= Energy profile
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{wiki}
= Reaction energy profile
{synonym}
An <energy profile (chemistry)> plots an explicitly specified <energy> against a <reaction coordinate>. Identify whether the vertical axis is <potential energy>, <enthalpy>, or <Gibbs free energy>; these descriptions are related but not interchangeable. A <potential-energy profile> describes a path through configurations, while a free-energy description also accounts for the relevant thermodynamic conditions.
\Image[/reaction-barriers-and-endpoints.png]
{title=Two schematic <potential-energy profiles> share endpoints but have different barriers}
{description=The reactant level is $0$, the product level is $-20\,\mathrm{kJ\,mol^{-1}}$, and the two maxima are $80$ and $50\,\mathrm{kJ\,mol^{-1}}$. These chosen curves illustrate endpoint differences and barrier heights; they are not measured surfaces, trajectories in time, or a complete catalytic cycle.}
For the two chosen profiles, the <reaction energy barriers> from the <reactants> are $80$ and $50\,\mathrm{kJ\,mol^{-1}}$. From the <reaction products>, the same maxima are $100$ and $70\,\mathrm{kJ\,mol^{-1}}$ higher. Both paths have endpoint difference $-20\,\mathrm{kJ\,mol^{-1}}$. The barrier is measured from the relevant starting endpoint, not from an arbitrary zero of <potential energy>.
The appeal of the diagram is that it separates two useful questions: which endpoint is lower, and which path is easier to traverse? <Catalysis> changes the route through <transition states> while retaining the endpoint thermodynamics of the same overall <chemical reaction>. A real catalytic route may have several steps and <reaction intermediates>; the two simple curves do not specify such a <reaction mechanism>. The plotted potential-energy difference alone is not an <equilibrium constant> or a calculated <rate constant>. See https://openstax.org/books/chemistry-2e/pages/12-7-catalysis[Chemistry 2e, 12.7: alternative reaction routes and catalysis].
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