

Reducing Agent

  • A substance that reduces another substance is known as a reducing agent.
  • It is basically an electron donor group.
  • Na(Hg) is an example of a common reducing agent.
  • At times, oxidation-reduction takes place in the absence of oxygen atoms as well. Formic acid, Oxalic acid and sulfites are examples of such reducing agents. 

Oxidising Agent and Reducing agent (in Redox Reaction)

Example: 2Al(s) + Fe2O3(s) 🡪 Al2O3(s) + 2Fe(I)

  • In this reaction, AI is reducing Fe2O3 by removing its oxygen atoms. Hence it is the reducing agent.
  • Meanwhile Fe2O3 is oxidizing AI by lending oxygen to it. Hence it is an oxidizing agent.  
  • The products of redox reactions will include a new oxidizing agent and a new reducing agent.

Oxidation Number

Oxidation number is an imaginary/apparent charge built over an atom of an element as it goes from its elemental free state to a combined state in molecules.

Rules (Calculation of Oxidation Number)

  • Several elements bear 0 as their oxidation number in their elemental state or allotropic form. Examples: Nitrogen ((N2), diamond and graphite.
  • The charge present on any monatomic ion will be its oxidation number.
  • Hydrogen, as it combines with non-metals, shows an oxidation state of +1. On the other hand, while combining with metals and metal hydrides, it displays the oxidation state of -1.
  • -2 is the most common oxidation state of oxygen.
  • While different elements unite to form compounds, more electronegative elements will display a negative oxidation state, whereas hardly any electronegative elements will show a positive oxidation state.
  • The sum of oxidation numbers of all the atoms in a compound will be zero. Let’s take the example of NaCl. The sum of oxidation states of Na and Cl is 0.
  • In case of complex ions, the sum of the oxidation numbers of all the atoms in the ionic state will be equal to the charge on the ion.
  • In the case of Radicals/Polyatomic ions, the oxidation state will be the charge on the ion, which in turn, will be equal to the algebraic sum of the charge on all the atoms.    

Different Types of Redox Reactions

Combination reactions, decomposition reactions, displacement reactions and disproportionation reactions are the four types of redox reactions.

Combination Reactions

Two or more molecules combine together chemically to form a new substance. Example: reaction between magnesium and nitrogen.

Decomposition reactions

In decomposition reactions, compounds break down into simpler substances. 

Example: Electrolysis of water. Water breaks down into hydrogen and oxygen.

Displacement reactions

In such reactions, more reactive metal will displace less reactive metal from its salt. 

The series in which elements are arranged in the decreasing order of their reactivity is known as the reactivity series. Inshort, elements present at the top of the reactivity series will be more reactive than the elements present at the bottom.

Disproportionation reactions

In this special type of redox reaction, an element in one oxidation state will be simultaneously oxidized and reduced.     

Example: Decomposition of Hydrogen Peroxide.

Equation: 2 CuBr 🡪 CuBr2 + Cu

It should be noted that oxidation number method and half-reaction method are used for balancing redox reactions. 

Check Your Knowledge

A redox reaction is a type of chemical reaction which involves transfer of electrons between two species. Oxidation and reduction takes place simultaneously in such reactions.

Oxygen, hydrogen peroxide, and halogens.

Combination reactions, decomposition reactions, displacement reactions and disproportionation reactions.   

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