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1. Hydrocarbons containing only single covalent bonds between carbon atoms are called:
A. Alkanes
B. Alkenes
C. Alkynes
D. Aromatic hydrocarbons
Answer: A. Alkanes
Explanation: Alkanes are saturated hydrocarbons, meaning all carbon-carbon bonds present are single bonds.
2. Hydrocarbons containing at least one carbon-carbon double bond are called:
A. Alkenes
B. Alkanes
C. Alkynes
D. Cycloalkanes only
Answer: A. Alkenes
Explanation: Alkenes are unsaturated hydrocarbons characterised by the presence of one or more carbon-carbon double bonds.
3. Hydrocarbons containing at least one carbon-carbon triple bond are called:
A. Alkynes
B. Alkanes
C. Alkenes
D. Aromatic hydrocarbons only
Answer: A. Alkynes
Explanation: Alkynes are unsaturated hydrocarbons distinguished by the presence of one or more carbon-carbon triple bonds.
4. The general formula for alkanes is:
A. CnH2n+2
B. CnH2n
C. CnH2n-2
D. CnHn
Answer: A. CnH2n+2
Explanation: Alkanes follow the general formula CnH2n+2, reflecting their fully saturated structure with only single bonds.
5. The general formula for alkenes is:
A. CnH2n
B. CnH2n+2
C. CnH2n-2
D. CnHn
Answer: A. CnH2n
Explanation: Alkenes follow the general formula CnH2n, reflecting the presence of one degree of unsaturation (the double bond).
6. The general formula for alkynes is:
A. CnH2n-2
B. CnH2n
C. CnH2n+2
D. CnHn+2
Answer: A. CnH2n-2
Explanation: Alkynes follow the general formula CnH2n-2, reflecting the two degrees of unsaturation introduced by the triple bond.
7. Hydrocarbons that form closed rings of carbon atoms are called:
A. Cyclic hydrocarbons
B. Straight-chain hydrocarbons
C. Branched hydrocarbons only
D. Aromatic hydrocarbons exclusively
Answer: A. Cyclic hydrocarbons
Explanation: Cyclic hydrocarbons are compounds whose carbon atoms are arranged in a closed ring structure, rather than an open chain.
8. Hydrocarbons characterised by a stable ring of alternating double bonds, exemplified by benzene, are called:
A. Aromatic hydrocarbons
B. Aliphatic hydrocarbons
C. Alicyclic hydrocarbons
D. Saturated hydrocarbons
Answer: A. Aromatic hydrocarbons
Explanation: Aromatic hydrocarbons, typified by benzene, possess a special ring structure with delocalised pi electrons, giving them enhanced stability.
9. The special stability of benzene arises due to the delocalisation of its:
A. Pi electrons around the ring
B. Sigma electrons only
C. Neutrons
D. Protons
Answer: A. Pi electrons around the ring
Explanation: Benzene's exceptional stability results from the delocalisation of six pi electrons evenly distributed across the entire ring structure.
10. The hybridisation of carbon atoms in alkanes is generally:
A. sp³
B. sp²
C. sp
D. sp³d
Answer: A. sp³
Explanation: Since alkane carbons form four single sigma bonds, they adopt sp³ hybridisation, resulting in tetrahedral geometry.
11. The hybridisation of carbon atoms involved in a double bond (as in alkenes) is generally:
A. sp²
B. sp³
C. sp
D. sp³d
Answer: A. sp²
Explanation: Carbon atoms forming a double bond adopt sp² hybridisation, resulting in trigonal planar geometry around each carbon.
12. The hybridisation of carbon atoms involved in a triple bond (as in alkynes) is generally:
A. sp
B. sp²
C. sp³
D. sp³d
Answer: A. sp
Explanation: Carbon atoms forming a triple bond adopt sp hybridisation, resulting in a linear geometry around each triple-bonded carbon.
13. The chemical reaction in which hydrogen atoms of an alkane are progressively replaced by halogen atoms is called:
A. Halogenation (substitution reaction)
B. Addition reaction
C. Elimination reaction
D. Polymerisation
Answer: A. Halogenation (substitution reaction)
Explanation: Halogenation of alkanes proceeds via a free radical substitution mechanism, replacing hydrogen atoms with halogen atoms.
14. The addition of hydrogen halide, hydrogen or halogens across the double bond of an alkene is generally classified as a/an:
A. Addition reaction
B. Substitution reaction
C. Elimination reaction only
D. Rearrangement reaction only
Answer: A. Addition reaction
Explanation: Alkenes typically undergo addition reactions, in which atoms or groups add directly across the double bond, breaking the pi bond.
15. Markovnikov's rule predicts the regiochemistry of addition of an unsymmetrical reagent (like HX) to an unsymmetrical alkene, stating that the hydrogen atom adds to the carbon with:
A. More hydrogen atoms already attached
B. Fewer hydrogen atoms already attached
C. No hydrogen atoms attached
D. Equal hydrogen atoms on both sides always
Answer: A. More hydrogen atoms already attached
Explanation: Markovnikov's rule states that in addition reactions, the hydrogen atom preferentially attaches to the carbon already bearing more hydrogen atoms.
16. Anti-Markovnikov addition, which occurs in the presence of peroxides during HBr addition, is also known as the:
A. Kharasch effect (peroxide effect)
B. Markovnikov effect
C. Saytzeff rule
D. Hofmann rule
Answer: A. Kharasch effect (peroxide effect)
Explanation: The Kharasch or peroxide effect describes the reversal of normal Markovnikov addition when peroxides are present, favouring anti-Markovnikov products.
17. The complete combustion of hydrocarbons in the presence of sufficient oxygen produces:
A. Carbon dioxide and water
B. Carbon monoxide only
C. Only carbon and hydrogen gas
D. Only soot
Answer: A. Carbon dioxide and water
Explanation: Complete combustion of hydrocarbons with adequate oxygen supply yields carbon dioxide and water as the primary products, along with energy release.
18. Incomplete combustion of hydrocarbons, due to insufficient oxygen, often produces harmful:
A. Carbon monoxide
B. Carbon dioxide only
C. Pure water only
D. Oxygen gas
Answer: A. Carbon monoxide
Explanation: When oxygen supply is limited, incomplete combustion generates toxic carbon monoxide gas instead of fully oxidised carbon dioxide.
19. The reaction mechanism involving the formation of a free radical intermediate, as seen in alkane halogenation, is called a:
A. Free radical mechanism
B. Ionic mechanism only
C. Pericyclic mechanism only
D. Nucleophilic substitution mechanism
Answer: A. Free radical mechanism
Explanation: Alkane halogenation typically proceeds via a free radical chain mechanism, involving initiation, propagation and termination steps.
20. Ozonolysis is a reaction used to identify the position of a double bond by cleaving it and converting it into:
A. Carbonyl compounds (aldehydes/ketones)
B. Alcohols only
C. Carboxylic acids only
D. Alkanes
Answer: A. Carbonyl compounds (aldehydes/ketones)
Explanation: Ozonolysis cleaves the carbon-carbon double bond, converting it into carbonyl compounds, helping determine the original position of unsaturation.
21. Aromatic hydrocarbons typically undergo which type of characteristic reaction, rather than addition?
A. Electrophilic substitution
B. Nucleophilic addition only
C. Free radical addition only
D. Elimination only
Answer: A. Electrophilic substitution
Explanation: Due to the stability provided by their delocalised pi system, aromatic compounds like benzene favour electrophilic substitution over addition reactions.
22. The process of converting straight-chain alkanes into branched-chain isomers to improve fuel quality is called:
A. Isomerisation
B. Cracking only
C. Polymerisation only
D. Reforming exclusively
Answer: A. Isomerisation
Explanation: Isomerisation converts linear hydrocarbon chains into branched forms, which generally improve the octane rating and combustion properties of fuels.
23. The breakdown of larger hydrocarbon molecules into smaller ones through heat and/or catalysts is called:
A. Cracking
B. Isomerisation only
C. Polymerisation only
D. Hydrogenation only
Answer: A. Cracking
Explanation: Cracking is an industrially important process that breaks down large hydrocarbon molecules into smaller, more useful fractions like petrol.
24. Wurtz reaction is used for the preparation of higher alkanes by treating alkyl halides with:
A. Sodium metal in dry ether
B. Water
C. Concentrated sulphuric acid
D. Oxygen gas
Answer: A. Sodium metal in dry ether
Explanation: The Wurtz reaction couples two alkyl halide molecules using sodium metal in dry ether to synthesise a higher alkane.
25. Alkynes are generally more reactive than alkenes towards addition reactions due to their:
A. Higher degree of unsaturation and more accessible pi electrons
B. Lower degree of unsaturation
C. Absence of any pi bonds
D. Larger molecular size
Answer: A. Higher degree of unsaturation and more accessible pi electrons
Explanation: The greater unsaturation and electron density around the triple bond typically make alkynes more reactive than alkenes in many addition reactions.
26. The catalytic hydrogenation of alkenes converts them into:
A. Alkanes
B. Alkynes
C. Aromatic compounds
D. Carboxylic acids
Answer: A. Alkanes
Explanation: Catalytic hydrogenation adds hydrogen across the double bond of an alkene, converting it into the corresponding saturated alkane.
27. Benzene's molecular formula is:
A. C₆H₆
B. C₆H₁₂
C. C₆H₁₄
D. C₅H₅
Answer: A. C₆H₆
Explanation: Benzene has the molecular formula C₆H₆, consisting of a six-membered carbon ring with alternating double bonds and one hydrogen per carbon.
28. Cis-trans (geometrical) isomerism is commonly observed in alkenes due to restricted rotation around the:
A. Carbon-carbon double bond
B. Carbon-carbon single bond
C. Carbon-hydrogen bond
D. Carbon-carbon triple bond
Answer: A. Carbon-carbon double bond
Explanation: The restricted rotation characteristic of a double bond allows substituents to be fixed on the same or opposite sides, giving rise to cis-trans isomerism.
29. Petroleum, a major natural source of hydrocarbons, is typically separated into useful fractions through:
A. Fractional distillation
B. Filtration only
C. Crystallisation only
D. Sublimation only
Answer: A. Fractional distillation
Explanation: Fractional distillation separates crude petroleum into various hydrocarbon fractions based on differences in their boiling points.
30. The presence of an extended conjugated pi-electron system generally makes a compound:
A. More stable due to delocalisation
B. Less stable due to increased strain
C. Non-reactive entirely
D. Impossible to synthesise
Answer: A. More stable due to delocalisation
Explanation: Conjugation allows for greater delocalisation of pi electrons, generally lowering the overall energy and increasing the stability of the molecule.
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