Class 12 , Chapter 8 : d and f block Elements

 The middle layer present in the periodic table is filled with the d block elements. The inner d orbits of group 3 to 13 are progressively filled. On the other side, the f block elements are found outside and at the bottom of the periodic table. In these elements, the 5f and 4f orbitals are progressively filled. The 3 series of transition elements are recognised by the filling of 3d, 4d, and 5d orbitals. They hold a high boiling and melting point. The metallic properties that are exhibited by the transition elements are given as follows:

• Electrical conductivity

• Thermal conductivity

• Malleability

• Metallic character

• High tensile strength

• Ductility

Class 12, Chapter 14 : Biomolecules

 

Sub-topics covered under Biomolecules

• Carbohydrates– This refers to sugars, starches, and fibres which exist in various kinds of grains, fruits, milk products, and vegetables.
• Monosaccharides– This refers to simple sugar and it certainly happens to be the simplest form of sugar.
• Disaccharides– Disaccharide refers to a sugar whose formation takes place when two the joining of two monosaccharides takes place by glyosidic linkage.
• Polysaccharides– This happens to be a specific type of carbohydrate whose molecules comprise a number of sugar molecules.
• Structure of Proteins– Students will learn about the structure of proteins in detail here.
• Amino Acids– These refer to organic compounds whose combination certainly results in the formation of proteins.
• Enzymes– These happen to be biological molecules that enhance up the rate of all chemical reactions which take place inside cells.
• Vitamins– Vitamins refers to organic compounds that are vital in small quantities for the sustenance of life.
• Nucleic Acids– These refer to biopolymers or small biomolecules whose presence is crucial to all the forms of life.
• Structure of Nucleic Acids– Students will study the structure of nucleic acids in detail under this sub-unit.

Class 12, Chapter 5 : Surface Chemistry

 

Surface Chemistry  

Surface Chemistry is the branch of chemistry which deals with the phenomenon that occurs on the surfaces or interfaces, such phenomenon includes corrosion. catalysis, crystallization, etc.

 Adsorption 

Due to unbalanced attraction forces, accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. The molecular species accumulates at the surface is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent, e.g.. 

(i) O2, H2, C12, NB3 gases are adsorbed on the surface of charcoal. 

(ii) Silica gels adsorb water molecules from air. 

Charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some adsorbents. 

Important Characteristics of Adsorption 

•  It is specific and selective in nature.
• Adsorption is spontaneous process, therefore change in free energy (ΔG)is negative. 
• ΔG= ΔH – TΔS, 
• For the negative value of ΔG,in a system, in which randomness decreases, ΔH must be negative.
• Hence, adsorption is always exothermic.
• Adsorption of hydrogen over Pt is called occlusion. 

Desorption 

It is a process of removing an adsorbed substance from a surface on which it is adsorbed, is known as desorption. 

Distinction between Adsorption and Absorption

Sorption 

It is a process in which both adsorption and absorption take place simultaneously, the term sorption is simply used. 

Positive and Negative Adsorption 

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption. On the other hand, if the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption, e.g., when a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption. 

Distinction between Physio sorption and Chemisorption 

Factors Affecting Adsorption 

(a) Nature of adsorbent

 

Same gas may be adsorbed to different extents on different adsorbent. 

(b) Surface area of the adsorbent 

Greater the surface area, greater is the extent of adsorption.

 

(c) Nature of the gas being adsorbed 

Greater is the critical temperature of a gas, greater are the van der Waals’ forces of attraction and thus, greater is the adsorption. 

(d) Temperature 

Adsorption is an exothermic process involving the equilibrium : 

Gas (adsorbate) + Solid (adsorbent) ⇔ Gas adsorbed on solid + Heat

 

Applying Le-Chatelier principle, increase of temperature decreases the adsorption and vice�versa. 

(e) Pressure 

Adsorption increases with pressure at constant temperature. The effect is large if temperature is kept constant at low value. 

(f) Activation of the solid adsorbent 

Activation means increasing the adsorbing power of the solid adsorbent. This can be done by subdividing the solid adsorbent or by removing the gases already adsorbed by passing superheated steam. 

Adsorption Isotherms 

It is the plot of the mass of gas adsorbed per gram of adsorbent (x / m) versus equilibrium pressure at constant temperature. 

Freundlich Adsorption Isotherm 

It gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. 

It can be expressed by the equation. 

      x / m = kp1/n …(i) 

Where, x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature.

At low pressure, n = 1, 

i.e., x / m = kp 

At high pressure, n > 1, 

i.e., x / m = k (independent of p) 

Taking logarithm of Eq. (i) 

Freundlich Adsorption Equation for Solutions 

x / m = kC1/n 

where, C is the equilibrium concentration. 

On taking logarithm of the above equation, 

we have langmuir 

Adsorption Isotherm 

According to Langmuir, the degree of adsorption is directly proportional to e, 

i.e., the fraction of surface area occupied. 

 x / m α θ = kθ 

 Adsorption Isobars 

These are plots of x / m us temperature t at constant pressure. 

For physical and chemical adsorption, they are shown below. 

Adsorption Isostere 

These are the plot of temperature versus pressure for a given amount of adsorption 

Applications of Adsorption 

1. For production of high vacuum. 

2. Gas masks containing activated charcoal is used for breathing in coalmines. They adsorb poisonous gases. 

3. Silica and aluminium gels are used as adsorbents for controlling humidity.

 4. Removal of colouring matter from solutions. 

5. It is used in heterogeneous catalysis. 

6. In separation of inert gas. 

7. As adsorption indicators. 

8. In chromatographic analysis. 

9. Qualitative analysis, e.g., lake test for Al3 +. Catalysis Catalyst is a chemical substance which can change the rate of reaction without being used up in that reaction and this process is known as catalysis  

A catalyst may be positive (i.e., increases rate of reaction) or negative (i.e., decreases rate of reaction). 

Types of Catalysis 

(a) Homogeneous catalysis In this catalysis, and the catalyst reactants are in the same physical state [phase], e.g., 

(b) Heterogeneous catalysis In heterogeneous catalysis, catalyst is present in a different phase than that of reactants, e.g., 

(c) Autocatalysis 

When one of the product of a reaction acts as catalyst, the process is called autocatalysis. 

Characteristics of Catalysts 

1 The catalyst remains unchanged in mass and chemical composition. 

2. In case of reversible reactions, the catalyst does not influence the composition of reaction mixture at equilibrium. 

It only helps to attain the equilibrium quickly. Promoters and Poisons 

Promoters are chemical substances that enhance the activity of a catalyst while poisons decreases the activity of a catalyst 

 Adsorption Theory of Heterogeneous Catalysis 

The mechanism involves five steps: 

(i) Diffusion of reactants to the surface of the catalyst 

(ii) Adsorption of reactant molecules on the surface of the catalyst. 

(ill) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate. 

(iv) Desorption of reaction products from t he catalyst surface. 

(v) Diffusion of reaction products away from the catalyst’s surface 

Important Features of Solid Catalysts 

(i) Activity 

The activity of a catalyst depends upon the strength of chemisorption to a large extent. The adsorption should be reasonably strong but not so strong that they become immobile and no space is available for other reactants to get adsorbed

. (ii) Selectivity 

The selectivity of a catalyst is its ability to direct a reaction to yield a particular product, e.g., starting with Hz and CO using different catalysts, we get different products. Shape–selective catalysis The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. 

 Cracking Isomerization of hydrocarbons in the presence of zeolites is an example of shape selective catalysis. 

An important zeolite catalyst used in the petroleum industry is ZSM-S.lt converts alcohols directly into gasoline. 

Enzyme Catalysis 

Enzymes are complex nitrogenous organic compounds which are Produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions in water. They are also known as biochemical catalysis. 

Mechanism of Enzyme Catalysis 

Some examples of enzyme catalysed reactions are:

(Source of invertase, zymase and maltose is yeast and that of diastase is malt. Soybean is the source of urease.) 

(v) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis.

 (vi) Lactobacilli is used to convert milk into curd. 

Characteristics of Enzyme Catalysis 

 High efficiency 

One molecule of an enzyme may transform one million molecule of reactant per minute. 

 Highly specific nature Each enzyme catalyst cannot catalyse more than one reaction. 

 Optimum temperature 

Enzyme catalyst gives higher yield at optimum temperature i.e., at 298-310 K. Human body temperature, i.e., at being 310 K is suited for enzyme catalysed reactions. 

 Optimum pH The rate of an enzyme catalysed reaction is maximum at optimum pH range 5 to 7. 

 Activators Activators like ions such as Na+ ,Ca 2+, Mn2+ help in the activation of enzymes which cannot act on their own strength. 

 Co-enzyme Co-enzymes are the substance having nature similar to the enzyme and their presence increases the enzyme activity. Mostly vitamins act as co-enzymes. 

 Effect of Inhibitors Inhibitors slow down the rate of enzymatic reaction. The use of many drugs is based on enzyme inhibition action of those drugs in the body.

1. This phenomenon of attracting and retaining the molecules of a substance by a solid (or a liquid) on its surface resulting into a higher concentration of the molecules on the surface is known as adsorption.
2. The substance that is adsorbed is called adsorbate and the substance which adsorbs is called adsorbent.
3. Desorption is a process of removing an adsorbed substance from a surface on which it is adsorbed.
4. Absorption is different from adsorption. In absorption, the substance is uniformly distributed throughout the body of a solid or a liquid.
5. When the adsorbate is held on the surface by weak van der Waals forces, the process is called physical adsorption or physical adsorption. This type of adsorption can be reversed by heating or decreasing the pressure.
6. When the forces holding the adsorbate on the surface are of the magnitude of chemical bond forces, the process is called chemical adsorption or chemisorption. This type of adsorption is irreversible.
7. Adsorption is generally accompanied by evolution of heat, i.e., it is an exothermic process.
8. The extent of adsorption of a gas on a solid depends upon the following factors:
(a) Nature of the adsorbate,
(b) Nature of the adsorbent,
(c) Temperature, and
(d) Pressure.
9. A relation or a graph between the magnitude of adsorption x/m and the pressure P of the gas at a constant temperature is called adsorption isotherm.
10. Freundlich adsorption isotherm:

Plot of log x/m Vs log P will be a straight line with a slope of 1 In. It holds good at moderate temperature. At low pressure, n = 1.
11. Langmuir adsorption isotherm is based on following assumptions:
(i) Every adsorption site is equivalent in all respect.
(ii) The ability of a particle to bind at a particular site is independent of whether the nearby sites are occupied or not.
12. Langmuir derived the following relation.

where a and b are Langmuir parameters.
13. A substance that can influence the rate of a chemical reaction but itself remains unchanged chemically at the end is called a catalyst.
14. In a homogeneous catalysis, the catalyst is present in the same phase as the reactants.
15. In heterogeneous catalysis, the catalyst is present in a different phase than that of the reactants.
16. Enzymes also called biological catalysts are proteins which catalyse the reactions in living systems.
17. The colloidal solutions are intermediate between true solutions and suspensions. The diameter of colloidal particles varies from 1 to 1000 nm.
18. A colloidal system is a heterogeneous system which consists of disperse phase and dispersion medium.
19. The disperse phase constitutes the colloidal particles whereas the dispersion medium constitutes the medium in which the colloidal particles are dispersed.
20. There are eight types of colloidal systems based on the disperse phase and the dispersion medium.
21. Sols are the colloidal system in which the solid is disperse phase and the liquid is dispersion medium.
22. Hydrosols-Colloids in water.
Alcosols – Colloids in alcohol.
23. Lyophillic colloids (solvent loving) are those substances that directly pass into the colloidal state when brought in contact with the solvent, e.g., proteins, starch, rubber, etc.
These sols are quite stable because of the strong attractive forces between the particles and the, dispersion medium.
24. Lyophobic colloids (solvent hating) are those substances that do not form the colloidal sol readily when mixed with the dispersion medium. These sols are less stable than the lyophilic sols.
25. The colloids are also classified as multimolecular, macro-molecular and associated colloids.
26. Lyophobic sols can be prepared by the following methods:
(a) Chemical methods:
(i) Oxidation,
(ii) Reduction,
(iii) Hydrolysis ,
(iv) Double decomposition,
(b) Physical methods:
(i) Exchange of solvent:
(ii) Excessive cooling: A colloidal sol of ice in an organic solvent (CHCl3 or ether) can be obtained by freezing a solution of water in the solvent.
(c) Dispersion methods:
(i) Mechanical dispersion:
(ii) Bredig’s arc method:
(iii) Peptization method:
27. Lyophilic sols are readily prepared by warming the substance with a dispersion medium, e.g., starch, gelatin, gumarabic, etc., are easily brought into the colloidal state by warming with water.
28. The process of separating a soluble crystalloid from a colloid is called dialysis.
29. Characteristics of colloidal solution:
(a) The zig-zag and random motion of the colloidal particles is called Brownian movement.
(b) When a beam of light is passed through a colloidal solution, its path becomes visible.
This phenomenon is known as Tyndall effect.
It is due to the scattering of light by colloidal particles.
(c) This movement of colloidal particles under applied electric field is known as
electrophoresis.
(d) Diffusion of colloidal particles takes place from a region of higher concentration to lower concentration.
30. Emulsions: It is a colloidal system in which both the dispersed phase and the dispersion medium are liquids, e.g., milk consists of small drops of liquid fat dispersed in water.
31. Emulsification is the process of making an emulsion.
32. Types of Emulsions
(a) Oil-in-water type in which small droplets of an oil are dispersed in water, e.g., milk, cod- liver oil, etc.
(b) Water-in-oil type in which water droplets are
dispersed in an oil medium, e.g., butter.

Class 12, Chapter 4 : Chemical Kinetics

 

Chapter 4 : Chemical Kinetics

The branch of chemistry, which deals with the rate of chemical reactions. the factors affecting the rate of reactions and the mechanism of the reaction. is called chemical kinetics.

Chemical Reactions on the Basis of Rate of Reaction

1. Fast/ Instantaneous reactions

Chemical reaction which completes in less than Ips (10^{- 12} s) time, IS known as fast reaction. It IS practically impossible to measure the speed of such reactions, e.g., ionic reactions. organic substitution reactions.

2. Slow reactions

Chemical re actions which completes in a long time from some minutes to some years are called slow reactions. e.g., rusting of iron. transformation of diamond etc.

3. Moderately slow reactions:

 Chemical reactions which are intermediate between slow and fast reactions are called moderately slow reactions.

Rate of Reaction

Rate of a chemical reaction IS the change in the concentration of any one of the reactants or products per unit time. It is expressed in mol L^-1 s^-1 or Ms^-1 or atm time^-1 units.

Rate of reaction

= (decrease/increase in the concentration of reactant/product/time taken)

This rate of reaction is known as average rate of reaction (r_av).(r_av can be calculated by dividing the concentration difference by the time interval).

For a chemical reaction,

Instantaneous Rate of Reaction

Rate of a chemical reaction at a particular moment of time, is known as instantaneous rate of reaction.

For reaction,

Methods for measuring reaction rate (i) pH measurement, (ii) change in optical activity, (iii) change in pressure, (iv) change in conductance.

Slowest step of a reaction was called rate determining step by van’t Hoff.

Factors Affecting Rate of Reaction

1. Nature and concentration of reactant
2. Temperature
3. Surface area of reactant
4. Radiations and catalyst
5. Pressure of gas

Rate Law Expressions

Law of mass action

According to the law of mass action,

For a chemical reaction,

a A + b B → Products

Rate α [A]^a [B]^b = k[A]^a [B]^b

Rate Law

But experimentally, it is observed that the rate of reaction is found to depend upon ‘α’ concentration terms of A and ‘β’ concentration terms of B.

 Then,

Rate α [A]^α [B]^β = k[A]^α [B]^β

where, [A] and [B] molar concentrations of A and B respectively and

 k is the velocity Constant or rate constant. 

The above expression is known as rate law.

Rate Constant

In the above expression, k is called rate constant or velocity constant.

Rate constant may be defined as the specific rate of reaction when the molar concentrations of the reactants is taken to be unity, i.e.,

Rate = k, if [A] = [B] = 1

Units

Units of rate constant or specific reaction rate for a nth order reaction is given as

K = (1/Time) x (1/[Conc.]^{n – 1})

Characteristics of rate constant

1. Greater the value of rate constant, faster is the reaction.
2. Each reaction has a particular value of rate constant at a particular temperature.
3. The value of rate constant for the same reaction changes with temperature.
4. The value of rate constant for a reaction doesn’t depend upon the concentration of the reactants.

Integrated Rate Equation for Zero Order Reactions

Integrated Rate Equation for First Order Reactions

Half-life period (t_1/2) : It is concentration independent term.

For first order chemical reactions,

(V_o, V_t, and _∞ are the volumes of NaOH solution used for the titration of same volume of the reaction mixture after times 0, t and ∞ respectively.)

Pseudo First Order Reaction

Chemical reactions which appear to be of higher order but actually are of the lower order are called pseudo order reactions. In case of pseudo first order reaction, chemical reaction between two sr” stances takes place and one of the reactant is present in execess. e.g., hydrolysis of ester.

[r_O r_t, and r_∞.. are the polarimetric readings at t = 0, t and ∞, respectively.]

Methods to Determine Order of Reaction

(i) Graphical method

(ii) Initial rate method In this method, the order of a reaction is determined by varying the concentration of one of the reactants while others are kept constant.
(iii) Integrated rate law method In this method out different integrated rate equation which gives the most constant value for the rate constant corresponds to a specific order of reaction.
(iv) Half-life period (t_1/2) method In general half-life period (t_1/2) of a reaction of nth order is related to initial concentration of the reactant as

This method is employed only when the rate law involved only one concentration term.

(v) Ostwald’s isolation method This method is employed in determining the order of complicated reactions by isolating one of the reactants so far as its influence on the reaction rate is concerned.

Temperature Dependence of Rate of a Reaction

For every 10°C rise in temperature, the rate of reaction becomes double, but only 16% collisions increases. It can be explained by Arrhenius equation.

Temperature coefficient is the ratio of rate constant of a reaction at two temperature differing by 10. Temperature selected are usually 298 K and 308 K

Temperature coefficient = _t + 10/_t ≈ 2 to 3

Arrhenius Equation

Arrhenius equation is a mathematical expression to give a quantitative relationship between rate constant and temperature, and the expression is

where, A = frequency or Arrhenius factor. It is also called pre-exponential factor

R = gas constant

E_a = activation energy

Activated complex (or transition state)

Activated complex is the highest energy unstable intermediate between the reactants and products and gets decomposed immediately (having very short life), to give the products. In this state, bonds of reactant are not completely broken while the bonds of products are not completely formed.

Threshold energy (ET) The minimum amount of energy which the reactant must possess in order to convert into products is known as threshold energy.

Activation energy (E_a) The additional amount of energy, required by the reactant so that their energy becomes equal to the threshold value is known as activation energy.

⇒ E_a = E_T – E_R Lower the activation energy, faster is the reaction.

Different reactions have different rates because their activation energies are different.

Larger the value of Eo, smaller the value of rate constant and greater is the effect of a given temperature rise on K

Important points about Arrhenius equation

(i) If ₂ and ₁ are rate constant at temperature T₂ and T₁; then

ii) Fraction of molecules with energy equal to or greater than the activation energy is called Boltzmann factor and is given by

(iii) E_a is constant for a particular reaction.
(iv) E_a does’t depend on temperature, volume, pressure, etc., but gets affected by catalyst.

In the Arrhenius equation, when T → ∞ then = Ae° = A when E_a = 0,k = A and the rate of reaction becomes independent temperature.

Role of Catalyst in a Chemical Reaction

A catalyst is a chemical substance which alters the rate of a reaction WIthout itself undergoing any permanent chemical change.

In the chemical reactions, catalyst provides an alternate pathway or reaction mechanism by reducing the activation energy between reactants and products and hence. lowering the potential energy barrier as shown.

In the presence of catalyst, activation energy decreases and hence.

where, P denotes presence of catalyst and a denotes absence of catalyst.

Theory of Reaction Rates

Collision Theory

According to this theory, the reactant molecules are assumed to be hard spheres and the reaction is postulated to occur, when molecules collide with each other.

The number of collisions between the reacting molecules taking place per second per unit volume is known as collision frequency (Z_AB)·

But only those collisions in which the colliding species are associated with certain minimum amount of energy and collide in proper orientation result in the product formation, such collisions are called fruitful collisions or effective collision.

Here, rate = – (dv/dt) = collision frequency x fraction of effective collision

= Z_AB x f = Z_AB x e^-E _a ^/RT

where, Z_AB represents the collision frequency of reactants, A and B e^-E _a ^/RT represents the fraction of molecules with energies equal to or greater than Ea.

So, to account for effective collisions, another factor, P called the probability or steric factor is introduced.

So, rate = PZ_ABe^-E _a ^/RT

The Activated Complex Theory or Transition State Theory

Reactants ⇔ Activated complex → Products

This theory is based on the fact that bond cleavage and bond formation, involved in a chemical reaction, must occur simultaneously. Hence, the reactants are not converted directly into the products. There is an energy barrier or activated complex [intermediate product with partially formed bond] between the reactants and products. The reactants must cross this energy barrier before converting into products. The height of the barrier determines the threshold energy.

Photochemical Reactions

Chemical reactions, that occur on exposure to visible radiation are called photochemical reactions.

1. The rate of a photochemical reactions is affected by the the intensity of light.
2. Temperature has little effect on photochemical reactions.

Quantum yield or quantum efficiency of a photochemical reaction,

φ = (number of reactant molecules reacting in a given time / number of photons (quanta) of light absorbed ill the same time)

Class 11- Chemistry: Chapter 14: Environmental Chemistry by Subhas C Chakra

 

Environmental Chemistry

 Environmental Chemistry
It is the branch of science which deals with the chemical changes in the environment. It includes our surroundings as air, water, soil, forest etc.
• Environmental Pollution
It is the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings.
• Pollutants
A substance, which causes pollution, is known as pollutant. Pollutants can be solid, liquid or gaseous substances. Present in higher concentration, it can be produced due to human activities or natural happenings.
• Troposphere
The lowest region of atmosphere, in which the human beings along with other organisms live, is called troposphere.
It extends to the height of about 10 km from the sea level. It contains air, water vapours, clouds etc. The pollution in this region is caused by some poisonous gases, smoke fumes, smog etc.
• Stratosphere
It extends from height of 10 to 50 km above the sea level. Ozone and some other gaseous substances present in this region are responsible for the pollution.
• Tropospheric Pollution
Pollution in this region is caused by the presence of undesirable gaseous particles like oxides of sulphur, nitrogen and carbon, hydrocarbons along with solid particles like dust, mist, fumes, smoke etc.
• Oxides of Sulphur
These are produced when coal containing sulphur is burnt.

It is also produced during volcanic eruptions.
Harmful effects:
(i) It is poisonous to both animals and plants.
(ii) A very high concentration of S02 may cause respiratory diseases e.g., asthma,bronchitis, emphysema in human beings.
(iii) It causes irritation to the eyes, resulting in tears and redness.
(iv) Its high concentration leads to the stiffness of flower buds.
(v) Particulate matter present in the air can catalyse the formation of sulphur trioxide from sulphur dioxide.

• Oxides of Nitrogen
Main oxides of nitrogen are nitric oxide (NO) and nitrogen dioxide (NO2).
Major Sources:
(i) Lightning discharge results in the combination of N2 and 02 to form NO.
(ii) Combustion of gasoline in automobilies, burning of hydrocarbons and coal etc.
Harmful effects:
Nitric oxide itself is not harmful to human beings, but it is very unstable and changes to nitrogen dioxide which is toxic in nature. These effects are as follows:
(i) It reacts with Ozone (03) present in the atmosphere and thus decrease the density of Ozone.
(ii) It affects the respiratory system and damages the lungs.
(iii) Higher concentrations of N02 damage the leaves of plants and retard the rate of photosynthesis.
(iv) It causes cracks in rubber.
(v) Nitrogen dioxide is also harmful to various textile fibres and metals.
• Hydrocarbons
Incomplete combustion of fossil fuel in industry and thermal power plants and the exhaust of automobiles release hydrocarbons into the atmosphere constantly causing pollution. Harmful Effects:
(i) They cause cancer.
(ii) Methane is one of the greenhouse gases.
(iii) They harm plants in various ways like breakdown of tissues, shedding of leaves etc.
• Oxides of Carbon
Carbon dioxide:
0.03% C02 is present in air by Volume.
Major Sources:
(i) By burning of fossil fuels.
(ii) By the decomposition of limestone during the manufacture of cement.
(iii) Emitted during volcanic eruptions.
(iv) C02 is released into the atmosphere by respiration.
Harmful effects:
Deforestation and burning of fossil fuel increases the C02 level which is mainly responsible for global warming.
Carbon Monoxide: Carbon Monoxide is a colourless and odourless gas.
Major Sources:
(i) Released by the automobile exhaust.
(ii) Incomplete combustion of coal, fire wood, petrol etc.
(iii) By the dissociation of C02 at high temperature.
Harmful effects:
(i) It binds to haemoglobin to form carboxyhaemoglobin which is more stable than oxygen-haemoglobin complex. Its concentration in blood when reaches to 3-4%, the oxygen carrying capacity of blood is greatly reduced.
The oxygen deficiency, results into headache, weak eyesight, nervousness etc.
(ii) It has harmful effects on plants when its concentration is (100 ppm or more).
• Global Wanning and Greenhouse Effect
Greenhouse Effect:
Some gases like carbondioxide, methane, ozone, water vapours, CFCs have the capacity to trap some of the heat radiations that are released from the earth or from sun. These gases are known as greenhouse gases and the effect is called greenhouse effect. This leads to global warming.
Consequences of global warming:
(i) It leads to melting of polar ice caps and flooding of low lying areas all over the earth.
(ii) Global rise in temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.
• Acid Rain
When the pH of the rain water drops below 5.6, it is known as acid rain.
Normal rain is slightly acidic due to dissolution of atmospheric carbon dioxide in water.

Oxides of nitrogen and sulphur released as a result of combustion of fossil fuels dissolve in water to form nitric acid and sulphuric acid.

Harmful Effects of Acid Rain:
(i) It has harmful effects on trees and plants as it dissolves and washes away nutrients needed for their growth.
(ii) It has very bad effect on aquatic ecosystem.
(iii) Acid rain damages buildings and other structures made of stone or metal. Taj Mahal in India has been affected by acid rain.

• Particulate Pollutants
Viable Particulates: They are minute living organisms that are dispersed in the atmosphere. e.g., bacteria, fungi) moulds, algae etc.
Non Viable Particulates:
(i) Smoke: It is the mixture of solid and liquid particles formed during combustion of organic matter,
Example: Cigarette smoke, smoke from burning of fossil fuel.
(ii) Dust: Composed of fine solid particles (over 2gm in diameter).
It is produced during crushing, grinding and attribution of solid particles.
(iii) Mist: These are produced due to the spray of liquids like herbicides and pesticides over the plants. They travel through air and form mist.
(iv) Fumes: They are generally released to the atmosphere by the metallurgical operations and also by several chemical reactions.
Harmful Effects of Particulate Pollutants:
(i) Fine particles less than 5 microns penetrate into the lungs. Inhalation of such particles can lead to serious lung diseases including lung cancer.
(ii) Suspended particles of bigger size can hinder the sun rays from reaching the earth surface. This can lower the temperature of earth and make the weather foggy.
• Smog
This is the common form of air pollution which is combination of smoke and fog.
Smog exists in two types:
(i) Classical Smog: Occurs in cool humid climate. It contains smoke, fog and sulphur dioxide. It is also called as reducing smog.
(ii) Photochemical Smog: This type of smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides released by the vehicles and industries. It has high concentration of oxidising agents and is therefore, called as oxidising smog.
Formation of Photochemical Smog

Harmful effects of photochemical smog:
(i) It can cause cough, bronchitis, irritation of respiratory system etc.
– To control this type of pollution the engines of the automobiles are fitted with catalytic converters to check the release of both oxides of nitrogen and hydrocarbons in the atmosphere.
– Some plants like Vitis, Pinus, Juniparus, Quercus, Pyrus can metabolise nitrogen
oxide and therefore, their plantation can be done.
• Stratospheric Pollution
Formation of Ozone: Ozone in the stratosphere is produced by UV radiations. When UV – radiations act on dioxygen (02) molecules, Ozone is produced.

Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus there exists an equilibrium between production and decomposition of Ozone molecules.
Depletion of Ozone layer: Ozone blanket in the upper atmosphere prevent the harmful UV radiations from reaching earth.
But in recent years, there have been reports of depletion of this layer due to presence of ,certain chemicals in the stratosphere. Chlorofluorocarbons (CFCs), nitrogen oxides, chloride, CCl4 etc. are the chemicals responsible for depletion.

Chlorofluorocarbons dissociate in the presence of light gives chlorine free radicals which catalyse the conversion of ozone into oxygen.
Effects of the depletion of Ozone layer:
(i) This leads to many diseases like skin cancer, sunburn, ageing of skin, cataract etc.
(ii) UV radiations can kill many phytoplanktons, damage the fish productivity.
(iii) It can decrease moisture content of the soil by increasing the evaporation of surface water.
(iv) UV radiations can damage paints and fibres, causing them to fade faster.
• Water Pollution
Presence of undesirable materials in water which is harmful for the human beings and plants is known as water pollution. Normal properties of the water can be changed by the presence of these foreign materials.


Causes of Water Pollution:
(i) Pathogens: Pathogens are the bacteria and the other organisms that enter water from domestic sewage and animal excreta.
Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis. It causes gastrointestinal diseases.
(ii) Organic Wastes: Organic matter such as leaves, grass, trash etc. can pollute water.
– Excessive growth of phytoplankton within water also pollute water.
– Large numbers of bacteria in water can consume oxygen dissolved in water by decomposing organic matter present in water.
– If the concentration of dissolved oxygen in water is below 6 ppm, the growth of fish gets inhibited.
– If too much of organic matter is added to water, all the available oxygen is used up. This can cause the death of the aquatic life.
• BOD (Biochemical Oxygen Demand)
It is defined as the amount of oxygen required by bacteria for the breakdown of the organic matter present in a certain volume of a sample of water.
The amount of BOD in water is a measure of the amount of organic material in the water. Clean water has BOD value of less than 5 ppm.
Highly polluted water could have a BOD value of 17 ppm or more.
• Chemical Pollutants
(i) Industrial Wastes: Chemical reactions carried in the industrial units also pollute water to a great extent. For example, lead, mercury, nickel, cobalt etc. These chemicals give very bad effect to the groundwater and waterbodies are polluted due’ to the chemical reactions known as leaching.
Organic chemicals like petroleum products also pollute many sources of water e.g., major oil spills in oceans.
(ii) Pesticides: These are mostly chlorinated hydrocarbons, organophosphates and metallic salts etc. They dissolve in water to small extent and pollute it. Since all the pesticides are toxic in nature, they are injurious to both plants and animals.
(iii) Polychlorinated biphenyls (PCBS): These are the chemical compounds used as fluids in transformers and capacitors. These are released in atmosphere as vapours. They mix with rain water and thus contaminate the water.
(iv) Eutrophication: The process in which algae like organisms reduce dissolved oxygen in water is called as eutrophication. It is harmful for aquatic life.
• International Standards for Drinking Water
Fluoride: Concentration of fluoride up to 1 ppm or 1 mg dm-3, is not harmful for human
beings if it is used as drinking water. The F~ ions make the enamel on teeth much harder by converting hydroxyapatite [3Ca3(P04)2- Ca(OH)2] the enamel on the surface of the teeth, into much harder fluorapatite, [3Ca3(P04)2- CaF2]. Concentration of F~ above 2 ppm causes brown mottling of teeth. Excess of fluoride is harmful to bones also.
Lead: Upper limit concentration of lead in drinking water is about 50 ppm. Lead can damage kidney, lever, reproductive system etc.
Sulphate: At moderate level it is harmless but excess is harmful.
Nitrate: The maximum limit of nitrate should be 50 ppm. Excess nitrate in drinking water can cause diseases such as methemoglobinemia (blue baby syndrome).
Chemical Oxygen Demand (COD): Water is treated with K2Cr207 in acidic medium to oxidise polluting substance which cannot be oxidised by microbial oxidation. The remaining  is determined by back titration with suitable reducing agent.
From the concentration of K2Cr207 consumed, the amount of O2 used in the oxidation is calculated.

• Soil Pollution—Sources of Soil Pollution
Pesticides: It can be classified as:
(i) Insecticide: The most common insecticides are chlorinated hydrocarbons like DDT, BHC etc.
As they are not much soluble in water, they stay in the soil for long time. They are ‘ absorbed by the soil and contaminate root crops like radish, carrot etc.
(ii) Herbicides: These are the compounds used to control weeds, namely, sodium chlorate (NaCl03) and sodium arsenite (Na3As03) are commonly used herbicides but arsenic compounds, being toxic are no longer preferred.
Fungicides: Organo-mercury compounds are the most common fungicides. Its dissociation in soil produces mercury which is highly toxic and harmful for the crops. i Industrial Waste: It has seen that most of the industrial wastes are thrown into water or dumped into the soil. These industrial wastes contain huge amounts of toxic chemicals which are mostly non-bidegradable. For example, metal processing industries, mining cement, glass industries, petroleum industry etc., fertilizer industry produce gypsum.
The disposal of non-biodegradable industrial solid waste is not done by suitable methods i and cause many serious problems.
Strategies to control environmental pollution:
(i) The improper disposal of wastes is one of the major causes of environmental I degradation. The management of wastes is very important.
(ii) All domestic wastes should be properly collected and disposed.
• Green Chemistry
Green Chemistry is a way of thinking and is about utilising the knowledge and principles of
chemistry that would control the increasing environmental pollution.
Green chemistry in day-to-day life:
(i) Dry-Cleaning of clothes and laundary: Replacement of halogenated solvent like (CCl4) by liquid C02 which is less harmful to groundwater.
Hydrogen peroxide (H202) is used for the purpose of bleaching clothes.
(ii) Bleaching of Paper: In place of chlorine H202 is used for the bleaching of paper,
(iii) Synthesis of Chemicals: Ethahal (CH3CHO) is prepared by step oxidation of ethene. Such as,

• Environmental pollution: It is the effect of undesirable changes in the surroundings that have harmful effects on plants, animals, and human beings.
• Troposphere: The lowest region of atmosphere which extends up to the height of 10 km from sea level in which man and other living organism exists.
• Stratosphere: It is above Troposphere between 10 to 50 km above the sea level.
• Acid rain: It is caused by the presence of oxides of Sulphur and nitrogen and C02 in the atmosphere. The pH of the rain drops below 5.6, and it becomes acidic.
• Greenhouse gases: Some gases like carbon dioxide, methane, ozone, water vapours, CFCs have the capacity to trap some of the heat radiations from the earth or from the sun. This leads to global warming.
• Eutrophication: When phosphate ion increases in water it increases the growth of algae which consume the dissolved oxygen in water consequently aquatic life is adversely affected. This results in loss of biodiversity and the phenomenon is known as Eutrophication.
• COD (Chemical Oxygen Demand): It is calculated as the amount of oxygen required to oxidise the polluting substances. It is measured by treating the given sample of water with an oxidising agent, generally K2Cr207in the presence of dil. H2S04.