A D is an endothermic reaction occurring in three steps (elementary). (i) A B H _ i =+ ve (ii) B C H _ i i =- ve (iii) C D H _ i i i =- ve Which of the following graphs between potential energy (y-axis) vs reaction coordinate (x -axis) correctly represents the reaction profile of A D ?
A.
The reaction A → D occurs through three elementary steps with energy changes: Step (i): A → B is endothermic ( H_I = +ve), so B has higher energy than A, creating the first peak. Step (ii): B → C is exothermic ( H_ II = -ve), so C has lower energy than B, creating a lower second peak. Step (iii): C → D is exothermic ( H_ III = -ve), so D has lower energy than C, creating an even lower third feature. Overall reaction: Since step (i) is endothermic and steps (ii) and (iii) are exothermic, the reaction can be endothermic overall depending on the magnitudes. The overall energy change is positive (A → D is endothermic), meaning D must be at higher energy than A. The reaction profile must show three distinct peaks/steps with the final product D positioned higher than the initial reactant A. (1) correctly depicts this three-step mechanism with proper energy relationships and overall endothermic character.
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