Chemistry · Hydrocarbons · chapter 9 of 9

Class 11 Chemistry Chapter 9: Hydrocarbons Assertion-reason questions with answers

2 assertion-reason questions on Hydrocarbons, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): For all practical purposes, rotation about the C–C bond in ethane is almost free at ordinary temperatures. Reason (R): The energy difference between the staggered and eclipsed conformations, about 12.5 kJ mol⁻¹, is supplied by intermolecular collisions.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. The barrier is small, so the thermal energy gained from collisions at ordinary temperatures is enough to cross it; this is exactly why the rotation is almost free, so R explains A.

Question 2
Assertion (A): Ethyne decolourises the reddish orange solution of bromine in carbon tetrachloride. Reason (R): Ethyne reacts with sodamide to form sodium acetylide.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. A is due to addition of bromine across the triple bond; R shows the acidic character of the hydrogens on sp carbons. R does not explain A.

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