Redox Reactions – Electron Transfer Reactions

Batteries are such common things. We require batteries in the day to day life and it is an inevitable part of our daily life. Innumerable devices run on batteries such as clocks, mobile phones, cars, flashlight, etc. However, do you know that electron transfer reactions take place inside the cell? This is the basic principle for the development of a regular battery/cell. Let’s learn about these electron transfer reactions.

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Definition of Redox Reaction

Redox reaction is a type of chemical reaction. A chemical reaction involves process change that happens when two or more molecules react with one another and result in the formation of a new product. In this article learn more about redox reactions with respect to electron transfer reactions.

It is necessary to understand how the transfer of electrons happens during a reaction for having a clear understanding of the electronic concept of oxidation and reduction. Let us take an example of magnesium when burnt with oxygen. The magnesium gets oxidized to magnesium oxide. Two electrons from magnesium are given to the oxygen atom during the reaction process. Refer to the example below

In this above reaction, we can clearly see the transfer of electrons. This process is called redox process. Now let us understand the definition of oxidation and reduction according to electron transfer.

Browse more Topics under Redox Reactions

• Balance Redox Reactions
• Classical Idea of Redox Reactions
• Oxidation Number
• Redox Reactions and Electrode Potential
• Types of Redox Reactions
• Redox Reactions as the Basis of Titrations

Oxidation in Terms of Electron Transfer Reactions

Oxidation is the process in which group of atoms or a single atom of a chemical reaction loses one or more electrons. Thus, oxidation results in the loss of electrons and increase in the positive charge of a species or decrease in the negative charge of the species. For example

Reducing agents or reductants are the species of a compound that undergoes electron loss during a chemical reaction. According to the example given above, Cl-, Fe²⁺, and Cu are reductants.

Reduction in Terms of Electron Transfer Reactions

The reduction is a process in which an atom or a group of it participating in a chemical reaction gains electrons. Gaining of electrons decreases the positive charge and increases the negative charge of the participating species. For example

Oxidizing agents or oxidants are the species undergoing gain of electrons during a chemical reaction. According to the above example of reactions, Ag⁺, Fe³⁺ ions, and Br² molecules are the oxidants.

Redox Reaction- Oxidation & Reduction

It is the combination of oxidation and reduction reactions. Oxidation is a type of reaction in which addition of electronegative element/oxygen to a substance occurs or elimination of electropositive element/hydrogen from a substance occurs.

A reduction is a type of reaction in which elimination of electronegative element/oxygen occurs or addition of electropositive element/hydrogen from a substance occurs.

Observe the reaction given below and identify what process (oxidation or reduction) is happening?

In the above example, the addition of oxygen to the electropositive element occur. Therefore, aluminium is oxidized. However, removal of oxygen is taking place from Fe3O4, thus the compound is reduced. Hence, both the reactions are taking place together and the above example is an example of redox-oxidation & reduction reaction.

The Occurrence of Combination of Oxidation & Reduction- Redox Reaction

By now we know that oxidation process involves loss of one or more electrons whereas reduction process involves gain of one or more electrons. Therefore, if a substance will lose an electron, there must be a substance which will gain electrons. This is because electrons cannot be the final product in any chemical reaction.

Hence, it implies that in a specific process oxidation will occur when there will be a simultaneous reduction and vice versa. Thus, it signifies that oxidation and reduction are complementary processes and neither of them can occur alone. This is the reason for naming this kind of chemical reaction as redox process/reactions. In a redox reaction, transfer of electrons takes place from the reducing agent to the oxidizing agent.

For instance, consider the reaction of zinc and copper ions given below:

In the above reaction, zinc atoms remove electrons and get oxidized to zinc ions. Similarly, cupric ions gain electrons and get reduced to copper atoms. Therefore, in the reaction zinc atoms act as reductant whereas cupric ions act as an oxidant.

Redox Reaction on the Basis of Electron Transfer

From the above explanations, we can state that oxidation and reduction is not just addition or removal of the electropositive element and electronegative element or addition and elimination of oxygen and hydrogen. It involves the change in the oxidation state of the element of a reaction which in turn involves electron transfer of the elements in the reaction. For instance, a redox reaction

There is a change in the oxidation state of zinc from 0 to +2. Similarly, there is a change in the oxidation state of H from +1 to 0.  Thus we can say, zinc is oxidized and H is reduced in the above reaction by losing and gaining electrons, respectively. Thus,

• Loss of electrons- Oxidation
• The gain of electrons- Reduction
• The simultaneous reaction of oxidation and reduction- Redox reaction

Therefore, a redox reaction consists of two halves or two parts. One part is the oxidation half reaction which demonstrates the oxidation reaction and the other part is the reduction half-reaction which demonstrates the reduction reaction.

Oxidation results in the increase of oxidation number whereas reduction results in the decrease of oxidation number. However, the overall reaction lost in electrons will be zero because the electrons given by reducing agent are accepted by oxidizing agents. The most famous example of a redox reaction is an electrochemical cell.

Competitive Electron Transfer Reactions

To understand this concept, let us go through one experiment.

Case 1

If we place a strip of zinc metal in a copper nitrate solution, we will observe that the oxidation of zinc and reduction of copper takes place. We can write the above reaction as

Zn (s) + Cu²⁺ (aq)   →   Zn²⁺ (aq) + Cu(s)

We will notice that in the reaction state of equilibrium favours products more than reactants.

Case 2

In another case when we place a strip of copper metal in silver nitrate solution, oxidation of copper and reduction of silver species take place. The reaction is

Cu(s) + 2Ag⁺ (aq)   →   Cu²⁺ (aq) + 2Ag (s)

In this reaction we will notice in the reaction state of equilibrium favours products more than reactants.

Case 3

If we place a cobalt metal strip in a Nickel sulphate solution, we will notice that oxidation of cobalt occurs and reduction of Nickel occur. Therefore, the reaction is,

Co (s) + Ni²⁺ (aq)   →   Co²⁺ (aq) + Ni (s)

It is quite noticeable that both nickel and cobalt are present in moderate concentration. Thus, the reaction does not exceptionally favour the reactants or the products. Thus, in all the above reactions, we can figure out that there is a competition for the release of electrons. This is easily comparable to release of protons in the case of acids.

There are tables that represent the strength of acids in a particular order. Therefore, it is important to develop a table where there is an arrangement of metals according to the tendency of metals to release electrons.

Relation of Electron Transfer Reactions & Electrochemical Cell

From the above reactions, it is very clear that zinc has the tendency to release electrons to copper. Similarly, copper has the tendency to release electrons to silver. Thus, on the basis of above experiment the order of metal activity arrangement will be Zn>Cu>Ag. Hence, there is a need for designing a metal activity series.

The order of reactivity of the metal activity series is known as electrochemical series. The electrochemical series helps to develop a particular class of cells (Galvanic cells). Thus, electron transfer reactions on the basis of electrochemical series form the basis of an electrochemical or galvanic cell in which chemical reactions produces electrical energy.

A Solved Question for You

Q. Explain the below reaction as a redox reaction.

Solution: It is a redox reaction. The product in the above reaction is an ionic compound. Therefore, we can also write it as Na⁺H^–. Therefore, there is two half of the reaction. The first half of the reaction is,

The other half of the reaction is,

The division of reaction in two parts/halves signifies that oxidation of sodium and reduction of hydrogen takes place. Therefore, the complete reaction is a redox reaction.