Balancing of Redox Reaction

Balancing the redox reaction is very easy. Redox reaction is an oxidation-reduction reaction. The oxidation number of a molecule/compound changes by gaining or losing electrons when an oxidation-reduction reaction takes place. Oxidation-reduction reactions can be observed in some of the essential life functions, including photosynthesis, respiration, combustion, and corrosion or rusting. In a redox reaction, oxidation and reduction co-occur for the same reaction. One compound goes into oxidation, and the other one gets reduced. During the reaction, transfer of electrons takes place; losing electrons leads to oxidation, and gaining electrons leads to reduction.

Four Types of Redox Reactions

• Decomposition reaction
• Combination reaction
• Displacement reaction
• Disproportionation reactions

Decomposition Reaction

As the word suggests, this process breaks down into different compounds. 

For example, 2NaH →2Na + H2.

Combination Reaction

This reaction takes place by combining more than two compounds into a single product, like (A + B —> AB)

For example, H2 + Cl2 —> 2HCl

C + O2 —> CO2.

Displacement Reaction

In this kind of redox reaction, an atom or an ion in a compound is displaced or moved by another element like (X + YZ → XZ + Y). 

Displacement reactions are of two types—one for the metal displacement and another for the nonmetal displacement. Hence, a metal present in the compound is displaced by another metal in metal displacement.

For example, CuSO4 + Zn → Cu + ZnSO4.

Oxidation and Reduction

In a redox reaction (the term redox is a short form of reduction-oxidation), reduction and oxidation co-occur in the same reaction. When an element or compound gains electrons, this gain is known as reduction, and when the element or compound loses electrons, this loss is known as oxidation. 

Adding oxygen leads to oxidation, and its removal leads to reduction. Moreover, the addition of hydrogen leads to reduction, and its removal leads to oxidation. 

Specifically, there are three types of transfers in redox reactions.

• In a redox reaction, one compound loses oxygen at the reactant side, and one of the products receives oxygen; this process is also called oxidation
• In a redox reaction, during hydrogen transfer, when one compound loses hydrogen and the other gains hydrogen, this process is also known as Reduction
• In a redox reaction, during electron transfer, when one molecule or compound loses electrons and another molecule or compound gains electrons; here, one gets reduced, and another gets oxidized

There are some oxidizing and reducing agents in a redox reaction. Oxidizing agents can get self-reduced by accepting electrons and oxidizing another compound. Reducing agents can get self-oxidized by donating electrons and reducing another compound. Reducing agents work as hydrogen suppliers or transfer other electropositive elements to another substance. 

Balancing of Redox Reaction

Let’s see how redox reactions are balanced; balancing means how the ions present at the reactant and product sides become equal.

There are different methods for balancing of redox reaction:

• Ion-electron method

• Oxidation number method

• Half-reaction method

Redox reactions are balanced either by the ion-electron method or the oxidation number method. Both methods lead to the correct form of the balanced equation. The ion-electron method has two advantages. Chemists prefer to use the ion-electron method for redox reactions in dilute aqueous solutions. Where free ions have a more or less independent existence. The oxidation state method for redox reactions is mainly used for solid chemicals or reactions in a concentrated acid medium.

• The ion-electron method is an analytical technique that uses ionic half-reactions to balance a chemical equation
• At the initial level of balancing ion, the electron method balances the number of oxygen and hydrogen atoms
• At the final level, it balances the number of electrons

• The oxidation number method is an analytical technique that uses the change in the oxidation of chemical elements to balance a chemical equation
• At the initial level of balancing, the oxidation number method balances oxidation numbers
• At the final balancing, it balances the oxygens and hydrogens by adding water to both reactants and products side

Balancing of Redox Reaction by Ion-Electron Method

• Firstly, write the skeletal equation containing all the oxidizing and reducing agents and the products in ionic form and assign their oxidation numbers to each atom
• Now, separate the equation into two half-reactions
• Balance all the atoms (except for  O and H) in each half-reaction separately
• This process is known as mass balance 
• For reactions in an acidic medium, add H2O to balance the oxygen atoms and enough H+ to balance the hydrogen atoms
• For reactions in a basic medium, first of all, balance the atoms as you would do for an acidic solution
• Then for each H+ ion, add OH- ion to both sides of the half-reaction
• Whenever  H+ and OH- appear on the same side, combine them to make H2O
• Now, add electrons to one side of each half-reaction to equalize the charges
• Electrons are added to the reactant side of a reduction half-reaction and the product side of an oxidation half-reaction
• The number of electrons added to one side of the half-reaction should make the total charge of that side equal to the charge on the other side
• This procedure is called a charge balance
• Then, multiply the half-reactions by the appropriate coefficients to balance the number of electrons required to balance the reaction equation
• And lastly, add the two half-reactions and then check the final equation by inspection
• Reduce the balanced equation to the lowest terms (coefficients)

 The oxidation-number-change method for balancing starts with the unbalanced skeleton equation (which must be balanced).

• Step 1: Assign the oxidation numbers to each atom in the equation and write the numbers above the atom

• Step 2: Identify all the atoms that undergo oxidation 
• Step 3: Use a line to connect the atoms changing oxidation numbers. On the line, write the oxidation-number change

• Step 4: Proper Use of coefficients is necessary to make the total increase in oxidation number equal to the total decrease in oxidation number

• Step 5: Checking the balancing for both sides of atoms and charges
• Sometimes very rarely, a coefficient may also be needed to be placed before a molecular formula that was not involved in the redox process.

Half-Reaction Method for Balancing Redox Reaction

Let’s see the half-reaction method for balancing the redox reaction.

To balance the redox reaction using the half-reaction method, The equation must be divided into two half-reactions; one should represent the oxidation, and the other should represent Reduction. The half-reactions are then balanced for mass and charge and, if necessary, then for both mass and charge so that the number of electrons transferred in each equation is the same. Finally, the half-reaction equations are added together, giving the balanced overall equation for the reaction.

CoX3+(aq) + Ni(s)→CoX2+(aq)+NiX2+(aq)

Here, the equation seems to be balanced by mass, but it’s still unbalanced by charge. The total equivalent charge on the left side of the equation is 3+, while the total equivalent charge on the right side is 4+.

In balancing of a redox reaction, the following procedure should be followed: 

•  Firstly, the equation must be divided into two half-reactions
•  Secondly, balance each half-reaction for mass/charge or, if required, then for both
•  For the third step, it’s necessary to ensure an equalized number of electrons are transferred in each half-reaction
•  Lastly, adding the half-reactions together provides us with fully balanced redox reactions

Conclusion

An oxidation-reduction reaction is a chemical reaction involving the transfer of electrons between two species. Oxidation is the process of losing electrons, and Reduction is gaining electrons. Where the oxidizing agent gains electrons and gets reduced, the reducing agent loses electrons and gets oxidized.

Oxidation and reduction will always occur together since the reduction of one molecule will cause the oxidation of another molecule. The reducing agent will become more positive or neutral and the oxidizing agent more negative (less positive). The half-reaction method is mainly used to balance the redox reaction, which requires that both mass and charge are balanced.