A→ D is an endothermic reaction occurring in three steps (elementary).
(i) A→ BΔHᵢ=+ve
(ii) B→ CΔHᵢᵢ=-ve
(iii) C→ DΔHᵢᵢᵢ=-ve
Which of the following graphs between potential energy ( y-axis) vs reaction coordinate ( x-axis) correctly represents the reaction profile of A→ D ?

Solution

Chemistry ma’am · 3 steps

Each valley is a species: let each Δ H sign tell you if the next valley sits higher or lower.

1. Read the graph. In a potential energy profile, each valley is a species (A, B, C, D) and each hump is a transition state. Three elementary steps give three humps and four valleys.
2. Translate the signs. Δ Hᵢ > 0, so B sits above A. Δ Hᵢᵢ < 0 and Δ Hᵢᵢᵢ < 0, so C sits below B and D sits below C. The overall reaction is endothermic, so D still ends above A.

   Graph	A to B	B to C	C to D	Verdict
   (a)	up, correct	up, should be down	down, correct	wrong
   (b)	up, correct	down, correct	up, should be down	wrong
   (c)	down, should be up	up	down	wrong
   (d)	up, correct	down, correct	down, correct	matches, and D ends above A

3. So. Only graph (d) goes up for step (i), down for steps (ii) and (iii), and still ends above A. The answer is (d).

The idea to keep. In a multi-step profile, compare each valley with the one before it. Uphill means that step is endothermic and downhill means it is exothermic. The hump heights only tell you activation energies.

Answer from the NTA final answer key. Solution by PrepWiser.