Chemical Kinetics

43 NEET-UG practice questions on Chemical Kinetics, part of Chemistry. The first 25 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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15. Question 15 · Easy

    For a zero-order reaction, the initial concentration of the reactant is 0.50 mol L⁻¹ and the rate constant is 0.010 mol L⁻¹ s⁻¹. Calculate the half-life.

    1. A69.3 s
    2. B50 s
    3. C12.5 s
    4. D25 s

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16. Question 16 · Medium

    Consider these statements about reaction rates: A. A catalyst provides an alternative pathway with a lower activation energy. B. A catalyst makes the Gibbs energy change of a reaction more negative. C. For many reactions, the rate constant nearly doubles for a 10 K rise in temperature. D. For a collision to be effective, the molecules need proper orientation as well as enough energy. Select the correct statements.

    1. AA, C and D only
    2. BA and C only
    3. CB, C and D only
    4. DA, B and D only

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17. Question 17 · Medium

    Assertion A: The rate of a zero-order reaction stays the same as the reaction proceeds, as long as some reactant remains. Reason R: The half-life of a zero-order reaction does not depend on the initial concentration of the reactant. Choose the correct answer.

    1. ABoth A and R are correct and R is the correct explanation of A
    2. BA is correct but R is not correct
    3. CBoth A and R are correct but R is not the correct explanation of A
    4. DA is not correct but R is correct

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18. Question 18 · Easy

    A first-order reaction has a rate constant of 2.303 × 10⁻² s⁻¹. How much time does it take for 90% of the reactant to be consumed?

    1. A30.1 s
    2. B4.58 s
    3. C100 s
    4. D43.4 s

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19. Question 19 · Medium

    In the reaction 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), water vapour is formed at a rate of 0.036 mol L⁻¹ s⁻¹. At what rate is oxygen being consumed?

    1. A0.030 mol L⁻¹ s⁻¹
    2. B0.036 mol L⁻¹ s⁻¹
    3. C0.024 mol L⁻¹ s⁻¹
    4. D0.043 mol L⁻¹ s⁻¹

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20. Question 20 · Medium

    A first-order reaction is 20% complete in 10 min. After how many minutes from the start will it be 36% complete?

    1. A20 min
    2. B46 min
    3. C30 min
    4. D18 min

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21. Question 21 · Hard

    Match List-I with List-II. List-I (plot) A. ln[A] against t, first-order reaction B. log([A]₀/[A]) against t, first-order reaction C. log k against 1/T D. ln k against 1/T List-II (slope) I. −Ea/R II. k/2.303 III. −Ea/(2.303R) IV. −k Choose the correct answer.

    1. AA-II, B-IV, C-III, D-I
    2. BA-IV, B-II, C-III, D-I
    3. CA-I, B-III, C-II, D-IV
    4. DA-IV, B-II, C-I, D-III

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22. Question 22 · Hard

    For a reaction A → products, the half-life is 50 s when [A]₀ = 0.40 mol L⁻¹ and 100 s when [A]₀ = 0.20 mol L⁻¹ at the same temperature. Which option gives the order and the rate constant?

    1. ASecond order; k = 20 L mol⁻¹ s⁻¹
    2. BZero order; k = 0.0040 mol L⁻¹ s⁻¹
    3. CSecond order; k = 0.050 L mol⁻¹ s⁻¹
    4. DFirst order; k = 0.014 s⁻¹

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23. Question 23 · Medium

    Assertion A: A catalyst increases the rate of a reversible reaction by increasing its equilibrium constant. Reason R: A catalyst provides an alternative pathway of lower activation energy. Choose the correct answer.

    1. ABoth A and R are correct but R is not the correct explanation of A
    2. BA is correct but R is not correct
    3. CBoth A and R are correct and R is the correct explanation of A
    4. DA is not correct but R is correct

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24. Question 24 · Medium

    For the gas-phase reaction X + 2Y → products, the rate law is rate = k[X][Y]². The volume of the vessel is suddenly reduced to one-third at constant temperature. By what factor does the rate change?

    1. AIt becomes 9 times
    2. BIt is unchanged
    3. CIt becomes 3 times
    4. DIt becomes 27 times

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25. Question 25 · Easy

    Acid hydrolysis of ethyl acetate in a large excess of water shows first-order kinetics, although water is also a reactant. Such a reaction is called a

    1. Apseudo first-order reaction
    2. Bfractional-order reaction
    3. Csecond-order reaction
    4. Dzero-order reaction

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