Amresh Academy: IMP QUE 11TH CHE

IMP QUE 11TH CHE

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Class 11th Chemistry Important Extra Questions with Answers Chapter Wise Pdf

Environmental Chemistry Important Extra Questions Very Short Answer Type

Question 1.
What is the percentage of CO2 in pure dry air?
Answer:
About 0.03%.

Question 2.
Which gas was responsible for the Bhopal tragedy?
Answer:
Methyl isocyanate (MIC).

Question 3.
What is smog?
Answer:
A combination of smoke and fog.

Question 4.
Name three natural sources of air pollution.
Answer:

Volcanic eruptions
Forest fires
Pollen grains of flowers.

Question 5.
What is that compound formed when CO combines with blood?
Answer:

Question 6.
Name three gases that are major air pollutants.
Answer:

CO2
NO2
SO2.

Question 7.
How does particulate help in cloud formation?
Answer:
The particulates work as nuclei for cloud formation.

Question 8.
Give one main reason for ozone depletion.
Answer:
The release of chlorofluorocarbons (CFCs) also known as freon.

Question 9.
Name two important sinks of CO.
Answer:
Oceans that dissolve it and plants use it for photosynthesis.

Question 10.
Which acids arc present in the added rain?
Answer:

H2SO4
HNO3
HCl.

Question 11.
What is the nature of London smog?
Answer:
It is reducing in nature.

Question 12.
Name two herbicides used to kill weeds.
Answer:

Sodium chlorate (NaClO3)
Sodium arsenite (Na3AsO3)

Question 13.
Name two gases that form acid rain.
Answer:

SO2
NO2.

Question 14.
What is the composition of photochemical smog?
Answer:
It is a mixture of irritation causing compounds NO2, O3, hydrocarbons, acrolein, formaldehyde, peroxyacetyl nitrate (PAN).

Question 15.
Which zone is called the ozonosphere?
Answer:
Stratospheric zone.

Question 16.
Name any four methods of waste management.
Answer:

Recycling
Burning and incineration
Sewage treatment
Dumping.

Question 17.
What is BOD?
Answer:
The amount of oxygen consumed by microorganisms in decomposing waste in a sample of sewage water is called BOD (Biochemical Oxygen Demand).

Question 18.
What is the size range of particulates?
Answer:
5 nm to 500,000 nm.

Question 19.
What type of aromatic compounds are present as particulates in the air?
Answer:
Poly cyclic aromatic hydrocarbons [PAH].

Question 20.
In which season and what time of day, there is London smog?
Answer:
In winter during morning hours.

Question 21.
Which disease is caused due to a hole in the ozone layer and why?
Answer:
Ultraviolet rays from the sun will reach the earth after passing through the ozone hole and cause skin cancer.

Question 22.
What are polar stratospherical clouds (PSC)?
Answer:
The special type of clouds present over Antarctica in winter are called polar stratospheric clouds.

Question 23.
What should be the tolerable limit of fluoride ions in drinking water? What happens if it is more than 10 ppm?
Answer:
One ppm or 1 mg dm-3. The higher concentration is harmful to bones and teeth.

Question 24.
Which main compounds are causing damage to the ozone layer?
Answer:
NO and freons.

Question 25.
What is the composition of London smog?
Answer:
Fog of H2SO4 droplets deposited on the particulates.

Question 26.
Why photochemical smog is so called?
Answer:
Because it is formed as a result of the photochemical reaction between oxides of nitrogen and hydrocarbons.

Question 27.
In which season and what time of day, there is photochemical smog?
Answer:
In summer, in the afternoon.

Question 28.
What is the ‘nature of photochemical smog?
Answer:
It is oxidising in nature.

Question 29.
Name three methods used in green chemistry.
Answer:
Use of sunlight and microwaves, use of sound waves and use of enzymes.

Question 30.
Give three examples in which green chemistry has been applied.
Answer:

Dry cleaning of clothes
Bleaching of paper
Synthesising chemicals.

Question 31.
What are the main advantages of using green chemistry?
Answer:
It is a cost-effective approach that involves a reduction in material, energy consumption and waste generation.

Question 32.
What is the troposphere?
Answer:
The troposphere is the lowest region of the atmosphere (~ 10 km) in which man along with other organisms including plants exist.

Question 33.
What is the stratosphere?
Answer:
The stratosphere extends above the troposphere up to 50 km above; sea level.

Question 34.
Name three scientists who got Noble Prize in chemistry in 2005 for their work in reducing hazardous waste in creating new chemicals.
Answer:

Yves Chauvin
Robert H. Grubbs
Richard R. Schrock.

Question 35.
What is meant by metathesis?
Answer:
Metathesis is an example of how important basic science has been applied for the benefit of man, society and the environment.

Question 36.
In what regions of the atmosphere, the temperature increases with altitude and in which regions it decreases?
Answer:
Temperature increases with altitude in the stratosphere and thermosphere and it decreases with altitude in the troposphere and mesosphere.

Question 37.
What do you mean by ‘Inversion temperature’ in different regions of the atmosphere?
Answer:
When we go from one region of the atmosphere to the next adjoining region, the trend of temperature changes from increase to decrease or vice-versa is called inversion temperature.

Question 38.
What is the most important sink of CO pollutant?
Answer:
Soil micro-organisms.

Question 39.
What is the compound formed when CO combines with blood?
Answer:
Carboxyhaemoglobin (HbCO).

Question 40.
What is anoxia starvation in the body (due to CO poisoning)?
Answer:
Actual oxygen starvation in the body (due to CO poisoning).

Question 41.
How are the flue gases from industries freed from oxides of N and S?
Answer:
By scrubbing them with a cone. H2SO4 or with alkaline solutions of Ca(OH)2 & Mg(OH)2.

Question 42.
What is chlorosis?
Answer:
Slowing down the formation of chlorophyll in plants due to the presence of SO2 as a pollutant is called chlorosis.

Question 43.
What is the cause of blue baby syndrome?
Answer:
Excess of nitrates in drinking water leads to methemoglobinemia which is commonly called ‘blue baby syndrome.

Question 44.
Give I.U.P.A.C. name of B.H.C.
Answer:
1, 2, 3, 4, 5, 6 Hexa- chloro cyclohexane.

Question 45.
What is meant by P.C.Bs.?
Answer:
P.C.Bs. are polychlorinated biphenyls. They are contaminants of water.

Question 46.
What is P.A.N.?
Answer:
Peroxy acetyl nitrate.

Question 47.
Name the oxidizing agent used in determining C.O.D.
Answer:
Potassium dichromate (K2C2O7).

Question 48.
What are fungicides?
Answer:
The chemicals which check the growth of fungi are called fungicides.

Question 49.
When does rainwater become Acid rain?
Answer:
When the pH of rainwater becomes as low as 2 to 3.5, it forms acid rain.

Question 50.
What is meant by Taj Trapezium?
Answer:
Taj Trapezium is the area that includes the town of Agra, Firozabad, Mathura & Bharatpur. Under this plan, more than 2000 polluting industries lying inside the Trapezium would switch over to the use of natural gas or L.P.G. instead of coal or oil.

Environmental Chemistry Important Extra Questions Short Answer Type

Question 1.
Fish do not grow as well in warm water as in cold water. Why?
Answer:
The amount of dissolved oxygen in warm water is less than in cold water. For the fish to survive in water, the concentration of dissolved O2 is 6 ppm which decreases in warm water.

Question 2.
Why does rainwater normally have a pH of about 5.6? When does it become acid rain?
Answer:
Normally rain has a pH of about 5.6 due to the dissolution of CO2 of the atmosphere into it.
CO2(g) + H20O(l) → H2CO3(al)
H2CO3(aq) ⇌ 2H+ + CO32-
When the pH of rain falls below 5.6, it becomes acid rain.

Question 3.
In which season the depletion of ozone in Antarctica takes place and when is it replenished?
Answer:
During spring seasons (i.e., in the months of September and October) depletion of ozone takes place and after spring (in the month of November) it is replenished.

Question 4.
What is the role of CO2 in the greenhouse effect?
Answer:
The heat from the sun after being absorbed by the earth is remitted by the earth and absorbed by CO2 and then radiated back to the earth, thereby warming it.

Question 5.
What are viable and non-viable particulates?
Answer:
Viable particulates are small-sized living organisms such as bacteria, fungi, moulds and algae etc. Non-viable particulates are formed by the disintegration of large-sized materials or condensation of small-sized particles or droplets e.g. mist, smoke, fume and dust.

Question 6.
Why there is ozone depletion mainly over Antarctica?
Answer:
This is because, in other parts of the stratosphere, chlorine-free radicals combine away but in Antarctica, the compounds formed are converted back to chlorine-free radicals which deplete the ozone layer.

Question 7.
Explain giving reasons. “The presence of CO reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells”.
Answer:
CO combines with haemoglobin of the red blood corpuscles (R.B.Cs) about 300 times more easily than oxygen to form carboxyhaemoglobin reversibly as follows:
Hb + CO ⇌ HbCO

Thus it is not able to combine with oxygen to form oxyhaemoglobin and transport of oxygen to different body cells cannot take place.

Question 8.
Why is acid rain considered a threat to the Taj Mahal?
Answer:
Taj Mahal is made of white marble (CaCO3). The acid rain contains H2SO4 in a very dilute form which attacks marble, thereby pitting it, discolouring it and making it lustreless.
CaCO3(s) + H2SO4 (aq) → CaSO4(aq) + CO2(g) + H2O(l)

Question 9.
What are the harmful effects of carbon monoxide?
Answer:
1. Carbon mono-oxide when inhaled passed through the lungs into the blood where it reacts with the haemoglobin of the die R.B.C. to form a complex. Known as Carboxy haemoglobin the latter is not in a position to transport the inhaled oxygen to various parts of the body. This will cause suffocation & will ultimately lead to death.

2. A high concentration of carbon mono-oxide (100 ppm) or more will harmfully affect the plants causing leaf drop reduction in leaf size & pre-mature ageing etc.

Question 10.
What are the harmful effects of oxides of nitrogen?
Answer:

A high concentration of NO2 in the atmosphere is harmful to plants resulting in leaf spotting, retardation of photosynthetic activity & also suppression the vegetation growth.
NO2 results in respiratory problems in human beings & lead to bronchitis.
NO2 has harmful effects on nylon, rayon & cotton yarns & also cause cracks in the rubber.
They also react with ozone present in the atmosphere & thus decrease the density of ozone.

Question 11.
What are the harmful effects of oxides of Sulphur?
Answer:

SO2 is a major source of irritation for the eyes & respiratory tract & leads to serious diseases such as bronchitis & lung cancer even when it is about 2.5 ppm. in the atmosphere.
London smog & sulphurous acid smog which mainly consists of SO2(0-40 ppm or above) is known as smog killer.
Plants are relatively more sensitive to the harmful effect of SO2 as compared to animals. Constant exposure to air containing a high level of SO2 kills leaf tissues, reduces plants productivity & also bleaches leaf pigments.
SO2 has harmful effects on buildings & statues made up of marble & limestone.
Air polluted with oxides of sulphur also accelerates the corrosion of metals like copper, zinc, iron etc.

Question 12.
How do detergents cause water pollution?
Answer:

Detergents are non-biodegradable & they cause water pollution.
They inhibit the oxidation of organic substances present in wastewaters because they form a sort of Envelope around them.
They form stable foam in rivers that extend over several hundred meters of the river water.

Question 13.
Why is add rain considered a threat to the Taj Mahal?
Answer:
The air of the city Agra, where the Taj Mahal is located, contains a very high level of Sulphur & nitrogen oxides. The resulting acid rain reacts with the marble of the Taj Mahal. Causing pitting in this wonderful monument that has attracted people from around the world. As a result, the monument is being slowly eaten away & the marble is getting decoloured & lusterless.

Question 14.
Explain giving reasons “The presence of co-reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells?
Answer:
Carbon mono-oxide binds itself with the haemoglobin of the R.B.Cs. about 200 times more easily than oxygen to form carboxyhaemoglobin reversibly as follows:
Hb + CO ⇌ HbCO

The presence of CO, therefore, reduces the amount of haemoglobin available in the blood for the transport of oxygen to the body cells & therefore, with fewer O2 levels, normal metabolism is impaired.

Question 15.
What do you understand by the greenhouse effect? What are the major greenhouse gases?
Answer:
The greenhouse effect is the phenomenon in which the earth’s atmosphere traps the heat from the sun, & prevents it from escaping into outer space. The warming of the earth or global warming due to re-emission of sun’s energy absorbed by the earth followed by its adsorption by CO2 molecules & H2O vapours present near the earth’s surface & then it’s radiation back to the earth is called “Greenhouse effect”. The major greenhouse gases are:

CO2, CH4, C.F.Cs & water vapours.

Question 16.
What do you understand by Mists, Smoke, Fumes & Dust?
Answer:
Mists: Mists are produced by particles of spray liquids & the condensation of vapours in the air. Examples are portions of herbicides & insecticides, that miss their targets & travel through the air to form mists.

Smoke: They are very small soot particles produced by burning & combustion of organic matter. Oil smoke, Tobacco smoke & Carbon smoke are typical examples.

Dust: It consists of fine particle produced during crushing, grinding & attribution of solid materials. Non-viable dust particulates in the atmosphere consist of ground limestone, sand tailing from floatation, pulverised coal, cement, fly ash & silica dust.

Question 17.
What is Pneumo coniosis? How does it occur?
Answer:
The smaller particulate pollutants are more likely to penetrate into the lungs. These fine particles are carcinogens. Inhalation of small particles irritates the lungs & exposure to such particles for long periods of time causes fibrosis of lung lining. This type of disease is termed “Pneumoconiosis”. This occurs due to exposure to such particles for a long time.

Question 18.
What measures should be taken to check pollution by sewage?
Answer:

Sewage must be churned by machines so that the large pieces may break into smaller ones & may get mixed thoroughly. The churned sewage is passed into a tank with a gentle slope. Heavier particles settle & the water flowing down is relatively pure.
Water must be sterilized with the help of chlorination. Chlorination is very essential, particularly in rainy session.
Treatment of water with alum, lime etc.

Question 19.
Discuss the water pollution caused by industrial wastes.
Answer:
The compounds of lead, mercury, cadmium, nickel, cobalt & zinc etc. Which are the products of chemical reactions; carried in the industrial units pollute water to a large extent & are responsible for many diseases. Mercury leads to Minamata disease, & lead poisoning leads to various types of deformities.

Question 20.
What remedial steps should be taken to save a person suffering from co-poisoning?
Answer:
Remedial treatment for CO poisoning:

Carry the patient into the fresh air immediately & do not allow him to walk.
Lose his clothes & take off his shoes.
Give artificial respiration if the patient is not able to breathe properly.
In the hospital the patient should be kept in a high-pressure chamber containing oxygen at 2-2.5 atm pressure under pressure, CO of carboxyhaemoglobin is replaced by O2 & thus the transport of O2 to different parts of the body starts.
HbCO + O2 ⇌ HbO2 + CO

Question 21.
How can pollution due to nitrogen & Sulphur oxides be controlled?
Answer:
The following steps are taken to control NO2 & SO2 pollution:
(i) The catalytic converters should be used in the automobile exhausts which is the first stage of converting the oxides of nitrogen, to free N2 or to a small amount of NH3.
(ii) The fumes of gases coming from power plants or industrial, units & containing NO2 & SO2 are freed from these gases by scrubbing the fumes with H2SO4. The following reactions take place:

I- step: NO2 + SO2 + H2O → H2SO4 + NO
II- step: NO + NO2 → N2O3
III- step: N2O3 + 2H2SO4 → 2NOHSO4 + H2O

the fumes of gases are thus freed from NO2 & SO2 & are released into the atmosphere. The reaction product NOH2O4 is decomposed to get H2SO4 which is then used again for scrubbing. As NOx & SOx are > acidic oxides, scrubbing can also be done with an alkaline solution such as Ca(OH)2 & Mg(OH)2.

Question 22.
What is groundwater pollution? How does it take place?
Answer:
Water below the surface of the earth is called groundwater. Most of the freshwater (> 90%) is present as groundwater. The remaining is present as groundwater. The remaining is present in lakes, ponds, rivers, streams etc. only 2% of water is present as soil moisture above the water table, which is needed for the growth of the plants.

Groundwater collected below the surface of the earth after passing through the pores of earthy materials which acts as a filter for it & is pure. It is for this reason that well water is spring water is used for drinking purpose in rural areas. However, due to the disposal of domestic wastes, industrial effluents, use of fertilizers & pesticides in agriculture, A number of harmful soluble substance dissolve into the rainwater & result in groundwater pollution especially where the water table is high.

Question 23.
How is the pollution of river water caused in India? What measures have been taken by the Government to check river pollution?
Answer:
The main reasons for pollution in rivers are as follows:

Industrial waste discharge: It includes those from paper, textiles, fertilizers, rayon, pesticides, detergents, drug industries & refineries.
Domestic sewage discharge: The government has enacted laws banning the discharge of industrial or domestic wastes into these rivers. It has started the following plans to clean up the water of these rivers.
(a) Ganga Action Plan I & II
(b) Yamuna Action Plan
(c) Plan to clean Hoogly water.

Question 24.
What are the effects of oil pollution on Sea Water?
Answer:
Effects of oil pollution on Sea Water:

oil spills cause heavy damage to fishes oil coating makes them unable to respire & clogs their gill slits. Aromatic compounds present in them are poison for fishes.
Emulsified oil goes deep down into the sea damaging aquatic animals & plants.
Oil spills result in a reduction of dissolved oxygen.
The most affected by oil pollution are the sea birds. Natural, insulating oil & waxes which shield the birds from water are broken down by the spilt oil. As a result due to loss of insulation, they start shivering & are freezing to death, especially in winter.

Question 25.
Which is the permitted safety limit of fluoride & lead concentration with respect to international standards of drinking water?
Answer:
The fluoride concentration should be about 1 ppm in drinking water. This concentration is within agreed safety limits & has been shown to protect teeth against decay. A high concentration of fluoride is poisonous & are harmful to bones & teeth at levels over 10 ppm.

The lead concentration should be about 50 ppm in drinking water.

Question 26.
Define the term pesticides? What are three categories of pesticides?
Answer:
Pesticides are substances that are used to control the reproductive process of unwanted organisms.

Three main categories of pesticides are:

Insecticides: These are used to control insects & curb diseases (Malaria, Yellow fever) & protect crops e.g. D.D.T.
Herbicides: These are used to kill weeds eg. NaClO3 & Na2AsO3
Fungicides: These are used to check the growth of fungi e.g. Methyl mercury.

Question 27.
Write four major pollutants of water, their source & effects.
Answer:

S.No. Major Pollutants of Water	Sources	Effects
1. Lead	Lead-acid batteries & leaded gasoline	Toxic to organisms causes phimosis
2. Acids	Mine drainages, Industrial wastes	Kills organisms, overgrowth of algae & aquatic weeds.
3. Detergents	Households & industries	Depletion of dissolved oxygen.
4. Pesticides & Insecticides	Agriculture & Mosquitoes repellants	Toxic to fishes, birds & mammals.

Question 28.
Discuss the mechanism of treatment of industrial wastes.
Answer:
The treatment of industrial waste depends upon the nature of the pollutant present. In order to ascertain it, the pH of a medium is first determined & the waste is then neutralized with the help of suitable acids or alkalies.

The chemical substances present in the industrial waste product, dissolved in water can be precipitated by suitable chemical reactions & removed later on from water. Quite recently, photo-catalysis & ion exchangers have been developed for the treatment of industrial wastes.

Question 29.
What measures are necessary to control soil pollution?
Answer:
In order to control soil pollution, the following measures are necessary:

Use of manures: Manures is semi-decayed organic matter which is added to the soil to maintain its fertility. These are mostly prepared from animal dung & another form of wastes. These are much better than the commonly used fertilizers.
Use of bio-fertilizers: These are organisms that are inoculated in order to bring about nutrient enrichment of the soil e.g. nitrogen-fixing bacteria & blue-green algae.
Proper sewerage System; A proper sewerage system must be employed & sewerage recycling plants must be installed in all town & cities.
Salvage & Recycling: Rag-picking removes a large no. of waste articles such s paper, polythene, cardboard rags, empty bottles & metallic, articles. These are subjected to recycling & this helps in checking soil pollution.

Question 30.
What are aerosols? Define the term green chemistry.
Answer:
Very small particles of solids & liquids dispersed in the air are called aerosols. Aerosol particles have a diameter of less than one micrometre (< 1μm). The particles of aerosols are so small that they remain suspended in the air.

Green Chemistry: The branch of chemistry which emphasizes the process & products that reduce or eliminate the use & generation of toxic/hazardous substances is called Green Chemistry.

Environmental Chemistry Important Extra Questions Long Answer Type

Question 1.
What is the difference between London (classical) smog and photochemical (Los Angeles) smog?
Answer:

Classical (London) smog	Photochemical (Los Angeles) Smog
1. This type of smog was first observed in London in 1952.	1. This type of smog was first observed in Los Angeles in 1950.
2. It is formed due to the presence of SO2 and humidity in the air which combines to form H2S04 fog which gets deposited on the particulates.	2. It is formed due to a photochemical reaction taking place when the air contains NO2 and hydrocarbons.
3. It involves smoke and fog.	3. It does not involve any smoke or fog. The word smog is a misnomer here.
4. It is formed in the months of winter particularly in the morning hours when the temperature is low.	4. It is formed during the months of summer during the afternoon when there is bright sunlight so that photochemical reaction can take place.
5. It causes a problem in the lungs.	5. It causes irritation in the eyes.
6. It is reducing in character.	6. It is oxidising in character.

Question 2.
(a) Mention some of the sources of soil pollution.
Answer:
Some of the sources of soil pollution are:

Industrial wastes.
Urban wastes
Agriculture pollutants include synthetic fertilizers, pesticides, insecticides like DDT, BHC, herbicides like NaClO3, Na3AsO3, fungicides soil conditioners, farm wastes.
Radioactive pollutants.

(b) Mention some ways to control environmental pollution.
Answer:
Environmental pollutants like household waste and industrial waste can be controlled in the following manner.

Recycling: Used glass bottles, iron scrap, plastic waste, polythene bags, used newspapers and magazines can all be properly recycled.
Burning and incineration.
Sewage treatment
Digesting.
Dumping
Green chemistry can be used to reduce pollution.

Hydrocarbons Important Extra Questions Very Short Answer Type

Question 1.
Give different isomers of C4H10 with their I.U.P.A.C. names.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 1

Question 2.
Give the I.U.P.A.C. name of the lowest molecular weight alkane that contains a quaternary carbon.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 2
It is and its I.U.P.A.C. name is 2, 2-Dimethylpropane.

Question 3.
Which of the following has the highest boiling point?
(i) 2-methylpentane
(ii) 2, 3 – dimethylbutane
(iii) 2, 2-dimethylbutane.
Answer:
(i) 2—methyl pentane has the largest surface area and hence has the highest boiling point.

Question 4.
Give the structure of the alkene (C4H8) which adds on HBr in the presence and in the absence of peroxide to give the same product C4H9Br.
Answer:
2-Butene with structure CH3 – CH = CH — CH3 being symmetrical gives the same product, i.e., 2-bromobutane CH3 CH (Br) CH2CH3.

Question 5.
How will you separate propene from propyne?
Answer:
Bypassing the mixture through ammoniacal AgNO3 solution when propyne reacts while propene passes over.

Question 6.
Name two reagents that can be used to distinguish \ between ethene and ethyne.
Answer:
Tollen’s reagent | Ammoniacal AgNO3 | and amm. CuCl solution.

Question 7.
How will you detect the presence of unsaturation in an organic compound?
Answer:
Either by Baeyer’s reagent
Hydrocarbons Class 11 Important Extra Questions Chemistry 3
or by Br, in CC14.

Question 8.
Arrange the following In order of increasing volatility: gasoline, kerosene, and diesel.
Answer:
Diesel, kerosene, gasoline.

Question 9.
Arrange the following: HCl, HBr, HI, HF in order of decreasing reactivity towards alkenes.
Answer:
HF, HCl, HBr, HI.

Question 10.
Out of ethylene and acetylene which is more acidic and why?
Answer:
Acetylene. Ethylene and acetylene have sp2, sp hybridized C atoms respectively. Due to the 50% S character of the C – H bond of acetylene rather than the 33% S-Character of the C – H bond in ethene, acetylene is more acidic.

Question 11.
Write the structure of the alkene which on reductive ozonolysis gives butanone and ethanal.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 4

Question 12.
Write the I.U.P.A.C. names of
Hydrocarbons Class 11 Important Extra Questions Chemistry 5
Answer:
3-methylpent-l— en—4-yne.

(ii) CH2 = CH – CH (CH3) – CH = CH – CH – CH2,
Answer:
3-methylhept-1, 4, 6—triene.

Question 13.
Draw the structures of the following:
(i) Dicyclopropyl methane
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 6

(ii) 2-methyl-3—isopropyl heptane.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 7

Question 14.
What effect the branching of an alkane has on its boiling point?
Answer:
Branching decreases the boiling point.

Question 15. Which of the following polymerizes most readily?
(i) Acetylene
(ii) Ethene
(iii) Buta —1, 3—diene
Answer:
(iii) Buta—1, 3—diene polymerizes most readily, being more reactive.

Question 16.
Arrange the following in increasing order of their release of energy on combustion
Hydrocarbons Class 11 Important Extra Questions Chemistry 8
Answer:
The more the number of C atoms having maximum hydrogen hydrogens, i.e., CH3 groups, the greater is the heat of combustion. Thus the increasing order of heat of combustion is (iii) < (iv) < (i) < (ii).

Question 17.
Arrangement of the following set of compounds in order of their decreasing relative reactivity with an electrophile E+.
(i) Chloro benzene, 2, 4 —dinitrochlorobenzene p—nitrochlorobenzene.
(ii) toluene, p – CH3 – C6H4 – CH3, p – CH3 – C6H4 – NO2, p – O2N – C6H4 – NO2
Answer:
Electron-donating groups increase the reactivity towards an electrophile E+, while electron-withdrawing groups decrease the reactivity. Thus
(i) Chlorobenzene > p — nitrochlorobenzene > 2. 4—di nitrochlorobenzene.
(ii) p – CH3 – C6H4 – CH3 > toluene > p – CH3 – C6H4 – NO2 > p – O2N – C6H4 – NO2.

Question 18.
What is the order of reactivity of halogen and alkyl groups in the dehydrohalogenation of alkyl halides to give alkenes?
Answer:

Halogens: Iodine > Bromine > Chlorine
Alkyl group: Tert > secondary > primary.

Question 19.
What products are formed when zinc reacts with
(i) vicinal C2H4Br2 and
Answer:
CH2Br – CH2Br + Zn → CH2 = CH2 + ZnBr2.

(ii) CH3CHBr – CH2Br.
Answer:
CH3 – CHBr – CH2Br + Zn → CH3 — CH = CH2 + ZnBr2.

Question 20.
What does L.P.G. stand for?
Answer:
L.P.G. stands for liquefied petroleum gas.

Question 21.
What do the terms (i) CNG and LPG stand for?
Answer:

CNG: Compressed natural gas
LPG: Liquefied Petroleum gas.

Question 22.
What are sources to obtain:
(i) LPG
Answer:
LPG is obtained by the fractional distillation of petroleum.

(ii) CNG?
Answer:
CNG is obtained by the fractional distillation of coal tar.

Question 23.
Write down the structures and names of all isomers with a molecular formula of C5H12.
Answer:

(i) CH3 — CH2 — CH2 — CH2 — CH3 is n-Pentane or Pentane.
is iso-pentane or 2-Methylbutane.
is neo-pentane or 2, 2-Dimethylpropane.

Question 24.
The sodium salt of which acid will give ethane on heating with soda-lime? Give reaction.
Answer:
Propionic acid.

Question 25.
What products are obtained by the acidic dehydration of
(i) Ethanol
Answer:
Ethene

(ii) Propan – 2—ol.
Answer:
Propene.

Question 26.
Out of cis-2-butene and trans-2-butene which is polar and which one is non-polar?
Answer:
Cis – 2 – butene is polar (μ = 0.33 D) and trans – 2 – butene is non-polar (μ = 0). .

Question 27.
Arrange the following in the decreasing order of acidic character.
(i) C2H4, C2H6, C2H2
Answer:
H – C ≡ C H > H2C = CH2 > H3C – CH3

(ii) CH3 – C = CH, C2H2, CH3 – C = C – CH3
Answer:
HC = CH > CH3 — C ≡ CH > > CH3 – C ≡ C – CH3.

Question 28.
Name two industrial sources of hydrocarbons.
Answer:

Petroleum
Coal.

Question 29.
Arrange the following in the increasing order of C – C bond length C2H6, C2H4, C2H22.
Answer:
C2H2 < C2H4 < C2H6.

Question 30.
What type of hydrocarbons is present in high octane gasoline?
Answer:
Branched-chain aliphatic and/or aromatic hydrocarbons.

Question 31.
What are the chief constituents of light oil fraction?
Answer:
Benzene, toluene, and xylenes.

Question 32.
Which of the following shows geometrical isomerism?
(i) CHCl = CHCl
(ii) CH2 = CCl2
(iii) CCl2 = l. Give the structures of cis-and transforms.
Answer:
(i) HC (Cl) = CH (Cl);
Hydrocarbons Class 11 Important Extra Questions Chemistry 12

Question 33.
Name the product formed when methyl bromide is treated with sodium and ether.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 13

Question 34.
Which of the following shows geometrical isomerism?
But-1—ene or but—2—ene.
Answer:
But-2—ene CH3 – CH = CH – CH3.

Question 35.
Write the structure and I.U.P.A.C. name of Acetonitrile.
Answer:
Acetonitrile is CH33 CN. Its I.U.P.A.C. name is Ethane nitrile.

Question 36.
Why does carbon has a larger tendency of catenation than silicon although they have the same number of valance electrons?
Answer:
It is due to the smaller length of the C – C bond which is stronger (335 kJ mol-1) than the Si-Si bond (225.7 kJ mol-1).

Question 37.
Give name atm structure to the first organic compound synthesized in the laboratory.
Answer:
Urea
Hydrocarbons Class 11 Important Extra Questions Chemistry 14

Question 38.
Benzene is highly unsaturated, yet it does not give usual addition reactions readily. Why?
Answer:
Benzene is highly unsaturated, yet, resonance imparts extra-stability to benzene and it does not give additional reactions.
Hydrocarbons Class 11 Important Extra Questions Chemistry 15

Question 39.
What is Lindlar’s catalyst? What is it used for?
Answer:
Pd/BaSO4 poisoned with quinoline. It is used for the partial reduction of alkynes to cis-alkenes.

Question 40.
How will you detect the presence of unsaturation in an organic compound?
Answer:
Either by Baeyer’s reagent or by Br2 in CCl4. The color is discharged.

Question 41.
How can ethylene be converted to ethane?
Answer:

Question 42.
Arrange the following in increasing order of C — C bond length C2H6, C2H4, C2H2.
Answer:C2H2 < C2H44 < C2H6.

Question 43.
What type of hybridization is involved in
(i) planar and
Answer:
sp2

(ii) linear molecules?
Answer:
sp.

Question 44.
Name the chain isomer of C55H12 which has a tertiary hydrogen atom.
Answer:
2-Methyl butane (CH3), CHCH2CH3.

Question 45.
What type of isomerism is shown by butane and isobutane.
Answer:
Chain or nuclear isomerism,

Question 46.
What do you mean by cracking?
Answer:
The thermal decomposition of higher hydrocarbons into lower hydrocarbons in the presence or absence of a catalyst is called cracking.

Question 47.
Complete the reaction HC ≡ CH

Answer:

Question 48.
Out of Octane and is heptane, which has a lower octane number?
Answer:
n-Octane.

Question 49.
What are the main components of LPG?
Answer:
Butane and isobutane.

Question 50.
Write a chemical reaction to illustrate the Saytzeffs rule.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 19

Hydrocarbons Important Extra Questions Short Answer Type

Question 1.
The following organic compounds are known by their common names
(i) Neopentane
Answer:
Neopentane is
Hydrocarbons Class 11 Important Extra Questions Chemistry 20
& its h.U.P.A.C. name is 2, 2—dimethyl propane.

(ii) Acetone
Answer:
Acetone is
Hydrocarbons Class 11 Important Extra Questions Chemistry 87
and its I.U.P.A.C. name is Propanone.

(iii) Vinyl chloride
Answer:
Vinyl chloride is CPU = CH — Cl and its I.U.P.A.C. name are chloroethene.

(iv) Tert butyl alcohol. Write their structural formulae and I.U.P.A.C. names.
Answer:
Tert; butyl alcohol
Hydrocarbons Class 11 Important Extra Questions Chemistry 21
and its I.U.P.A.C. name is 2-methyl propan-2-ol.

Question 2.
What are the various products expected when propane reacts with fuming nitric acid?
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 22

Question 3.
How will you convert methane into
(i) Methanol
Answer:
Conversion of methane into methanol:
Hydrocarbons Class 11 Important Extra Questions Chemistry 23

(ii) Methanal.
Answer:

Question 4.
What is aromatization? How will you convert ^hexane into benzene?
Answer:
Aromatization. It is the process that involves cyclization, isomerization, and dehydrogenation with the application of heat and catalyst to convert alkanes containing six or more carbon atoms into aromatic hydrocarbons.
Hydrocarbons Class 11 Important Extra Questions Chemistry 25

Question 5.
Give the different conformations of ethane with their
(i) Sawhorse representation and
(ii) Newmann Projection formulae.
Answer:
Sawhorse representation Newmann projection models
Hydrocarbons Class 11 Important Extra Questions Chemistry 26

Question 6.
What are the relative stabilities of different conforma¬tions of ethane? Is it possible to isolate these at room temperature?
Answer:
The staggering form of ethane is more stable than the eclipsed form because the force of repulsion between hydrogen atoms on adjacent C atoms is minimum. The energy difference between the staggered form and eclipsed form of ethane is just 12.55 kJ mol-1. Therefore, it is not possible to separate these two forms of ethane at room temperature.

Question 7.
What is Saytzeff Rule? What are the expected products when 2-Bromobutane is dehydrohalogenation with ale. KOH?
Answer:
Saytzeff Rule. Whatever two alkenes are theoretically possible during a dehydrohalogenation reaction, it is always the more highly substituted alkene that predominates.
Hydrocarbons Class 11 Important Extra Questions Chemistry 28

Question 8.
What is the order of reactivity of H2C = CH2, (CH3)2, H2C = CH2, CH3 – CH = CH2, CH3 – CH = CH – CH3, (CH3)2 C = C (CH3)2, (CH3)2 C = CH CH3 towards electrophilic addition reactions?
Answer:
The order of reactivity of the above alkenes towards electrophilic addition reactions decreases in the order.
(CH3)2 C = C (CH3)2 > (CH33)2 C = CH CH3 > (CH3)2 C = CH2 > CH3 CH – CH – CH3 > CH3 – CH = CH, > CH2 = CH2.

Question 9.
Define Markownikov rule. Explain it with an example.
Answer:
Markownikov rule states. The negative part of the addendum adding to an unsymmetric alkene goes to that C atom of the double bond which is attached to a lesser number of C atoms.
Hydrocarbons Class 11 Important Extra Questions Chemistry 29

Question 10.
What is the Peroxide effect/Kharasch Effect? Illustrate with an example.
Answer:
In the presence of peroxides such as benzoyl peroxide, the addition of HBr (but not of HCl or HI) to an unsymmetrical alkene takes place contrary to the Markownikov rule. This is known as the peroxide/Kharsch effect.
Hydrocarbons Class 11 Important Extra Questions Chemistry 30

Question 11.
An alkene with the molecular formula C7H14 gives propanone and butanal on ozonolysis. Write down its structural formula and its I.U.P.A.C. name.
Answer:
The structures of the compounds on ozonolysis of C7H14 and

Remove the oxygen atoms and connect them by a double bond, the structure of the alkene is
Hydrocarbons Class 11 Important Extra Questions Chemistry 32

Question 12.
How will you prepare propyne and I-Butyne from acetylene;?
Answer:
(i) Preparation of propyne from acetylene
Hydrocarbons Class 11 Important Extra Questions Chemistry 33

(ii) Preparation of 1-butyne from acetylene
Hydrocarbons Class 11 Important Extra Questions Chemistry 34

Question 13.
Describe a method to distinguish between ethane, ethene, ethyne.
Answer:
(i) Ethene (C2H4) and ethyne (C2H2) decolorize bromine in carbon tetrachloride whereas ethane (C2H6) does not.
Hydrocarbons Class 11 Important Extra Questions Chemistry 35

(ii) Ethyne (and not ethane, ethene) reacts with ammoniacal AgNO3 (Tollen’s reagent) to form white precipitates.
Hydrocarbons Class 11 Important Extra Questions Chemistry 36

Question 14.
Give the mechanism of an electrophilic addition of chlorine into propene.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 37
Cl4 (chloronium ion) formed by the heterolytic fission of Cl2 in step (i) being an electrophile attacks propene in (ii) step, propene undergoes electrometric effect combined with + I effect to form carbocation which, is a slow step. In (iii) step Cl ion being a. nucleophile attacks carbocation and forms the product. It is a fast step.

Question 15.
Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds: [I.I.T. 2002]
Hydrocarbons Class 11 Important Extra Questions Chemistry 38
Answer:
— NO2 group in structure IV is an electron attracting group, it deactivates the benzene ring largely towards electrophilic substitution reactions. Cl group in III is also a deactivating group, but its deactivation is lower too — NO2, where—CH3, a group in II is an electron-releasing group and so activates the benzene ring towards electrophilic substitution. Therefore, the order of reactivity is
Hydrocarbons Class 11 Important Extra Questions Chemistry 39

Question 16.
What is meant by (i) delocalization
Answer:
Delocalisation: Delocalisation means that pairs of 7t electrons extend over 3 or more atoms. They belong to the whole molecule. For example, 6n electrons present in benzene are delocalized and are spread on the whole of the ring and this imparts extra stability to the molecule.

(ii) resonance energy.
Answer:
Resonance energy: The difference between the energy of the most stable contributing/canonical structural and the energy of the resonance hybrid is known as resonance energy. In the case of benzene, the resonance hybrid has 147 kJ mol-1 than either A or B below. Thus resonance energy of benzene is 147 kJ mol-1.
Hydrocarbons Class 11 Important Extra Questions Chemistry 40

Question 17.
Describe Friedel’s craft reaction with suitable examples.
Answer:
When an alkyl or acid halide is treated with benzene or its derivative in the presence of anhydrous AlCl3 as a catalyst, we got alkyl or acyl benzene.
Hydrocarbons Class 11 Important Extra Questions Chemistry 41

Question 18.
Classify the following hydrocarbons into alkanes, alkenes, alkynes, and arenes.
(i) (CH3)4C
Answer:
Alkane

(ii) C2H2
Answer:
Alkyne

(iii) C3H6
Answer:
Alkene

Hydrocarbons Class 11 Important Extra Questions Chemistry 42
Answer:
Arene.

Question 19.
Write all possible structures of C5H8 and give their I.U.P.A.C. names.
Answer:
Structural isomers of C5H8 (Pentyne) are
(i) CH3 – CH2 – CH2 – C ≡ CH (Pent-l-yne)
(ii) CH3 – CH2 – C ≡ C – CH3 (Pent-2-yne)
(iii) Hydrocarbons Class 11 Important Extra Questions Chemistry 43
(3-Methylbut-l-yne)

Question 20.
How does ethylene undergo polymerization? What is the use to which the polymer obtained is put?
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 44
Low-density polythene [LDPE] and high-density polyethylene [HDPE] as used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio, and T.V. cabinets, etc.

Question 21.
What is the action of water on
(i) Ethyne
(ii) Propyne? Name the end products obtained.
Answer:
(i) Action of water on ethyne: When ethyne is warmed with dilute H2SO4 at 333K and H8SO4 as catalyst ethanol is obtained.
Hydrocarbons Class 11 Important Extra Questions Chemistry 45
(ii) Action of water on Propyne CH3 — C ≡ CH + H2O

Question 22.
Propene reacts with HBr to give Isopropyl bromide but does not give n-propyl bromide. Why?
Answer:
The addition of unsymmetrical addendum (HBr) to unsymmetrical olefines (CH3CH = CH2) takes place according to Markownikov rule, the negative part of reagent (i.e. Br-) adds on the carbon atom having a minimum number of hydrogen atoms. Hence Isopropyl bromide will be formed.
Hydrocarbons Class 11 Important Extra Questions Chemistry 47

Question 23.
An oxidizing agent is needed in the iodination of methane but not in the chlorination or bromination. Give reason.
Answer:
In the iodination of methane, H — I is also formed as the product with iodomethane, since it is a strong reducing agent, it reduces iodomethane back to methane & makes the reaction reversible. In order to destroy HI, an oxidizing agent like HIO3 (or HNO3) is needed. But HCl& HBr formed in the chlorination & bromination reactions of methane & not in a position to react with the monosubstituted products (CH3Cl & CH3Br) since they are comparatively weak reducing agents.

Therefore, no oxidizing agent is needed for these reactions.
CH4 + I2 ⇌ CH3I + HI
5HI + HIO3 → 3H2O + 3I2

Question 24.
The dipole moment of trans 1, 2-chloroethene is less than the cis isomer. Explain.
Answer:
The structure of the trans isomer is more symmetrical to the cis isomer. In the trans isomer, the dipole moments of the polar C—Cl bonds are likely to cancel with each other & the resultant dipole moment of the molecule is nearly zero. But in the cis isomer, these do not cancel. Therefore, the cis isomer has a specific moment but is zero in the case of the trans isomer.
Hydrocarbons Class 11 Important Extra Questions Chemistry 48

Question 25.
Write l.U.P.A.C. names of the products obtained by addition reactions of HBr to hex-l-ene.
(a) In the absence of Peroxide
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 49
(b) In the presence of Peroxide.
Answer:

Question 26.
How will you distinguish between the following:
(a) Butyne-l & Butyne-2 ,
Answer:
Butyne-1 having an acetylene hydrogen atom will give white ppt. With ammoniacal silver nitrate & red ppt. with ammonical cuprous chloride. On the other hand, butyne-2-having no acetylenic hydrogen atom does not respond to either of the two reagents.

(b) Butene-1 & Butene-2
Answer:
Butene-1 & butene-2 can be distinguished either by ozonolysis or by oxidation with acidic KMnO2 solution with which they give different carbonyl compounds.
Hydrocarbons Class 11 Important Extra Questions Chemistry 51

Question 27.
Alkynes are less reactive than alkenes towards addition reaction even though they contain 2-7t bond. Give reason.
Answer:
This is due to

greater electronegativity of sp-hybridized carbon of alkynes than sp2 hybridized carbon atoms of alkenes which holds the π-electrons of alkynes more tightly and
greater delocalization of π-electrons in alkynes (because of the cylindrical nature of their n electron cloud) than in alkenes. As a result, n electrons of alkynes are less easily available for addition reactions than those of alkenes.

Consequently, alkynes are less reactive than alkenes towards addition reactions.

Question 28.
Why do addition reactions occur more readily with alkenes & alkynes than with aromatic hydrocarbons?
Answer:
The energy gained by forming two sigma bonds (of four sigma bonds) more than compensates for the loss of one or two n bonds when addition occurs to an alkene or alkyne. However, in aromatic hydrocarbons, the aromatic ring is specially stabilized by the delocalization of n electrons about the ring.

It, therefore, requires substantial activation energy to cause the loss of its aromatic character. The most usual reaction in arenes is thus substitution rather than addition, since substitution does not result in loss of aromatic character.

Question 29.
A Hydrocarbon A, adds one mole of hydrogen in presence of platinum catalyst from n-Hexane. When A is oxidized vigorously with KMnO4, a single carboxylic acid, containing three carbon atoms is isolated. Give the structure of A & explain.
Answer:

Since hydrocarbon A adds one molecule of H2 in presence of platinum to form n-hexane. A must be a hexene.
Since A on vigorous oxidation with KMnO4 gives a single carboxylic acid containing three carbon atoms, therefore, A must be asymmetrical hexene i.e. hex-3-ene.

Thus, the given hydrocarbon A is hex-3-ene.

Question 30.
How would you carry out the following conversion?
Answer:
Propene to Ethyne
Hydrocarbons Class 11 Important Extra Questions Chemistry 53

Hydrocarbons Important Extra Questions Long Answer Type

Question 1.
How would you convert the following compounds to benzene?
(i) Acetylene
Answer:
Acetylene into benzene. Ethyne (Acetylene) in passing through a red hot iron tube at 873 K undergoes cyclic polymerization as shown below.
Hydrocarbons Class 11 Important Extra Questions Chemistry 54

(ii) Benzoic acid
Answer:
Benzoic acid into benzene
Hydrocarbons Class 11 Important Extra Questions Chemistry 55

(iii) Hexane
Answer:
Hexane into benzene
Hydrocarbons Class 11 Important Extra Questions Chemistry 56

(iv) Benzene diazonium chloride
Answer:
Benzene diazonium chloride into benzene
Hydrocarbons Class 11 Important Extra Questions Chemistry 57

Question 2.
How will you convert benzene into
(i) Nitrobenzene
Answer:
Benzene into Nitrobenzene
Hydrocarbons Class 11 Important Extra Questions Chemistry 58

(ii) Benzene sulphonic acid
Answer:
Benzene into benzene sulphonic acid
Hydrocarbons Class 11 Important Extra Questions Chemistry 59

(iii) Toluene
Answer:
Benzene into Toluene
Hydrocarbons Class 11 Important Extra Questions Chemistry 60

(iv) Acetophenone?
Answer:
Benzene into Acetophenone
Hydrocarbons Class 11 Important Extra Questions Chemistry 61

Question 3.
What is the mechanism of nitration of benzene?
Answer:
Nitration of benzene. It is carried out by treating benzene with a mixture of cones. HNO3+ Cone. H2SO4. The various steps involved are:
Step I: Generation of an electrophile, i.e., NOt (nitronium ion)
Hydrocarbons Class 11 Important Extra Questions Chemistry 62

Step II: Formation of complex or carbocation intermediate
Hydrocarbons Class 11 Important Extra Questions Chemistry 63
This step is slow and hence is the rate-determining step of the reaction.

Step III: Loss of a proton from the carbocation intermediate
Hydrocarbons Class 11 Important Extra Questions Chemistry 64
This step is fast and does not affect the rate of the reaction.

Question 4.
(a) What is the general formula of Alkynes?
Answer:
The general formula of alkynes ¡s CnH2n-2.

(b) Give the I.U.P.A.C. names and structure of all alkynes having the molecular formula C2H8.
Answer:
C5H8 has the following isomers.
(i) CH3 — CH2 — CH2 — C ≡ CH Pent-1-yne
(ii) CH3 CH, C ≡ C — CH3 Pent-2-yen
(iii) Hydrocarbons Class 11 Important Extra Questions Chemistry 65
3-Methyl but-1-yen

(c) Give any two methods for preparing acetylene.
Answer:
Acetylene can be prepared by the following two methods
Hydrocarbons Class 11 Important Extra Questions Chemistry 66
(ii) By dehydrohalogenation of dihaloalkanes

(d) Discuss any three chemical properties of acetylene.
Answer:
(i) Addition of Hydrogen:

(ii) On oxidation with alkaline KMnO4, it gives oxalic acid.
Hydrocarbons Class 11 Important Extra Questions Chemistry 69

(iii) When acetylene is passed through a red hot iron tube, it trickeries to give benzene
Hydrocarbons Class 11 Important Extra Questions Chemistry 70

Question 5.
(a) Write structures of different chain isomers of alkanes corresponding to the molecular formula C6H14. Also, write their I.U.P.A.C. names.
(b) Write structures of different isomeric alkyl groups corresponding to the molecular formula C5H11. Write IUPAC names of alcohols obtained by attachment of —OH groups at different carbons of the chain.
Answer:
Hydrocarbons Class 11 Important Extra Questions Chemistry 71
(b) Structures of – C5H11 group Corresponding alcohols

Question 6.
How will you prepare Alkanes by
(i) Wurtz reaction
Answer:
Methods of preparation of Alkanes
Wurtz reaction. Alkyl halides on treatment with sodium in dry ether give higher alkanes, preferably containing an even number of carbon atoms.
Hydrocarbons Class 11 Important Extra Questions Chemistry 74
(ii) Decarboxylation of sodium salts of fatty acids
Answer:
Sodium salts of fatty acids on heating with-soda lime (a mixture of NaOH and CaO) give alkanes containing one carbon atom less than the carboxylic acid. The process of elimination of carbon dioxide from a carboxylic acid is known as Decarboxylation
Hydrocarbons Class 11 Important Extra Questions Chemistry 75

(iii) Kolbe’s electrolytic method.
Answer:
Kolbe’s electrolytic method. An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkanes containing an even number of carbon atoms at the anode.

The probable mechanism for the reaction is
Hydrocarbons Class 11 Important Extra Questions Chemistry 77

(b) How do alkanes udergo:
(i) substitution reactions with halogens
Answer:
Substitution reactions with Halogens. The order of reactivity of halogens is F2 > Cl2 > Br2 > I2
Hydrocarbons Class 11 Important Extra Questions Chemistry 78
Bromination is similar. With fluorine, the reaction is too violent to be controlled. Iodination is very slow and a reversible reaction.
It can proceed in the presence of oxidizing agents like HNO3, HIO3.
CH4 + I2 ⇌ CH3I + HI
HIO3 + 5HI → I3 + 3H2O

Halogenation of alkanes proceeds via a free-radical mechanism which consists of three steps.

Chain initiation step
Chain propagation step
Chain termination step

1. Chain initiating step. Cl2 undergoes hemolysis in the presence of heat and light.
Hydrocarbons Class 11 Important Extra Questions Chemistry 79
2. Chain propagation step

3. Chain termination step

(ii) oxidation by combustion
Answer:
Oxidation of alkanes by combustion: Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of a large amounts of heat.
Hydrocarbons Class 11 Important Extra Questions Chemistry 82
In the presence of an insufficient amount of air or dioxygen, carbon black is formed.

(iii) Isomerisation
Answer:
Isomerization reactions of alkanes, n-Alkanes on heating in the presence of anhydrous aluminum chloride and hydrogen chloride gas isomerize to branched-chain alkanes.
Hydrocarbons Class 11 Important Extra Questions Chemistry 84

(iv) Aromatisation.
Answer:
Aromatisation. n-Alkanes having six or more C atoms on heating to 773 K at 10-20 atmospheric pressure in. the presence of oxides of vanadium, molybdenum get dehydrogenated and cyclized. This reaction is called aromatization or reforming.
Hydrocarbons Class 11 Important Extra Questions Chemistry 85