Enthalpy Diagram Plotter — Reaction Coordinate, Activation Energy & Catalyst Effect

A reaction coordinate (enthalpy) diagram visually unifies two topics that are often taught separately — kinetics (activation energy, reaction rate) and thermodynamics (ΔH, spontaneity) — making it a genuinely cross-cutting tool spanning AP Chemistry Units 5 and 6.

The diagram plots energy on the vertical axis against reaction progress on the horizontal axis, with three key points: the reactants' energy level, a peak representing the transition state (the highest-energy, least stable arrangement of atoms during the reaction), and the products' energy level. Activation energy for the forward reaction, Ea(forward), is the energy gap between reactants and the transition state peak; activation energy for the reverse reaction, Ea(reverse), is the gap between products and that same peak.

Overall reaction enthalpy, ΔH = E(products) - E(reactants), is read directly from the diagram's shape: if products sit lower than reactants, the reaction is exothermic (ΔH negative) and released net energy overall; if products sit higher, the reaction is endothermic (ΔH positive) and absorbed net energy. Critically, ΔH depends only on the reactant and product energy levels — the height and shape of the transition-state peak in between has zero effect on ΔH.

This last point is exactly why catalysts work the way they do, and it's the single most heavily tested fact from this topic: a catalyst provides an alternative reaction pathway with a lower-energy transition state, lowering both Ea(forward) and Ea(reverse) by the same amount — but it does not change the energy of either the reactants or the products, so ΔH remains completely unchanged. A catalyst speeds up how fast a reaction reaches equilibrium; it has no effect whatsoever on where that equilibrium ends up or how much energy the reaction releases overall.

This plotter lets you drag reactant, transition state, and product energy levels to build a custom reaction coordinate diagram, then add a catalyst and watch both activation energies drop while ΔH stays fixed — making the catalyst distinction visually unmistakable rather than just a memorized rule.