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Consider the following plots of log of rate constant k (log k) vs frac1mathrmT for three different reactions. The correct order of activation energies of these reactions is
Arrhenius plots of log k vs 1 over T for Q40 - JEE Main 2025 Evening
The graph depicts three linear curves with distinct negative slopes showing the temperature dependence of rate constants.

Solution & Explanation

### Related Formula log k = log A - fracE_a2.303 R T textSlope of the line = -fracE_a2.303 R implies |textSlope| propto E_a ### Core Logic From the given graph, we look at the steepness (magnitude of the negative slope) of lines 1, 2, and 3: - Line 2 is the steepest, meaning it has the largest slope magnitude. - Line 1 has an intermediate slope. - Line 3 is the flattest, indicating the smallest slope magnitude. Since the activation energy E_a is directly proportional to the magnitude of this slope: |textSlope_2| > |textSlope_1| > |textSlope_3| implies E_a2 > E_a1 > E_a3 ### Pattern Recognition In Arrhenius coordinates, steepness equals barriers. A steeper line means the reaction rate is highly sensitive to temperature because it has a higher activation energy (E_a). ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Chemistry: Chemical Kinetics

Reference Study Guides

More Chemical Kinetics Previous-Year Questions — Page 5

Q39 2025 Reaction Mechanism and Rate Law
The reaction mathrmA_2 + mathrmB_2 ightarrow 2 AB follows the mechanism mathrm A _ 2 xrightarrow [ mathrm k _ - 1 ]mathrm k _ 1 mathrm A + mathrm A text (fast) mathrm A + mathrm B _ 2 xrightarrow mathrm k _ 2 mathrm A B + mathrm B text (slow) mathrm A + mathrm B ightarrow mathrm A B text (fast) The overall order of the reaction is :
  • A. 1.5
  • B. 3
  • C. 2.5
  • D. 2

Solution

### Related Formula textRate = k cdot [textReactants]^textorder ### Core Logic The slowest elementary step controls the net kinetic pathway rate law : textRate = k_2[mathrmA][mathrmB2] quad dots textEquation (1) Since [mathrmA] behaves as a transient intermediate species, replace it using the prior fast equilibrium step : frack_1k-1 = frac[mathrmA]^2[mathrmA2] implies [mathrmA]^2 = left(frack_1k-1 ight) [mathrmA2] [mathrmA] = sqrtfrack_1k-1 cdot [mathrmA2]^1/2 Substitute [mathrmA] back into Equation (1) : textRate = k_2 sqrtfrack_1k-1 cdot [mathrmA2]^1/2[mathrmB2] Sum of powers determining overall order: textOrder = frac12 + 1 = 1.5 Hence, Option (1) is correct. ### Pattern Recognition Whenever a fast initial step dissociates a molecule into matching independent halves, it always injects a fractional order component of 0.5 relative to that parent species. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Chemistry: Chemical Kinetics

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