Reactant A converts to product D through the given mechanism (with the net evolution of heat):
A → B slow; ΔH = +ve
B → C fast; ΔH = -ve
C → D fast; ΔH = -ve
Which of the following represents the above reaction mechanism?

Solution

Chemistry ma’am · 3 steps

Read the plot from left to right and check every step: its barrier height and whether it goes up or down.

1. What to look for. A slow step has the largest activation energy, so A → B must climb the tallest barrier. A step with Δ H > 0 ends higher than it starts, and a step with Δ H < 0 ends lower.
2. Four conditions. So B must sit above A, C must sit below B, and D must sit below C. Since heat is evolved overall, D must also end below A. The barriers after B should be small, because those steps are fast.

   Option	B → C	D compared with A	Verdict
   (a)	C above B, so endothermic	D above A, so net endothermic	fails twice
   (b)	C below B	D above A, so net endothermic	fails
   (c)	C above B, so endothermic	D below A	fails
   (d)	C below B	D below A	fits every condition

3. So. In (d), A → B climbs the tallest barrier and ends higher. The next two barriers are small, and each step goes downhill, so D ends below A. The answer is (d).

The idea to keep. On an energy profile, the slow step has the tallest barrier measured from its own starting level. The sign of Δ H tells you whether each step ends higher or lower than it started.

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