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Temperature effect, Arrhenius theory and activation energy: NEET MCQs with answers

88 questions on this topic of Chemical Kinetics (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following expressions correctly represents the threshold energy (Eₜₕ) of an elementary chemical reaction?

  1. Eₜₕ = Activation energy (Eₐ) - Average kinetic energy of reactant molecules
  2. Eₜₕ = Enthalpy of reaction (ΔH) - Activation energy (Eₐ)
  3. Eₜₕ = Activation energy (Eₐ) - Enthalpy of reaction (ΔH)
  4. Eₜₕ = Activation energy (Eₐ) + Average kinetic energy of reactant molecules
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Answer: (D) Eₜₕ = Activation energy (Eₐ) + Average kinetic energy of reactant molecules

Threshold energy is the minimum total energy that the colliding molecules must possess to yield an effective collision leading to products. It is the sum of the activation energy (energy barrier that must be supplied) and the average initial kinetic energy already possessed by the reactant molecules.

medium

2. The slope of the line in the plot of log₁₀ k versus 1/T for a chemical reaction is -5000 K. What is the activation energy of the reaction? (Take R = 8.314 J K⁻¹ mol⁻¹)

  1. 41.57 kJ mol⁻¹
  2. 115.1 kJ mol⁻¹
  3. 95.7 kJ mol⁻¹
  4. 21.6 kJ mol⁻¹
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Answer: (C) 95.7 kJ mol⁻¹

For a plot of log₁₀ k vs 1/T, slope = -Ea / (2.303 R). Given slope = -5000 K, we have Ea = 5000 × 2.303 × 8.314 = 95735 J mol⁻¹ ≈ 95.7 kJ mol⁻¹.

hard

3. In a reaction mechanism, the rate constant k of an overall process is given by k = (k₁)² / k₂. If the activation energy of step 1 is 45 kJ mol⁻¹ and that of step 2 is 30 kJ mol⁻¹, what is the overall activation energy of the reaction?

  1. 15 kJ mol⁻¹
  2. 60 kJ mol⁻¹
  3. 75 kJ mol⁻¹
  4. 120 kJ mol⁻¹
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Answer: (B) 60 kJ mol⁻¹

Taking the natural logarithm of both sides: ln k = 2 ln k₁ - ln k₂. Differentiating with respect to temperature or substituting the Arrhenius relation ln k = ln A - Ea / RT gives: -Ea / RT = 2(-Ea₁ / RT) - (-Ea₂ / RT). Multiplying by -RT yields: Ea = 2 Ea₁ - Ea₂ = 2(45) - 30 = 90 - 30 = 60 kJ mol⁻¹.

medium

4. For a reaction, the slope of a plot of log₁₀ k versus 1/T is -4000 K. What is the activation energy of the reaction? (Take R = 8.314 J K⁻¹ mol⁻¹)

  1. 76.57 kJ mol⁻¹
  2. 33.26 kJ mol⁻¹
  3. 48.11 kJ mol⁻¹
  4. 96.22 kJ mol⁻¹
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Answer: (A) 76.57 kJ mol⁻¹

According to the Arrhenius equation, log₁₀ k = log₁₀ A - Ea / (2.303 R T). The slope of the log₁₀ k versus 1/T plot is -Ea / (2.303 R). Therefore, Ea = -slope × 2.303 × R = -(-4000) × 2.303 × 8.314 = 76572 J mol⁻¹ ≈ 76.57 kJ mol⁻¹.

easy

5. In the Maxwell-Boltzmann distribution curve of kinetic energy at two temperatures T₁ and T₂ (where T₂ > T₁), what happens to the total area under the curve?

  1. Increases because more molecules attain higher kinetic energy
  2. Remains constant because the total number of reacting molecules is conserved
  3. Decreases because the height of the maximum decreases
  4. First increases and then decreases with increasing molecular speed
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Answer: (B) Remains constant because the total number of reacting molecules is conserved

The total area under the Maxwell-Boltzmann distribution curve represents the total fraction or number of molecules in the system. As temperature increases, the curve broadens and flattens toward higher energy, but the total area under the curve must remain constant as the total number of molecules does not change.

hard

6. Consider the following statements regarding the effect of temperature on reaction rates: Statement I: The exponential factor e^(-Ea / RT) in the Arrhenius equation represents the fraction of molecules with kinetic energy greater than or equal to Ea. Statement II: With an increase in temperature, the peak of the Maxwell-Boltzmann distribution curve shifts towards higher energy and its height increases. In light of the above statements, choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct; the Boltzmann factor e^(-Ea / RT) gives the fraction of molecules having energy ≥ Ea. Statement II is incorrect because as temperature rises, the peak of the curve shifts to the right (higher energy) but broadens and flattens out, meaning its height (fraction of molecules with the most probable speed) decreases.

easy

7. The rate of a reaction doubles for every 10 °C rise in temperature. If the temperature is raised by 10 °C, the rate increases by a factor of:

  1. 4
  2. 3
  3. 1
  4. 2
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Answer: (D) 2

10 °C is 1 steps of 10 °C, so the factor is 2¹ = 2.

easy

8. The rate of a reaction is measured at 300 K and 310 K. The fraction of molecules having energy equal to or greater than the activation energy increases significantly. Which term in the Arrhenius equation represents this fraction?

  1. Ea / RT
  2. A
  3. ln (k₂ / k₁)
  4. e^(-Ea / RT)
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Answer: (D) e^(-Ea / RT)

According to the Maxwell-Boltzmann distribution and the Arrhenius formulation, the exponential factor e^(-Ea / RT) directly represents the fraction of molecules that possess kinetic energy equal to or greater than the activation energy Ea at temperature T.

easy

9. Which of the following expressions correctly gives the fraction (x) of molecules possessing energy equal to or greater than activation energy according to the Arrhenius theory?

  1. x = e^(Ea / RT)
  2. x = ln(Ea / RT)
  3. x = e^(-Ea / RT)
  4. x = -Ea / (2.303 RT)
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Answer: (C) x = e^(-Ea / RT)

In the Arrhenius equation k = A e^(-Ea / RT), the exponential factor e^(-Ea / RT) corresponds to the Boltzmann factor, representing the fraction of molecules with kinetic energy greater than or equal to the activation energy Ea.

easy

10. In the Arrhenius equation, the pre-exponential factor 'A' represents the frequency factor. For a bimolecular elementary gaseous reaction, which statement best describes 'A' according to collision theory?

  1. It is solely determined by the fraction of molecules having energy greater than Ea.
  2. It is directly proportional to temperature raised to the power of activation energy.
  3. It accounts for both the collision frequency (Z_AB) and the steric factor (P).
  4. It represents the minimum energy required for bond cleavage in reactants.
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Answer: (C) It accounts for both the collision frequency (Z_AB) and the steric factor (P).

According to modified collision theory, rate = P × Z_AB × e^(-Ea/RT). Comparing this with the Arrhenius equation k = A e^(-Ea/RT), the Arrhenius factor A incorporates both the collision frequency of reactants (Z_AB) and the steric or orientation factor (P).

easy

11. Consider the following statements regarding the collision theory of bimolecular reactions: Statement I: The factor e^(-Ea / RT) corresponds to the fraction of molecules that have kinetic energy greater than or equal to Ea. Statement II: According to collision theory, every collision possessing energy greater than activation energy leads directly to product formation, irrespective of orientation. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true.
  2. Both Statement I and Statement II are false.
  3. Statement I is true but Statement II is false.
  4. Statement I is false but Statement II is true.
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Answer: (C) Statement I is true but Statement II is false.

Statement I is true: the Boltzmann factor e^(-Ea / RT) represents the fraction of molecules with kinetic energy equal to or exceeding Ea. Statement II is false because effective collisions require not only sufficient activation energy but also proper mutual orientation of the colliding species, accounted for by the steric factor P.

easy

12. According to the Arrhenius equation, a plot of ln k versus 1/T yields a straight line. What does the slope of this line represent?

  1. -Ea / (2.303 R)
  2. Ea / R
  3. -Ea / R
  4. ln A
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Answer: (C) -Ea / R

The logarithmic form of the Arrhenius equation is ln k = ln A - Ea / (RT). Plotting ln k on the y-axis against 1/T on the x-axis gives a straight line with slope equal to -Ea / R and y-intercept ln A.

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