U5.10 · Multistep Reaction Energy ProfilePractice · 多步反应能量图

Each peak is a transition state, one elementary step: 3 peaks, 3 steps. The valleys between the ends (20 and 35) are intermediates, 2 in all, always one fewer than the steps.

Measure each step from its own starting valley to its peak: step 1, 50 − 0 = 50; step 2, 85 − 20 = 65; step 3, 60 − 35 = 25. Step 2 has the largest activation energy and the highest transition state, so it is the slowest.

Step 2 starts from the first intermediate (20) and climbs to the second transition state (85): 85 − 20 = 65 kJ/mol. Measuring from the reactants (0) gives 85, which is wrong.

ΔH depends only on the ends: products (−40) − reactants (0) = −40 kJ/mol, exothermic. The peaks and valleys in between don’t affect ΔH.

An intermediate sits at a local energy minimum (a valley) and lives long enough, in principle, to be detected. A transition state sits at an energy maximum (a peak), with old bonds partly broken and new ones partly formed; it lasts too short a time to isolate. There is one transition state per step, one more than the number of intermediates.