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Question #44795

Difficulty: 8/10 | 🟢 Free

The Question

For a complex gas-phase reaction, the temperature dependence of the rate constant kk deviates from classical Arrhenius behavior and is accurately described by the modified equation k=A0Tme−E0/RTk = A_0 T^m e^{-E_0/RT}, where A0A_0, mm, and E0E_0 are temperature-independent constants. A student plots ln⁡k\ln k versus 1/T1/T and observes a non-linear curve. By drawing tangents to the curve, the student determines the instantaneous apparent activation energy (Ea,appE_{a,app}) to be 54.98 kJ mol−154.98 \text{ kJ mol}^{-1} at 300 K300 \text{ K} and 62.47 kJ mol−162.47 \text{ kJ mol}^{-1} at 600 K600 \text{ K}. Assuming ideal gas behavior, what are the values of the true threshold energy (E0E_0) and the temperature exponent (mm)? (Given: R=8.314 J K−1 mol−1R = 8.314 \text{ J K}^{-1} \text{ mol}^{-1})

Options

  • A.

    E0=47.50 kJ mol−1,m=3E_0 = 47.50 \text{ kJ mol}^{-1}, m = 3

  • B.

    E0=54.98 kJ mol−1,m=1E_0 = 54.98 \text{ kJ mol}^{-1}, m = 1

  • C.

    E0=40.02 kJ mol−1,m=2E_0 = 40.02 \text{ kJ mol}^{-1}, m = 2

  • D.

    E0=62.47 kJ mol−1,m=−3E_0 = 62.47 \text{ kJ mol}^{-1}, m = -3

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Instantaneous vs. Average Rate|Arrhenius Pre-exponential Factor

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Chemical Exception' of the temperature-dependent frequency factor in complex reactions.

Students often blindly apply the linear Arrhenius equation (ln⁡k=ln⁡A−Ea/RT\ln k = \ln A - E_a/RT) and assume EaE_a is a strict constant.

By introducing A(T)=A0TmA(T) = A_0 T^m, the slope of ln⁡k\ln k vs 1/T1/T becomes temperature-dependent.

Students failing to differentiate the modified Arrhenius equation will either assume E0=Ea,appE_0 = E_{a,app} or incorrectly apply standard Transition State Theory (m=1m=1) without deriving the empirical mm from the provided instantaneous rates.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterChemical Kinetics - Class 12
ConceptInstantaneous vs. Average Rate
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree