JEE Chemistry Hydrocarbons MCQs Set 10

Answer: (d) It is immiscible and lighter than water

Explanation:
1. To float, a liquid must be both insoluble in water and less dense
2. Cyclohexane has a density of about 0.78 g/mL

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (a) Methane

Explanation:
1. Natural gas is about 80-95% methane
2. Smaller amounts of ethane, propane and butane are present

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) \( \ce{CCl4} \)

Explanation:
1. Carbon tetrachloride cannot burn because carbon is already highly oxidised by chlorine
2. The vapour is used in fire extinguishers

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (d) Catalytic hydrogenation

Explanation:
1. Propene adds \( \ce{H2} \) over Ni, Pd or Pt to give propane
2. The other methods are not reductions of an alkene

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (a) \( \ce{(C2H5)4Pb} \)

Explanation:
1. Tetraethyl lead is added to raise the octane number
2. TNT is an explosive and the others are not used in fuel

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) Pt/Ni

Explanation:
1. Finely divided Ni, Pt or Pd adsorb hydrogen and the alkene
2. Pt and Ni are the usual catalysts here

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (c) \( \ce{Br-CH2-CH2-Br} \)

Explanation:
1. Bromine adds across the C=C bond
2. The product is 1,2-dibromoethane

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (a) Fractional distillation

Explanation:
1. Crude oil is separated into fractions of different boiling ranges in a fractionating tower
2. Gasoline is the fraction boiling at about 40-200 degrees C

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (d) Reaction of ethyl chloride with alcoholic KOH

Explanation:
1. Alcoholic KOH eliminates HCl from ethyl chloride to give ethene, an alkene
2. The other three give ethane, methane and ethane

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) \( \ce{C4H10 + C3H8} \)

Explanation:
1. Liquefied petroleum gas consists mainly of butane and propane
2. Both are easily liquefied under pressure

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (d) Methane

Explanation:
1. Methane burned in a limited supply of air (or thermally decomposed) gives finely divided carbon
2. This carbon black is used in ink, paint and shoe polish

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (b) Raises its octane number

Explanation:
1. Tetraethyl lead slows the radical chain that causes knocking
2. So it raises the octane number

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (a) n-Hexane

Explanation:
1. Boiling point rises with the number of carbons and falls with branching
2. n-Hexane is the largest and unbranched

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (b) Ignites rapidly

Explanation:
1. Knocking is sudden, irregular burning of the fuel-air mixture
2. It makes the piston jerk and produces a violent sound

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (a) Alkanes

Explanation:
1. Crude oil consists mainly of straight and branched alkanes with some cycloalkanes
2. Alkenes and aromatics are minor

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) n-Octane

Explanation:
1. For equal molecular mass (octane isomers), the straight chain has more surface contact and a higher boiling point
2. n-Butane is much lighter

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (c) Ethane

Explanation:
1. Kolbe electrolysis: \( \ce{2 CH3COO^- -> CH3-CH3 + 2 CO2 + 2e^-} \) at the anode
2. Hydrogen and KOH form at the cathode

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (c) Ethane

Explanation:
1. Ethane is saturated and does not add bromine
2. Alkenes and alkynes decolourise it by addition

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (b) Methane

Explanation:
1. Decarboxylation: \( \ce{CH3COONa + NaOH -> CH4 + Na2CO3} \) (CaO)
2. The product has one carbon fewer than the acid

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (c) \( \ce{CO + H2} \)

Explanation:
1. Steam passed over red-hot coke gives \( \ce{C + H2O -> CO + H2} \)
2. This mixture is water gas

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.

Answer: (b) 81

Explanation:
1. Octane number equals the percentage by volume of iso-octane in the mixture
2. So it is 81

Teacher's Note:
1. Recall the key fact or reaction first, then eliminate options that contradict it.
2. Link each answer to the underlying bonding or mechanism rather than memorising it.

Answer: (d) All of these

Explanation:
1. Crude oil contains alkanes, cycloalkanes and small amounts of sulphur compounds
2. So all three are present

Teacher's Note:
1. Check the type of reaction involved and what it does to the carbon skeleton.
2. Compare the options one by one against that.

Answer: (c) Kolbe's synthesis

Explanation:
1. Hermann Kolbe developed this electrolytic coupling of carboxylate salts
2. Sabatier-Senderens is catalytic hydrogenation

Teacher's Note:
1. Trends such as boiling point and stability depend on molecular size, branching and substituent effects.
2. Apply the trend directly to rank the options.