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Redox Reactions, also known as Reduction Oxidation reactions or Oxidation-Reduction reactions are the type of reactions where both these processes (Oxidation and reduction) occur simultaneously. Oxidation is a process which involves the loss of electrons from a species while reduction is a process which involves the gain of electrons to a species. So, in redox reactions, basically, there are two (or more) reactants, out of which one is losing electrons (and hence is getting oxidized), while the other is gaining electrons (and hence is getting reduced), simultaneously. In one of the special cases of redox reactions, there may be only one reactant involved which is getting oxidized as well as reduced simultaneously in a single reaction. Thus, overall there exists a balance of the electrons amongst the reactants involved in the reactions i.e., there is neither loss of electrons or gain of electrons from this overall total reaction.
Redox Reactions, also known as Reduction Oxidation reactions or Oxidation-Reduction reactions are the type of reactions where both these processes (Oxidation and reduction) occur simultaneously. Oxidation is a process which involves the loss of electrons from a species while reduction is a process which involves the gain of electrons to a species. So, in redox reactions, basically, there are two (or more) reactants, out of which one is losing electrons (and hence is getting oxidized), while the other is gaining electrons (and hence is getting reduced), simultaneously. In one of the special cases of redox reactions, there may be only one reactant involved which is getting oxidized as well as reduced simultaneously in a single reaction. Thus, overall there exists a balance of the electrons amongst the reactants involved in the reactions i.e., there is neither loss of electrons or gain of electrons from this overall total reaction.
Redox Reactions may be of three types, depending on the number of reactants involved in the reaction. These are – intermolecular redox reactions, intramolecular redox reactions, and disproportionation redox reactions. Also, the redox reactions may be classified as direct and indirect redox reactions depending on the number of systems involved to carry out the reaction.
Applications of Redox Reactions
The redox reactions find a wide number of applications in varied types of industries. These applications are discussed below:
Reduction Combustion Process: In a combustion reaction, the reactant compound which is undergoing combustion reacts with molecular oxygen. The molecular oxygen involved in this reaction generally goes from its oxidation state of “zero” to a lower oxidation state of “-2” whereas the substance which is undergoing combustion gains energy (from the heat energy being supplied to it for the process of combustion) and goes from its lower oxidation state (its original form) to a higher positive valued oxidation state. In this manner, the combustion process involves both oxidation (of the substance undergoing combustion) and reduction (of molecular oxygen) and hence is considered as a redox reaction. Taking an example of methane getting oxidized, we describe the reaction occurring as follows:
CH₄ + 2O₂ → CO₂ + 2H₂O + Huge amount of Energy
Oxidation
The same concept of redox reaction in combustion reactions is also applied in the space shuttles for the launch of rockets. The fuel present in the rockets is allowed to undergo combustion, which due to the oxidation of fuel and reduction of the oxidizing agent (generally molecular oxygen) releases an immense amount of energy which is required to launch the rocket from the ground into the air and then finally into outer space.
Electrochemistry Reactions: Basically, electrochemistry is the study of the relationship between the chemical and electrical relationship of reactants involved in a particular reaction. The concept of electrochemistry is fully based on the redox reactions that occur between two species which have different ionic potentials or oxidation states. In electrochemistry reactions, two solutions of chemical species having different electrical energies are taken. These chemical species have different oxidation states or ionization energies which make them electrically different from each other. Since we know that electrons have a tendency to flow from a region of high potential to low potential, there is a flow of current between these two solutions (of different chemical species) which takes place from the chemical species which is at a higher ionization potential to the chemical species with lower ionization potential, thereby leading to oxidation of one species while reducing the other.
A very common example of electrochemical reaction is done between zinc and copper, where zinc is present at the anode and copper is present at the cathode. The flow of electrons takes place from zinc (which gets oxidized from an oxidation state of “zero” to an oxidation state of “+2”) towards the copper (which gets reduced from +2 oxidation state to zero oxidation state). The reaction occurs as follows:
Reduction
Zn +2 Cu + → 2 Zn + Cu
Oxidation
Electrochemical Cells or Batteries: The electrochemical cells or batteries that we use in our daily lives to operate a lot of devices, appliances, and even vehicles are based on the application of redox reactions. The chemical substance used in these electrochemical cells or batteries stores chemical energy which when operated upon undergoes a redox reaction to produce electrical energy. These electrochemical cells or batteries comprise of a voltaic cell (also known as the galvanic cell) which is made up of two half cells and is linked together by the means of a semi-permeable membrane and has a wire connected to it. Both half cells contain a metal of different ionization potential acting as two electrodes, one anode, and one cathode. The reaction is initiated by means of an electrolyte solution present in these electrochemical cells or batteries which act as a concentration gradient of electrons for the half cells. There occurs a chemical reaction between these two half cells, which leads to the reduction at the cathode and oxidation at the anode. As the flow of electrons proceeds with the reaction, this electron flow is utilized as our source of electrical energy to supply power to the device or appliance we need to power by means of the wire or connecting surface.
Photosynthesis Process: The example of photosynthesis is probably the one which everybody would be able to associate with as it is one of the most related processes we observe around us in our daily lives. We have been studying about this process since childhood. Green plants utilize sunlight and are able to prepare their own food by processing a chemical reaction in their leaves. But little did you know that the chemical process was actually a redox reaction.
Green plants get sensitized towards sunlight due to a pigment, chlorophyll, present in their leaves and absorb it. This absorbed sunlight acts as the activation energy to convert carbon dioxide (absorbed by the plants from the surroundings) and water (absorbed by the plant from the soil) into carbohydrates. In this reaction, carbon dioxide gets reduced to carbohydrates (which acts a source of food and energy for the plants), while the water is oxidized to oxygen (which is released into their surrounding air). The redox reaction described above is elucidated as follows:
Reduction
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Oxidation
This reaction is a source of food for both plants themselves as well the organisms which feed on plants (
such as herbivorous animals, microorganisms, and humans).
Extraction of Metals: The redox reactions find a great deal of application in the extraction industry to extract metals or minerals from the natural ores. Metals usually exist in an oxidized state in nature (due to their long-term exposure to the oxygen present in the air surrounding them). Hence, they need to be reduced in order to extract the required metal out of them. This is done in the industry on a large scale with the help of a suitable reducing agent, depending on the metal or ore which is to be refined. For example, iron is extracted from the oxidized ore of ferric oxide in a large blast furnace in the iron extracting and refining industries using coke as a reducing agent. The reaction takes of iron metal extraction from its oxidized natural ore takes place as follows:
Reduction
Fe2O₃ + 3C → 2Fe + 3CO
Oxidation
Similarly, aluminum is extracted from its ore, aluminum oxide [Al(OH)_{3}] by means of reduction. Other metals extracted in the same manner include magnesium, sodium, calcium, potassium, lithium, and many others.
Quantitative Analysis: The redox reactions form the basic principle of the redox titrations carried out for the quantitative analysis of various substances. Redox titrations are carried out to find the concentration of any electrically charged species present in the sample solution. These are done by titrating the unknown substance against a standardized titrant whose concentration is already known to us. In this type of titration, the solution containing the unknown substance is kept in the bottom flask and the solution of the known titrant is filled in the burette. The solution from the burette is allowed to fall drop-wise in the bottom flask (containing the unknown substance and an indicator) till the indicator changes the colour of the solution in the flask, indicating that the end point of the titration has reached. At the endpoint, the volume of the standardized titrant is noted down. This volume is equal to the volume of unknown substances required to neutralize the standardized titrant. The volume remaining in the sample solution is back-calculated by the total amount of the sample taken and thereby we can calculate the concentration of the unknown substance present in that solution. These reactions are quite useful in the pharmaceutical industry.