More practice questions

4. Q4. From NCERT Table 3.1, butyl chloride falls from 0.0671 mol L⁻¹ at 200 s to 0.0549 at 300 s and 0.0439 at 400 s. Find the average rate over each 100 s interval.

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   0.0122/100 = 1.22 × 10⁻⁴ and 0.0110/100 = 1.10 × 10⁻⁴ mol L⁻¹ s⁻¹ (NCERT Table 3.1, page 64).

5. Q5. In NCERT Example 3.2, at what rate is O₂ produced?

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   O₂ has coefficient 1 in 2N₂O₅ → 4NO₂ + O₂, so Δ[O₂]/Δt equals the rate of reaction, 6.79 × 10⁻⁴ mol L⁻¹ min⁻¹ (NCERT page 66).

6. Q6. In N₂ + 3H₂ → 2NH₃, ammonia forms at 2.0 × 10⁻⁴ mol L⁻¹ s⁻¹. How fast are N₂ and H₂ used up?

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   Rate of reaction = ½ × 2.0 × 10⁻⁴ = 1.0 × 10⁻⁴ mol L⁻¹ s⁻¹. N₂ is used at 1.0 × 10⁻⁴ and H₂ at 3 × 1.0 × 10⁻⁴ = 3.0 × 10⁻⁴ mol L⁻¹ s⁻¹ (method of NCERT page 65).

7. Q7. The partial pressure of a gaseous reactant falls by 0.12 bar in 60 s at 300 K. Give its rate of disappearance in bar s⁻¹ and in mol L⁻¹ s⁻¹. (R = 0.0831 L bar K⁻¹ mol⁻¹.)

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   0.12/60 = 2.0 × 10⁻³ bar s⁻¹. Dividing by RT = 0.0831 × 300 = 24.9 L bar mol⁻¹ gives 8.0 × 10⁻⁵ mol L⁻¹ s⁻¹ (reasoned from NCERT page 65).

8. Q8. Assertion: for butyl chloride, the average rate over 0–50 s is larger than over 700–800 s. Reason: the rate of this reaction falls as the concentration of butyl chloride falls. Are both true, and does the reason explain the assertion?

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   Both are true, and the reason explains the assertion: 1.90 × 10⁻⁴ against 0.4 × 10⁻⁴ mol L⁻¹ s⁻¹ (NCERT Table 3.1, page 64; NCERT Section 3.2.2, page 66). A zero-order reaction keeps the same rate (NCERT Section 3.3.1, page 71).

9. Q9. Thermodynamics says diamond should change into graphite. Why do people still say “diamond is forever”?

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   The change is feasible, but its rate is so slow that it cannot be noticed at all; thermodynamics tells only whether a reaction can happen, kinetics how fast (NCERT pages 61–62).

10. Q10. What are the units of rate if concentration is in mol L⁻¹ and time in minutes? And for a gas followed in atm and seconds?

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    mol L⁻¹ min⁻¹ and atm s⁻¹ (NCERT page 63).

11. Q11. For 2N₂O₅ → 4NO₂ + O₂, the rate of loss of N₂O₅ and the rates of formation of NO₂ and O₂ are written k₁[N₂O₅], k₂[N₂O₅] and k₃[N₂O₅]. How are k₁, k₂ and k₃ related?

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    The reaction has one rate, −½ d[N₂O₅]/dt = ¼ d[NO₂]/dt = d[O₂]/dt, so k₁/2 = k₂/4 = k₃ (reasoned from NCERT page 65).