Q27 of 30 Page 188

Assertion (A) : Ozone is destroyed by solar radiations in upper stratosphere.

Reason (R) : Thinning of ozone layer allows excessive UV radiations to reach the surface of earth.


A. A and R both are correct but R is not the explanation of A.


B. A is false but R is correct.


C. A and R both are correct and R is the correct explanation of A.


D. A and R both are incorrect.


B. A is false but R is correct.


The upper stratosphere consists of considerable amount of ozone (O3), which protects us from the harmful ultraviolet (UV) radiations coming from the sun. Ozone is not destroyed by solar radiations rather it is destroyed because of release of chlorofluorocarbon compounds (CFCs), also known as freons.


Due to CFCs ozone layer is depleting and allows excessive UV radiations to reach the surface of earth. CFCs mix with the normal atmospheric gases and reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical which then reacts with ozone and results in depletion of ozone.


CF2Cl2 UV→ Cl. + F2Cl


Cl. + O3 → ClO. + O2


More from this chapter

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25

Assertion (A) : The first ionization enthalpy of alkali metals decreases down the group.

Reason (R) : Increase in number of orbitals increases the shielding effect which outweighs the increasing nuclear charge, therefore, the removal of outermost electron requires less energy on moving down the group.


A. A and R both are correct but R is not the explanation of A.


B. A is false but R is correct.


C. A and R both are correct and R is the correct explanation of A.


D. A and R both are incorrect.


 26

Assertion (A): Nitration of benzene requires the use of concentrated sulphuric acid and nitric acid.

Reason (R): The mixture of acids produces the electrophile for the reaction.


A. A and R both are correct but R is not the explanation of A.


B. A is false but R is correct.


C. A and R both are correct and R is the correct explanation of A.


D. A and R both are incorrect.


 28

(a) Liquids can be considered as very dense gases. When a liquid phase changes to gas phase, the liquid and the gas phases are in equilibrium and a surface separates the two phases. This surface is visible if both phases are in equilibrium and are below critical tempertaure and pressure. However, it is possible to interconvert liquid and gas wherein two phases are never present together.

With the help of a well-labelled diagram show that CO2 gas can be liquified by changing the pressure and temperature without passing through the situation when both gaseous and liquid CO2 are at equilibrium.


(b) Arrange the following liquids in increasing order of their viscosities. Give reason for your answer.


Water, benzene, ethane-1,2-diol.


 29

(a) Explain why :

(i) BCl3 is a Lewis acid.


(ii) Boric acid is a monobasic acid.


(b) Compound ‘A’ of boron reacts with excess NH3 to give a compound ‘B’. Compound ‘B’ on heating gives cyclic compound ‘C’. Compound C is called inorganic benzene.


(i) Identify compounds ‘A’, ‘B’ and ‘C’


(ii) Give the reactions involved in these processes.