Chemistry Class 11 NCERT Solutions: Chapter 8 Redox Reactions Part 7

Chemistry Class 11 NCERT Solutions Chapter 8 Redox Reactions Part 7

Q: 11. Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the oxidising agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess. Justify this statement giving three illustrations.

Answer

Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess. This can be illustrated as follows:

(I) Equation and Equation are reducing and oxidising agents respectively.

If an excess of Equation is treated with Equation , then Equation will be produced, wherein the oxidation number (O.N.) of P is +3.

Equation

However, if Equation is treated with an excess of Equation , then Equation will be produced, wherein the O.N. of P is +5.

Equation

(II) K acts as a reducing agent, whereas Equation is an oxidising agent.

If an excess of K reacts with Equation , then Equation will be formed, wherein the O.N. of O is –2.

Equation

However, if K reacts with an excess of Equation , then Equation will be formed, wherein the O.N. of O is –1.

Equation

(III) C is a reducing agent, while Equation acts as an oxidising agent. If an excess of C is burnt in the presence of insufficient amount of Equation , then CO will be produced, wherein the O.N. of C is +2.

Equation

On the other hand, if C is burnt in an excess of Equation , then Equation will be produced, wherein the O.N. of C is +4.

Equation

Q: 12. How do you count for the following observations?

(A) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.

(B) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why?

Answer:

(A) In the manufacture of benzoic acid from toluene, alcoholic potassium permanganate is used as an oxidant because of the following reasons.

(I) In a neutral medium, OH– ions are produced in the reaction itself. As a result, the cost of adding an acid or a base can be reduced.

(II)KMnO4 and alcohol are homogeneous to each other since both are polar. Toluene and alcohol are also homogeneous to each other because both are organic compounds. Reactions can proceed at a faster rate in a homogeneous medium than in a heterogeneous medium. Hence, in alcohol, KMnO4 and toluene can react at a faster rate. The balanced redox equation for the reaction in a neutral medium is give as below:

(B) When conc. Equation is added to an inorganic mixture containing bromide, initially Equation is produced. Equation , being a strong reducing agent reduces Equation to Equation with the evolution of red vapour of bromine.

Equation

Equation

Equation

But, when conc. H2SO4 is added to an inorganic mixture containing chloride, a pungent smelling gas (HCl) is evolved. HCl, being a weak reducing agent, cannot reduce H2SO4 to SO2.

Equation

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