How can I draw an elementary reaction in a potential energy diagram?

Jun 26, 2015

You join the potential energy diagrams into one diagram.

Explanation:

A potential energy diagram represents the energy pathway for an elementary reaction — a single-step process.

Many reactions take place in a series of steps.

For example, the starting materials ($\text{SM}$) may first react to form some intermediate
($\text{I}$), which then reacts further to form the products ($\text{P}$).

We would write the reaction as occurring in two steps:

Step 1: $\text{SM" → "I}$

Step 2: $\text{I" → "P}$

Overall: $\text{SM" → "P}$

Step 1 is an elementary reaction, so we draw a potential energy diagram going from $\text{SM}$ to $\text{I}$ through a transition state ${\text{TS}}_{1}$ — the left portion of the diagram below.

Step 2 is another elementary reaction, so we draw a potential energy diagram going from $\text{I}$ to $\text{P}$ through a transition state ${\text{TS}}_{2}$.

But the starting point for Step 2 is the ending point for step 1, so we join the two diagrams together to make a double-hump curve as shown above.

Each hump represents the transition state for a separate elementary reaction.