Chemical Kinetics

14 NEET-UG practice questions on Chemical Kinetics, part of Chemistry. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   For a zero-order reaction with [A]₀ = 0.80 mol/L and k = 0.020 mol L⁻¹ min⁻¹, the time needed for all of A to be used up is

   1. A34.7 min
   2. B80 min
   3. C20 min
   4. D40 min

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   Answer: D · 40 min

   For zero order, [A] = [A]₀ − kt. Setting [A] = 0 gives t = 0.80/0.020 = 40 min.

   • A: Uses the first-order half-life formula, 0.693/k.
   • B: Multiplies by 2 and takes t = 2[A]₀/k.
   • C: Gives the half-life, [A]₀/2k, not the completion time.

   Remember this

   A zero-order reaction finishes in a finite time, [A]₀/k, and its half-life is proportional to [A]₀.

2. Question 2 · Hard

   The gas-phase first-order reaction A(g) → B(g) + C(g) is run at constant volume and temperature, starting from pure A at 100 mmHg. The total pressure is 150 mmHg after 20 min. What will the total pressure be after 40 min?

   1. A175 mmHg
   2. B200 mmHg
   3. C150 mmHg
   4. D125 mmHg

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   Answer: A · 175 mmHg

   If A falls by x, the total is 100 + x. At 20 min x = 50, so A = 50 mmHg and t½ = 20 min. At 40 min A = 25 mmHg, so x = 75 and the total is 175 mmHg. Check: total = 2(100) − p_A = 200 − 25 = 175.

   • B: Assumes the reaction is complete after two half-lives, as in zero order.
   • C: Assumes the pressure stays constant after 20 min.
   • D: Adds the remaining A (25) to 100 instead of the amount reacted (75).

   Remember this

   For A → B + C, the total pressure is 2P₀ − p_A, so find p_A from half-lives and then convert.

3. Question 3 · Medium

   Given below are two statements: Statement I: A plot of ln k against 1/T for a reaction following the Arrhenius equation is a straight line with a negative slope. Statement II: Of two reactions compared on such a plot, the one with the larger activation energy gives the less steep line. Choose the correct answer.

   1. ABoth Statement I and Statement II are correct
   2. BStatement I is correct but Statement II is incorrect
   3. CBoth Statement I and Statement II are incorrect
   4. DStatement I is incorrect but Statement II is correct

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   Answer: B · Statement I is correct but Statement II is incorrect

   ln k = ln A − Eₐ/(RT), so the slope is −Eₐ/R, a negative value, which makes Statement I true. A larger Eₐ makes the slope larger in magnitude, so the line is steeper and Statement II is false.

   • A: Statement II is false; a larger Eₐ gives a steeper line.
   • C: Statement I is true; the slope is −Eₐ/R.
   • D: Statement I is true and Statement II is false, not the other way round.

   Remember this

   On an Arrhenius plot, the slope gives −Eₐ/R and the intercept gives ln A.

4. Question 4 · Medium

   Assertion A: The rate constant of a reaction generally increases with a rise in temperature. Reason R: At a higher temperature, a larger fraction of molecules has energy equal to or above the activation energy. Choose the correct answer.

   1. AA is correct but R is not correct
   2. BBoth A and R are correct and R is the correct explanation of A
   3. CA is not correct but R is correct
   4. DBoth A and R are correct but R is not the correct explanation of A

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   Answer: B · Both A and R are correct and R is the correct explanation of A

   A rise in temperature shifts the Maxwell-Boltzmann distribution towards higher energies, so more collisions can cross the barrier. This is the reason k increases, so R explains A.

   • A: R is correct; the fraction above Eₐ does rise with T.
   • C: A is correct; k generally rises with T.
   • D: R is the actual cause of the rise in k, not an unrelated fact.

   Remember this

   The exponential factor e^(−Eₐ/RT) is the fraction of molecules at or above Eₐ, and it rises with T.

5. Question 5 · Medium

   For a first-order reaction, arrange the following times in increasing order. A. Time for 75% completion B. Time for 99% completion C. Time for 50% completion D. Time for 90% completion

   1. AA, C, D, B
   2. BB, D, A, C
   3. CC, D, A, B
   4. DC, A, D, B

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   Answer: D · C, A, D, B

   t = (2.303/k) log([A]₀/[A]) gives 0.693/k, 1.386/k, 2.303/k and 4.606/k for 50%, 75%, 90% and 99% completion. The larger the percentage completed, the longer the time.

   • A: Places the 75% time ahead of the 50% time.
   • B: Gives the decreasing order, the reverse of what is asked.
   • C: Places the 90% time ahead of the 75% time.

   Remember this

   For first order, t₉₉ is twice t₉₀, and t₇₅ is twice t₅₀.

6. Question 6 · Medium

   Study these statements about rate equations. A. The half-life of a first-order reaction does not depend on the initial concentration. B. The half-life of a zero-order reaction does not depend on the initial concentration. C. The rate constant of a zero-order reaction has the unit s⁻¹. D. A plot of ln[A] against time for a first-order reaction is a straight line. Select the correct statements.

   1. AA and D only
   2. BB and C only
   3. CA, B and D only
   4. DA, C and D only

   Answer and explanation in LearnFree account

7. Question 7 · Medium

   If a first-order reaction reaches 75% completion in 30 min, how long will it take to reach 87.5% completion?

   1. A37.5 min
   2. B60 min
   3. C45 min
   4. D35 min

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8. Question 8 · Hard

   At 300 K, a catalyst lowers the activation energy of a reaction by 11.4 kJ/mol, leaving the frequency factor unchanged. By what factor does the rate constant increase? (Take 2.303R = 19 J K⁻¹ mol⁻¹.)

   1. A2
   2. B20
   3. C100
   4. D10⁴

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9. Question 9 · Medium

   Three kinetic runs of A + B → products gave these initial rates. Run 1: [A] = 0.10 M, [B] = 0.10 M, rate = 2.0 × 10⁻³ M/s Run 2: [A] = 0.20 M, [B] = 0.10 M, rate = 8.0 × 10⁻³ M/s Run 3: [A] = 0.10 M, [B] = 0.30 M, rate = 6.0 × 10⁻³ M/s The rate law consistent with these data is

   1. ARate = k[A]²[B]²
   2. BRate = k[A][B]
   3. CRate = k[A]²[B]
   4. DRate = k[A][B]²

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10. Question 10 · Easy

    A first-order reaction has a rate constant of 0.0231 min⁻¹. Its half-life is

    1. A30 min
    2. B15 min
    3. C43 min
    4. D60 min

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11. Question 11 · Easy

    Which unit is correct for the rate constant of a second-order reaction?

    1. AL mol⁻¹ s⁻¹
    2. Bs⁻¹
    3. Cmol L⁻¹ s⁻¹
    4. DL² mol⁻² s⁻¹

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12. Question 12 · Hard

    The Arrhenius plot of a reaction is shown; note that the horizontal axis does not start at zero. Taking R = 8.3 J K⁻¹ mol⁻¹, find the activation energy and ln A.

    

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    1. A36 kJ mol⁻¹, ln A = 20
    2. B36 kJ mol⁻¹, ln A = 40
    3. C83 kJ mol⁻¹, ln A = 40
    4. D83 kJ mol⁻¹, ln A = 20

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13. Question 13 · Medium

    The concentration of reactant A during its decomposition at constant temperature is plotted. If the same trend continues, what will [A] be at 80 min?

    

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    1. A0 mol L⁻¹
    2. B0.050 mol L⁻¹
    3. C0.025 mol L⁻¹
    4. D0.100 mol L⁻¹

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14. Question 14 · Easy

    Which labelled double arrow in the energy profile gives the activation energy of the reverse reaction?

    

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    1. AArrow b
    2. BArrow d
    3. CArrow c
    4. DArrow a

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