Multistep reactions proceed through several elementary steps, each with its own energy change and barrier. If you know the energetics of each step, you can sketch the full energy profile consistently and interpret what it implies about the pathway.

What a multistep energy profile represents

A two-step reaction coordinate diagram showing reactants and products at different potential energies, with two labeled transition states (peaks) and an intermediate (local minimum) between them. This makes the “one hump per elementary step” idea explicit and helps you see how intermediates correspond to minima on the energy profile. Source

In this subtopic, the key skill is building the qualitative shape of that plot from step energetics: the energy change for each step and the barrier height for each step.

Energetic quantities you may be given

To construct the profile, you typically need (in words, a diagram, or data):

• The relative energies of reactants and products (overall energy change)

• For each step:

  • Whether it is exothermic (energy goes down) or endothermic (energy goes up)

  • The activation energy barrier height for that step (how high the “hump” is above the step’s starting point)

Building the profile from step energetics (process)

Step 1: Set a baseline and choose an energy scale

• Draw axes: potential energy (vertical) vs reaction coordinate (horizontal).

A basic reaction coordinate diagram for a one-step reaction, showing reactants rising to a transition state and then descending to products. It provides a reference picture for identifying activation energy as the vertical gap from a minimum to the peak, and overall energy change as the difference between the reactant and product energy levels. Source

• Place the reactants at a chosen reference energy (often arbitrary, e.g., 0 kJ/mol), since only relative energies matter.

Step 2: Add step-by-step energy changes (the “stairs”)

For each elementary step, use the sign and magnitude of the step energy change to locate the next minimum relative to the previous one:

• If step 1 is exothermic by a certain amount, the next minimum must be that amount lower than the reactants.

• If step 2 is endothermic, the next minimum must be that amount higher than the previous minimum.

• Continue cumulatively so that each minimum is consistent with the running total of energy changes across steps.

This cumulative bookkeeping ensures the final products land at the correct overall energy relative to reactants.

In practice, you can use this relationship as a consistency check: the sum of the step energy changes must match the overall energy change shown by the endpoints of your profile.

Step 3: Add barriers for each step (the “humps”)

Each step has a barrier that rises above the step’s starting minimum:

• For step 1, draw a peak that is Ea₁ above the reactant energy level, then descend to the next minimum set by step 1’s energy change.

• For step 2, start from the step 2 minimum, rise by Ea₂, then drop to the next minimum, and so on.

Key construction rule: barriers are measured from the local minimum immediately before that step, not necessarily from the original reactants.

Step 4: Check for energetic realism and internal consistency

A correctly constructed multistep profile should satisfy:

• The number of humps equals the number of elementary steps.

• The final product energy matches the cumulative result of the step energy changes.

• Large endothermic steps place subsequent minima higher; large exothermic steps place them lower.

• If two steps have very different barriers, the taller hump must visibly be the larger barrier relative to its own starting minimum.

Common construction pitfalls (and how to avoid them)

Mixing up “overall” vs “step” quantities

• Do not place products using only the largest step’s energy change; use the sum of all step energy changes.

• Do not measure every barrier from the original reactants; measure each from the preceding minimum.

Confusing magnitude with direction

• Exothermic step: minimum goes down.

• Endothermic step: minimum goes up.

• A step can be endothermic but still have a small or large barrier; these are independent pieces of information that both must appear in the sketch.

What your constructed profile can immediately communicate

Without invoking any extra mechanism details, a multistep profile built from step energetics can show:

• Whether the overall reaction is net exothermic or net endothermic

• Which step has the largest energy barrier (visually the tallest hump above its preceding minimum)

• Whether energy is released/absorbed early or late in the pathway based on the sequence of minima