What are Redox Reactions? 

Redox reactions are considered to be the chemical reactions involving both oxidation and reduction processes. They are actually oxidation-reduction reactions. The term redox is made from two words where “red” comes from reduction and “ox” comes from oxidation. During a redox reaction electron transfer takes place in such a way that one chemical compound gets reduced and the other one gets oxidized. 

Reduction and Oxidation Reactions

Reduction is the process where oxygen or an electronegative component is removed from a compound or the process where hydrogen or an electropositive component is added to the compound. 

On the other hand, oxidation is the process where oxygen or an electronegative component is added to the compound or the process involving removal of an electropositive component or hydrogen atom from the compound. 

It was observed that for any given reaction, oxidation and reduction both occur simultaneously hence the word redox reaction was coined for such reactions. 

A lot of examples of redox reactions can be seen in everyday lives. The most common example is the addition of preservatives in pickle. The preservatives here act as a reducing agent which stops the reaction to occur. Another example is the prevention of browning of apple slices when kept for sometime by squeezing some lemon juice on it which can prevent its oxidation due to atmospheric oxygen. In human body the process of cellular respiration also involves redox reactions. This process has electron carriers which carry electron from glycolysis to electron transport chain. It has two electron carriers namely NAD⁺ and FAD⁺ which undergoes redox reactions throughout the process of cellular respiration 

Redox Reaction Types 

1. Combination reaction 
2. Decomposition reaction 
3. Displacement reaction 
4. Disproportionation reaction 

1. Combination reaction  

In a combination reaction, two compounds will combine together to create a new compound. 

Combination reaction can be expressed as C+D → CD. 

For example: 

C(s) + O₂ (g) → CO₂ (g) 

2. Decomposition reaction 

In decomposition reactions, one compound is broken down into its respective constituent products. Decomposition reaction is entirely opposite to that of combination reactions. 

For example: 

2 H₂O (l) → 2 H₂ (g) + O₂ (g) 

3. Displacement reaction 

Replacement of an element or an ion in one compound by another element or an ion is known as the displacement reaction.  

This process can be expressed as A+ BC → AC+B.            

Displacement reactions can be further classified into two categories as follows: 

• Metal displacement:  

In this process, a metal present in a compound could be displaced by a different metal that is present in an uncombined form. 

CuSO₄ (aq) + Zn (s) → Cu (s) + ZnSO₄ (aq) 

Application of metal displacement reactions is mostly found in metallurgical activities. 

• Non-metal displacement:  

In this process, a non-metal is displaced from a compound. 

Displacement of hydrogen or displacement of oxygen from compounds is an example of non-metal displacement redox reaction. 

2 Na (s) + 2 H₂O (l) → 2 NaOH (aq) + H₂ (g) 

4. Disproportionation reaction 

In disproportionation reactions, an element which is present in a particular oxidation form undergoes simultaneous oxidation and reduction. One of the reacting compound present in a disproportionation reaction occurs in minimum three states of oxidation. The reacting substance is of intermediate state of oxidation whereas the oxidation state higher and lower to that will be formed during the reaction.  

2H₂O₂ (aq) → 2H₂O (l) + O₂ (g) 

In the above reaction it can be observed that the oxygen present in the peroxide is present in the -1 state whereas the oxygen in O₂ is in zero oxidation state and the oxygen in H₂O is in -2 oxidation state. 

Redox Reaction Balancing 

Redox reactions are generally balanced using two process. The first method is based on the redox reaction’s division into two reactions where one is involved in oxidation and the other in reduction (half reaction method) and the second one consists of the method that is based on the reducing and oxidizing molecule’s oxidation number variation (oxidation number method). The two methods are as follows: 

Half reaction method  

In this method the process occurs in two steps where each half is balanced individually and later combined together to give us a balanced equation. 

Following is an example to balance a redox equation by a half reaction method: 

Consider the following redox equation: 

Al + Ag⁺ → Al³⁺ + Ag 

The first half involves oxidation which can be written as Al → Al³⁺. 

This reaction is not completely balanced as Al loses three electrons on oxidation, so the balanced equation can be written as:  

Al → Al³⁺ + 3 e⁻ 

The second half reaction with reduction is Ag⁺ → Ag. 

But this is still unbalanced as Ag will have to accept one electron in order to become a neutral Ag atom. 

Ag⁺ + e⁻ → Ag 

Now multiply one of the sides with an integer in order to cancel out the common molecule. 

3 × [Ag⁺ + e⁻ → Ag] 

3 Ag⁺ + 3 e⁻ → 3 Ag 

Now the both the half reactions are balanced and can be written as: 

Al + 3 Ag⁺ + 3 e⁻ → Al³⁺ + 3 Ag + 3 e⁻ 

Cancelling put the common molecules the final redox reaction can be written as: 

Al + 3 Ag⁺ → Al³⁺ + 3 Ag 

Oxidation number method 

In this, the equation is balanced using the oxidation number of the atoms involved in the redox reaction. 

Consider the following equation. 

Fe₂O₃ (s) + CO (g) → Fe (s) + CO₂ (g) 

Now write the oxidation numbers for all the atoms above the atom itself. 

Fe₂^3- O₃^2-(s)   +C²⁺O^2- (g) → Fe⁰ (s) + C⁴⁺O₂^2- (g) 

In the above equation it can be seen that the oxidation of carbon atom is increased by 2 whereas on the other hand the oxidation number of iron reduces by 3. But it can also be observed that the equation is not balanced on both the sides hence an integer must be multiplied to form an balanced equation. 

According to the above equation, the rise in the oxidation number has to be multiplied from 3 and the reduction in oxidation number should be multiplied from 2 to form a balanced equation.  

The final balanced equation which will be obtained as follows: 

Fe₂O₃(s) + 3CO (g) → 2Fe(s) + 3CO₂ (g) 

Common mistakes 

Not writing the formulas correctly while balancing an equation for example sodium nitrate is written as NaNO₃ and not as Na₂(NO₃)₂ 

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry. 

Practice Problem 

Express the balanced half reactions for the reaction in basic solution. 

NiO₂ + 2 H₂O + Fe → Ni (OH)₂ + Fe(OH)₂ 

Solution: 

2 H₂O + NiO₂ + 2 e^- → Ni(OH)₂ + 2 OH^- 

2 OH^- + Fe → Fe(OH)₂ + 2 e^-