6.Redox Reaction

2. 
   

4. 
   

NOTES OF REDOX REACTION

 • Oxidation

Oxidation is defined as the addition of oxygen/electronegative element to a substance or removal of hydrogen/ electropositive element from a susbtance.

• Reduction

Reduction is defined as the removal of oxygen/electronegative element from a substance or addition of hydrogen or electropositive element to a substance.

What is a Redox Reaction?

Redox is a term used for the oxidation-reduction reaction. Redox reaction is a chemical reaction where a change in the oxidation state of atoms occurs. It involves electron transfer, i.e. if one chemical species gains electrons, then another chemical species gives or loses electrons. The species from which the electron is lost is said to be oxidized whereas the species to which the electron is added is said to be reduced. Example zinc displaces copper in an aqueous solution called copper sulfate.

Redox reactions can be classified into four different categories:

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

1. (A) Combination reaction: When two or more reactants react to form single product.

i.e   a + b --> ab (in this single product is formed).

For the reaction to be Redox, both the elements should be in elemental form.

For example: All combustion reactions are Redox reactions.

C0        + O2 -->   CO2

Carbon  Oxygen  Carbondioxide

2. (B) Decomposition reaction: It is the type of reaction, in which a reactant breaks up into its constituents either by heating, passing current or in presence of light.

2. (C) Displacement reaction: It is the reaction between element and a compound. In This more reactive element displaces less reactive from its compound.

CuSO4            + Zn         --> ZnSO4 + Cu

Copper Sulphate  Zinc                Zinc Sulphate Copper

4. (D) Disproportionate reaction: A reaction in which the same species is simultaneously reduced and oxidized .One of the reacting species in this reaction is always an element, that can exist in at least three oxidation states .The element in the form of reacting species is in the intermediate oxidation state .In this higher and lower both oxidation states of the element are formed.
5. For Example:- 

      H2O2         -->   H2O    +  H2

Hydrogen Peroxide  Water Oxygen Gas

In above example, the oxidation number of Oxygen which is -1 in peroxide increases to zero in O2 and decreases to -2 in water.

Oxidizing And Reducing Agents

The role of an oxidizing agent (Oxidant) in a chemical reaction is to gain electrons. They are highly electronegative. Once they gain electrons, these substances are reduced. An oxidant is also known as an electron acceptor. Example nitric acid, and halogen.

The role of a reducing agent (Reductant) in a chemical reaction is to lose electrons. Once they lose electrons, these substances are oxidized. A reductant is also known as an electron donor. Examples include sulfite compounds and earth metals.

2. 
   

• Balancing of Redox Reactions

(i) Oxidation Number Method. Following steps are involved

(ii) Write the correct formula for each reactant and product.

(b) By assigning the oxidation change in oxidation number can be identified.

(c) Calculate the increase and decrease in oxidation number per atom with respect to the reactants. If more than one atom is present then multiply by suitable coefficient.

(d) Balance the equation with respect to all atoms. Balance hydrogen and oxygen  atoms also.

(e) If the reaction is carried out in acidic medium, use H+ ions in the equation. If it is          in basic medium use OH– ions.

(f) Hydrogen atoms in the expression can be balanced by adding (H20) molecules to  the reactants or products.

(g) If there are the same number of oxygen atoms on the both side of equation then it   represents the balanced redox reaction.

(ii) Half Reaction Method. In this method two half equation are balanced separately  and than added together to give balanced equation.