Important questions of Science (SEE2076)

1. Describe the mechanism of formation and depletion of the ozone layer.

I. Ozone layer formation

Ozone (O3) is a pale bluish non-toxic gas formed by the photolytic dissociation of molecular oxygen and combination of nascent oxygen with the molecular oxygen.

a. O2→O+O

b. O2+O→O3 (ozone)

In the first step, the molecular oxygen is dissociated into two atomic or nascent oxygen (O) oxygen due to UV rays. The nascent oxygen is very reactive and in turn, gets combined with molecular oxygen to give ozone (O3). Actually, the ozone molecules are formed on the surface of the earth and they rise up in the upper level of the stratosphere to form an umbrella-like protective layer now called the ozone layer.

II. Ozone layer depletion

Formation of the hole on the ozone layer or thinning of the ozone layer is called the ozone layer depletion. The ozone layer is depleted by chlorofluorocarbon (CFC) and oxides of nitrogen.

a. By Chlorofluorocarbon (CFCl3)

First of all the chlorofluorocarbon undergoes the photolytic dissociation in the presence of the UV rays to give nascent chlorine (Cl) and the nascent chlorine is responsible for the ozone layer depletion.

i. CFCl3→CFCl2+Cl

ii. O3+Cl→ClO+O2

In the first step, the chlorofluorocarbon such as trichloro fluorocarbon (CFCl3) is dissociated into dichlorofluoro carbon (CFCl2) and nascent chlorine (Cl) due to the photolytic effect of UV rays. In the second step, the nascent chlorine attacks the ozone molecule (O3) and dissociates into oxychloride (ClO) and molecular oxygen (O2) thus depleting the ozone layer.

Moreover, the oxychloride (ClO) is further dissociated to give nascent chlorine to destroy more ozone thus leading the reaction in a never-ending chain process. One molecule of CFC destroys about 100,000 of the ozone molecules.

b. by the oxides of nitrogen

The nitric oxides released from the nitrogenous fertilizers and supersonic jet planes are responsible for depleting the ozone layer by reducing ozone into molecular oxygen.

i. NO+O3→NO2+O2

ii. 2NO2+O3→N2O3+2O2

First of all, nitric oxide (NO) combines with ozone (O3) and gives nitrogen dioxide (NO2) and molecular oxygen (O2) and the nitrogen dioxide again reacts with ozone to give molecular oxygen in the same way thus repeating the reaction in multiple times. Thus, oxides of the nitrogen are responsible for the ozone layer depletion.

2. Explain the process of making alkali/ base/ antacid from a magnesium ribbon.

In the first step, the magnesium ribbon is burnt with the air (O2) to get magnesium oxide (MgO).

2Mg+O2→2MgO

In the second step, the magnesium oxide is treated with water to get magnesium hydroxide [Mg (OH)2] which is alkali and acts as the antacid to reduce the hyperacidity.

MgO+H2O→Mg (OH)2

3. How does the fire extinguisher work?

Fire extinguisher consists of two chemicals namely saturated sodium bicarbonate (NaHCO3) and concentrated sulphuric acid (H2SO4). At the time of use, the fire extinguisher is inclined or inverted and shaked or dropped on the floor to get mixed above mentioned two chemicals.

When saturated sodium bicarbonate and concentrated sulphuric acid react to each other it gives sodium sulphate, water and carbon dioxide. A large amount of carbon dioxide is sprayed out in very high pressure to extinguish the burning flame.

2NaHCO3+H2SO4 (conc.) →Na2SO4+2H2O+2CO2

The carbon dioxide is very ideal to extinguish fire because it is neither combustible nor the supporter of the combustion. Also, it is heavier than air and the carbondioxide displaces the oxygen coming in contact of flame and covers the flame just like a blanket. Thus, carbon dioxide acts as the fire extinguisher.

4. Explain the naming codes of hydrocarbons according to IUPAC.

International union of pure and applied chemistry (IUPAC) is the common organization of global chemists. This organization has produced a number of codes regarding the nomenclature of the hydrocarbons. The major are as follows:

a. The naming of the hydrocarbon is associated with the specific prefix on the basis of a number of carbon atom present. For instance:

i. 1 carbon atom: Meth

ii. 2 carbon atoms: Eth

iii. 3 carbon atoms: Prop

iv. 4 carbon atoms: But

v. 5 carbon atoms: Pent

vi. 6 carbon atoms: Hex

vii. 7 carbon atoms: Hept

viii. 8 carbon atoms: Oct

ix. 9 carbon atoms: Non

x. 10 carbon atoms: Dec

b. A saturated hydrocarbon series having the carbon carbon single bond (C-C) has got the suffix-ane. So that, the hydrocarbons are named as:

i. Meth+ane=Methane

ii. Eth+ane= Ethane

iii. Prop+ane= Propane

And, Butane, pentane, Hexane, Heptane, Octane , Nonane , Decane, etc.

c. An unsaturated hydrocarbon series having the carbon-carbon double bonds (C=C) has got the suffix-ene. So that, the hydrocarbons are named as:

i. Eth+ene=Ethene

ii. Prop+ene= Propene

Similarly, Butene, pentene, Hexene, Heptene, Octene , Nonene , Decene, etc.

d. An unsaturated hydrocarbon series having the carbon carbon triple bonds (C≡C) has got the suffix- yne. So that, the hydrocarbons are named as:

i. Eth+yne=Ethyne

ii. Prop+yne= Propyne

Similarly, Butyne, pentyne, Hexyne, Heptyne, Octyne , Nonyne , Decyne, etc.

5. Define neutralization reaction? Wite any 2 examples of such reaction. Also, mention any 4 applications of acid base reaction.

When acid and base react each other then both acids and base lose their properties and form two neutral substances namely salt and water. Such chemical reaction is called neutralization reaction.

Examples:

Acid + Base → Salt + Water

i. HCl+NaOH→NaCl+H2O

ii. H2SO4+2KOH→K2SO4+2H2O

There a number of applications of acid base reactions. Any 4 of them are given as follows:

a. Farmers use the lime (base) to treat the acidity of the soil.

b. Human beings use antacids such as magnesium hydroxide and aluminum hydroxide (base) to treat hyperacidity.

c. Honey bees and ants inject formic acid when they sting which can be treated by washing hand with soap (base)

d. Excessive sourness of the food and fruits can be reduced by using baking soda i.e. sodium bicarbonate (base)

6. Define isomers and isomerism. Write the possible isomers of butane and pentane. Also, draw their structural formula.

The organic compounds having the same molecular formula but different structural formula(e) are called isomers.

The phenomenon of having a number of isomers of a compound is called isomerism.

The possible isomers of butane are:

a. n-butane

b. iso-butane

The possible isomers of pentane are:

a. n-pentane

b. iso-pentane

c. neo-pentane

Home work

7. What do you mean by chemical reaction? Write the types of chemical reaction, their definitions and one example of each.

8. What happens when carbon dioxide is passed to thee lime water for long time? Write with balanced chemical equations.

9. We can not use dil sulphuric acid instead of dil hydrochloric acid for the lab preparation of carbondioxide, why?

10. What happens when iron is mixed to the coppersulphate solution?

11. Water acts as both acid and base (amphoteric) liquid, why/how?

12. What do you mean by metallurgy? Describe the process involved in the metallurgy in brief.

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