All Free Chemistry MCQs with Answers

211. According to Markovnikov's rule, when hydrogen bromide adds to propene the major product is

• A. 1-bromopropane
• B. 2-bromopropane
• C. 1,2-dibromopropane
• D. propan-2-ol

Explanation: The hydrogen adds to the carbon that already carries more hydrogens, so the bromine ends up on the middle carbon. The underlying reason is that this route passes through the more stable secondary carbocation rather than a primary one, since alkyl groups release electron density and spread the positive charge. In the presence of peroxides the addition becomes a radical process and the anti Markovnikov product dominates instead.

212. The hybridisation of the carbon atoms in ethene and the bond angle about each are

• A. sp3 and 109.5 degrees
• B. sp and 180 degrees
• C. sp2 and 120 degrees
• D. sp2 and 109.5 degrees

Explanation: Each carbon forms three sigma bonds using sp2 hybrids arranged in a plane at 120 degrees, and the remaining unhybridised p orbitals overlap sideways to form the pi bond. That sideways overlap is what prevents rotation and makes geometric isomerism possible. The whole ethene molecule is therefore flat.

213. Ethyne differs from ethene in that ethyne contains

• A. one sigma and two pi bonds between the carbon atoms, with sp hybridisation
• B. two sigma bonds between the carbon atoms
• C. only sigma bonds
• D. three pi bonds between the carbon atoms

Explanation: A triple bond is always one sigma bond plus two pi bonds, formed from two unhybridised p orbitals on each sp hybridised carbon, and the molecule is linear as a result. Ethyne also shows weak acidity, unlike ethene, because the higher s character of the sp orbital holds the bonding electrons closer to carbon. Any multiple bond, however high the order, contains exactly one sigma bond.

214. Ethyne can be prepared in the laboratory by the action of water on

• A. calcium carbonate
• B. calcium oxide
• C. calcium hydroxide
• D. calcium carbide

Explanation: Calcium carbide reacts vigorously with water to give ethyne and calcium hydroxide, which was once used in carbide lamps and is still the classic laboratory preparation. Calcium carbonate with acid would give carbon dioxide instead, which is the tempting confusion between the carbide and the carbonate. The ethyne produced this way smells unpleasant because of phosphine impurities.

215. Terminal alkynes such as ethyne react with ammoniacal silver nitrate to give a precipitate because

• A. the triple bond adds silver
• B. the hydrogen attached to the triply bonded carbon is weakly acidic and is replaced by silver
• C. silver catalyses polymerisation
• D. alkynes are strong bases

Explanation: The sp hybridised carbon holds its bonding electrons closer, so the attached hydrogen is slightly acidic and can be replaced by a metal to form an acetylide precipitate, which is the standard test distinguishing a terminal alkyne from an internal one. Alkenes and alkanes give no such reaction. The dry precipitates are explosive and must be destroyed with acid.

216. The Kekule structure of benzene failed to explain

• A. the molecular formula C6H6
• B. the ring shape of the molecule
• C. why benzene resists addition reactions and has all its carbon to carbon bonds of equal length
• D. why benzene contains only carbon and hydrogen

Explanation: Alternating single and double bonds would predict two different bond lengths and the ready addition reactions of an alkene, but experiment shows a single intermediate bond length of 0.139 nm and a strong preference for substitution. The modern picture replaces the alternating bonds with a delocalised ring of six pi electrons above and below the plane. That delocalisation is what makes benzene unusually stable.

217. Benzene undergoes electrophilic substitution rather than addition because substitution

• A. preserves the stable delocalised pi system
• B. produces a smaller molecule
• C. requires no catalyst
• D. is always exothermic

Explanation: Addition would destroy the aromatic sextet and lose the large resonance stabilisation, whereas replacing a hydrogen leaves the delocalised ring intact. This is why benzene reacts with bromine only in the presence of a halogen carrier and gives bromobenzene rather than a dibromo addition product. Nitration, sulphonation and Friedel Crafts reactions follow the same pattern.

218. The nitration of benzene is carried out using

• A. dilute nitric acid alone at room temperature
• B. a mixture of concentrated nitric and concentrated sulphuric acids at about 50 to 60 degrees Celsius
• C. sodium nitrate solution
• D. nitrogen dioxide gas in sunlight

Explanation: Sulphuric acid protonates nitric acid so that it loses water and generates the nitronium ion, which is the actual electrophile attacking the ring. The temperature is kept near 55 degrees because higher temperatures give unwanted dinitrobenzene. Nitrobenzene is important industrially as the starting point for aniline and hence for dyes.

219. The general formula of the alkane homologous series is

• A. CnH2n
• B. CnH2n-2
• C. CnH2n+2
• D. CnH2n+1

Explanation: Every carbon in a saturated open chain carries the maximum hydrogen, giving two per carbon plus two extra for the ends of the chain, so hexane is C6H14. CnH2n belongs to alkenes and cycloalkanes and CnH2n-2 to alkynes. CnH2n+1 is not a molecule at all but an alkyl group, such as methyl or ethyl.

220. The boiling points of the alkanes increase with increasing chain length because

• A. the covalent bonds within the molecules become stronger
• B. the molecules become more polar
• C. hydrogen bonding develops between longer chains
• D. the larger surface area increases the dispersion forces between molecules

Explanation: Longer chains touch each other over a greater area, so the temporary induced dipole attractions between molecules add up to more, and more energy is needed to separate them. This is why methane is a gas, hexane a liquid and long chain alkanes waxy solids. Branching lowers the boiling point of an isomer, because a compact molecule presents less contact area.