A D is an endothermic reaction occurring in three steps (elementary). (i) A B, Hi = +ve (ii) B C, Hii = -ve (iii) C D, Hiii = -ve Which of the following graphs between potential energy (y-axis) vs reaction coordinate (x-axis) correctly represents the reaction profile of A D?
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Answer
(4) [figure]
Solution
In a potential energy profile, the level after each hump is the energy of the product of that step.
Step (i) endothermic E(B) > E(A)
Step (ii) exothermic E(C) < E(B)
Step (iii) exothermic E(D) < E(C)
Overall endothermic E(D) > E(A)
E(B) > E(C) > E(D) > E(A)
Graph (1): E(C) > E(B), wrong. Graph (2): E(D) > E(C), wrong. Graph (3): E(B) < E(A), wrong.
Graph (4): the minimum after each hump is lower than the previous one, but the final level D is still above the initial level A. Correct.
Answer: (4)