CBSE Class 11 Chemistry VBQs Hydrocarbons

Question. Which of the following has dipole moment?
a. cis-2-butene
b. trans-2-butene
c. cis-1,2-dibromoethene
d. trans-1,2-dibromoethene
Answer : A, C

Question. Which of the following reactions can be used to prepare methane?
a. Clemmensen reduction
b. Wurtz reaction
c. Catalytic hydrogenation of methyl iodide
d. Reduction of methyl iodide by using a zinc-copper couple
Answer : C, D

Question. Sodium acetate can be converted to ethane by:
a. Heating with LiAlH₄
b. Electrolysing its aqueous solution
c. Heating with sodalime
d. Heating with calcium acetate
Answer : B

Question. Alkenes usually show which type of reaction?
a. Addition
b. Substitution
c. Elimination
d. Superposition
Answer : A

Question. Which of the following gases is used for welding?
a. Methane
b. Ethane
c. Acetylene
d. Ethene
Answer : C

Question. A mixture of ethyl iodide and methyl iodide is subjected to the Wurtz reaction. The products formed are:
a. ethane
b. butane
c. propane
d. 2-methylpropane
Answer : A, B, C

Question. Amongst the following the most basic compound is:
a. Benzylamine
b. Aniline
c. Acetanilide
d. p-nitroaniline
Answer : A

Question. Among the following compound which one is planar in shape?
a. Methane
b. Acetylene
c. Benzene
d. Isobutane
Answer : C

Question. The cis hydroxylation of alkenes can be effected by:
a. the addition of a 1% KMnO₄ solution
b. reaction with osmium tetraoxide followed by treatment with water
c. the addition of ozone to the alkene, followed by treatment of the ozonide with H₂O₂
d. none of these
Answer : A, B

Question. Benzene molecule is:
a. Tetrahedral
b. Planar
c. Pyramidal
d. Trigonal
Answer : B

Question. Which of the following has highest knocking property?
a. Aromatic hydrocarbons
b. Olefins
c. Branched chain paraffins
d. Straight chain paraffins
Answer : D

Question. Oils are converted into fats by:
a. Hydration
b. Decarboxylation
c. Hydrogentation
d. Dehydrogenation
Answer : C

Question. Which represents an alkane?
a. C₅H₈
b. C₈H₆
c. C₉H₁₀
d. C₇H₁₆
Answer : D

Question. Which of the C–C bond is strongest?
a. Formed by sp³–sp³ hybridised carbon atoms (as in alkanes)
b. Formed by sp²–sp² hybridised carbon atoms (as in alkenes)
c. Formed by sp–sp hybridised carbon atoms (as in alkynes)
d. All are equal
Answer : C

Question. Cyclic hydrocarbon molecule A has all the carbon and hydrogens in a single plane. All the carbon-carbon bonds are of same length and less that 1.54 Å but more than 1.34 Å. C−C−C bond angles will be:
a. 120°
b. 180°
c. 100°
d. 109° 28′
Answer : A

Question. Hydrocarbon C₆H₆ decolourise Br₂ water and gives ppt. with ammonical AgNO₃ Hydrocarbon can be
a. 1, 3, 5 Cyclohexatriene
b. 1, 5 Hexadiyne
c. 2, 4 Hexadiyne
d. None
Answer : D

Assertion and Reason

Note: Read the Assertion (A) and Reason (R) carefully to mark the correct option out of the options given below:
a. If both assertion and reason are true and the reason is the correct explanation of the assertion.
b. If both assertion and reason are true but reason is not the correct explanation of the assertion.
c. If assertion is true but reason is false.
d. If the assertion and reason both are false.
e. If assertion is false but reason is true.

Question. Assertion: 1-Butene on reaction with HBr in the presence of peroxide produces 1-bromo-butane.
Reason: It involves the free radical mechanism.
Answer : A

Question. Assertion: Addition of Br₂ to 1-butene gives two optical isomers.
Reason: The product contains one asymmetric carbon.
Answer : A

Question. Assertion: Styrene on reaction with HBr gives 2-bromo- 2-phenyl-ethane.
Reason: Benzyl radical is more stable than alkyl radical.
Answer : B

Question. Assertion: Iodination of alkanes is reversible.
Reason: Iodination is carried out in presence of iodic acid.
Answer : B

Question. Assertion: 2-Butanol on heating with H₂SO₄ gives 1-butene and 2-butene.
Reason: Dehydration of 2-butanol follows saytzeff rule.
Answer : A

Question. Assertion: Neopentane forms only one monosubstituted compound.
Reason: Neopentane has high bond energy.
Answer : C

Question. Assertion: Pyrrole is an aromatic heterocyclic compound.
Reason: It has a cyclic, delocalised 6π electrons.
Answer : A

Question. Assertion: Isobutane on oxidation with KMnO₄ gives tert-butyl alcohol.
Reason: Oxidising agents have no effect on alkanes.
Answer : B

Question. Assertion: CH4 does not react with Cl₂ in dark.
Reason: Chlorination of CH₄ takes place in sunlight.
Answer : B

Question. Assertion: Alkyl benzene is not prepared by Friedel- Crafts alkylation of benzene.
Reason: Alkyl halides are less reactive than acyl halides.
Answer : B

Question. Assertion: All the hydrogen atoms in CH₂ == C == CH₂ lie in one plane.
Reason: All the carbon atoms in it are sp2 hybridized.
Answer : C

Question. Assertion: Nitration of toluene is easier than benzene.
Reason: The methyl group in toluene is electron-releasing
Answer : A

Comprehension Based

Paragraph –I

The term arene is used to benzene and its homologues. The term aromatic refers for the organic compounds which obey Huckel rule, i.e., all aromatic compounds must possess π electrons equal to (4n+2) where n is any positive integer. Due to aromatic nature benzene and it homologues show electrophilic substitution reactions producing o-and p-substituted derivatives or m-substituted derivatives. The orientation (new incoming group’s substitution) in benzene nucleus is explained in terms of directive influence of the groups.

Question. Side chain of a nucleus can be oxidized to—COOH group only when it has atleast one:
a. vinylic H
b. benzylic H
c. allylic H
d. olefinic H
Answer : B

Question. The general formula for arenes having monocylic nature where y represents number of 6-membered rings is?
a. C_nH_{2n −6y}
b. C_nH_{2n −8y}
c. C_nH_2n−4y
d. C_nH_{2n −5y}
Answer : A

Question. Which of the following are m-directing groups?
(I) —NH₃⁺ (II) —NH₂ in acid
(III) —CCl₃ (IV) —COO−
(V) —NC (VI) —CN
a. I, II, III, VI
b. IV, V, VI
c. III, IV, V
d. III, IV, VI
Answer : A

Question. Which of the following statements regarding Sg reaction in C₅H₅ is wrong?

a. AlCl₃ acts as Lewis acid
b. AlCl₃ acts to produce free radical
c. AlCl₃ acts to produce electrophile
d. AlCl₃ acts as halogen carrier
Answer : B

Match the Column

Question. Match the statement of Column I with those in Column II:

a.A→1; B→2; C→3; D→4; E→5
b. A→2, B→3, C→1, D→5; E→4
c. A→3, B→2, C→1, D→4; E→5
d. A→2, B→1, C→4, D→3; E→5
Answer : B

Integer

Question. The total number of cyclic isomers possible for a hydrocarbon with the molecular formula C₆H₄ is :
Answer : 5

Question. The maximum number of isomers (including stereoisomerism) that are possible on mono-chlorination of the following compound is CH₃CH₂CH(CH₃)CH₂CH₂ .
Answer : 6

Question. Number of acidic hydrogen atoms in butyne-1 are:
Answer : 3

Question. The A sample of gasoline contains 81% iso-octane and 19% n-heptane. Its octane number will be:
Answer : 81

Question. A fuel contains 25 % n-heptane and 75 % iso-octane. Its octane number is:
Answer : 75

1. Saturated aliphatic hydrocarbons are called alkanes. They are mainly used as fuels. Depending upon their physical properties such as boiling point, density, solubility, they have many other uses. Now answer the following questions.

(a) Out of propane and butane why is propane used as fuel for barbecue grill? 
Answer: Barbecue is usually done in open. In cold countries, although both propane and butane can be used but propane is preferred because it works better at low temperatures. The reason being that due to lower boiling point (231K), propane always remains as gas in the cylinder in spite of cooling produced by Joule Thomson effect during its use and thus ensures a constant supply of gas. Incontrast ,butane due to its higher boiling point(272.5K)and cooling produced by Joule-Thomson effect, it becomes a liquid in the cylinder and thus the supply of gas becomes slower and slower as the gas is being used.

(b) Which alkane is used in cigarette lighters?
Answer:  Butane is used in cigarette lighters since it can be easily liquefied at room temperature.

(c) What is mineral oil? Give its uses. 
Answer:  Mineral oil is a mixture of liquid alkanes containing 16-18 carbon atoms and is used as a laxative and as a lubricant.

(d) What are the uses of solid alkanes? 
Answer:  Alkanes with 18 or more carbon atoms are waxy solids. These are used as waxy coatings on fruits and vegetables to help them to retain moisture, inhibit mould growth and enhance appearance.

(e) What is Vaseline? What are its uses?
Answer:  Vaseline is a mixture of solid alkanes having low melting points. It can prevent moisture loss and chapped lips and soften nail cuticles. It is also used in ointments and cosmetics.

2. Benzene is the simplest and ideal aromatic compound. It is also one of the most basic petrochemicals. It is mainly used to prepare a number of important chemicals such as toluene, phenol, aniline, biphenyletc which are used in the manufacture of dyes, detergents, drugs, explosives, pesticides, rubbersandlubricants.But it is carcinogenic. Now answer the following questions.

(a) Mention 3 uses of benzene in petrochemical industry. (b)Why benzene is used as additive in gasoline?
Answer:  Benzene is mainly used as an intermediate to make other chemicals. About 85 of benzene is consumed in the production of 3 chemicals, ethyl benzene, cumene and cyclohexane. Ethyl benzene is a precursor to styrene, which is used to make polymers and plastics. Cumene is converted to phenol and acetone. Cyclohexane is used to manufacture nylon.

(b)Why benzene is used as additive in gasoline?
Answer: Because of its high octane number and anti-knocking properties benzene is used as an additive in gasoline.

(c) Comment on the bad effects and carcinogenicity of benzene.
Answer:  Exposure of benzene vapours to humans is a global problem. It affects liver, kidney, lungs and heart. Benzene increases the risk of cancer and other illness. In the body, it gets oxidized to benzene oxide which is not excreted readily and can interact with DNA to produce harmful mutations. Benzene is also a notorious cause of bone marrow failure.

(d) In view of toxicity and carcinogenicity of benzene, should its production be banned? Give your suggestions.
Answer: Although benzene is a potential carcinogen, its production cannot be banned because it’s wide spread application in the manufacture of dyes, detergents, nylons, explosives and pesticides.Instead,benzene should be used in fuming cupboards and ventilated laboratories.

3. Kamala and Reena are best friends. One evening Kamala went to Reena’s house and found her in the kitchen. She saw that the burner was emitting yellow flame and a black soot was getting deposited on the bottom of the cooking vessel .She advised Kamala to put off the gas and helped her in cleaning the holes of the burner. The gas was then switched on and they found that the burner was now emitting clean blue flame.

(i) Why was the burner emitting yellow flame?
Answer:  The holes of the burner were blocked due to soot or some oily material. This caused incomplete combustion of the fuel.

(ii) What was the purpose of cleaning the holes of the burner?
Answer:  By cleaning the holes of the burner the deposits were removed which led to the complete combustion of the fuel. 

(iii) What value is expressed by Kamala?
Answer: Kamala helped her friend in saving fuel and in preventing wastage of gas.

Please refer to the link below for CBSE Class 11 Chemistry VBQs Hydrocarbons