Chemistry · Equilibrium · chapter 6 of 9

Class 11 Chemistry Chapter 6: Equilibrium Assertion-reason questions with answers

2 assertion-reason questions on Equilibrium, 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): In CaCO₃(s) ⇌ CaO(s) + CO₂(g) at equilibrium at a fixed temperature, putting more CaCO₃(s) into the vessel does not change the equilibrium pressure of CO₂. Reason (R): The molar concentration of a pure solid is constant, whatever amount of it is present.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Because [CaCO₃(s)] and [CaO(s)] are constant, they are absorbed into the constant and Kp = p(CO₂). At a given temperature p(CO₂) is fixed, so adding solid changes nothing. R is the reason for A.

Question 2
Assertion (A): For carbonic acid, Ka₂ is much smaller than Ka₁. Reason (R): In a solution of carbonic acid, the H₃O⁺ comes mainly from the first dissociation step.
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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 true because a proton is harder to remove from the negatively charged HCO₃⁻ than from neutral H₂CO₃ (electrostatic attraction). R is a consequence of Ka₁ ≫ Ka₂, not the reason why Ka₂ is small.

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