Oxidation and Reduction

Have you ever noticed that sliced apples turn brown when left in contact with air? Well not only sliced apples, to make your favorite French fries when we peel and slice potatoes and leave them in contact with air for some time then they also become grey or black in color. Do you know why this is happening? Well, the answer is oxidation. Sliced apples turn brown due to oxidation of phenolic compounds present in apples under the action of an enzyme called polyphenol oxidase and peeled or sliced potatoes become grey or brown in color due to oxidation of starch present in potatoes. In the same way you can find many examples of reduction as well in your daily life. Such as in photosynthesis carbon dioxide converts into carbohydrate by the process of reduction. The reactions in which oxidation and reduction both occur together are named as redox reactions. You can see many examples of redox reactions also in your everyday life. Such as respiration, photosynthesis, corrosion, combustion, bleach etc. all processes involve both oxidation and reduction. 

What is Oxidation? 

Oxidation is a chemical process which can be explained by following four point of views –

  • In terms of oxygen transfer 

  • In terms of electron transfer 

  • In terms of hydrogen transfer 

  • In terms of oxidation number 

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.

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. 

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? 

Reduction is a chemical process which can be explained by following four point of views –

  • In terms of oxygen transfer 

  • In terms of electron transfer 

  • In terms of hydrogen transfer 

  • In terms of oxidation number 

Reduction in terms of oxygen transfer –Reduction is loss of oxygen. 

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 gets reduced.  

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

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

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 Redox Reaction? 

A chemical reaction which involves transfer of electrons or change in oxidation number of atoms is called redox reaction. In redox reactions oxidation and reduction takes place simultaneously. Oxidation and reduction cannot take place independently of one another. Few examples of redox reactions are given below –

  • 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 –

  • 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 –

  • 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 –

  • 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 –

  • 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 –


What is Oxidizing Agent?

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. 

Oxidizing agents can be defined as those groups which transfer oxygen atoms to the substrate although it's not true in every case as in many redox reactions, oxidation – reduction takes place in absence of oxygen atoms. 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. 

What is a Reducing Agent? 

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 atoms 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 –

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 agents 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 –

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. 

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