Real Thermochemistry in Reaction Coordinate Diagrams

ChemIllusion now overlays curated experimental ΔH values from the NIST Chemistry WebBook on reaction coordinate diagrams for canonical Organic Chemistry 1 reactions — so the product well depth reflects real thermochemistry, not an arbitrary sketch.

A reaction coordinate diagram is one of the most common figures in organic chemistry, and one of the most loosely drawn. The curve goes up, then down. The barrier looks steep or shallow depending on how the instructor felt that morning. The product well sits wherever there is room on the page.

That is fine for qualitative discussion. But it misses an opportunity: for many canonical Organic Chemistry 1 reactions, the net ΔH for the reaction as written is known experimentally. The product well depth is not arbitrary. We can show a real number.

The reaction-coordinate editor showing a two-step free-energy diagram labeled Reactants, Intermediate 1 and Products, with per-step controls for exergonic or endergonic and barrier size.

Choose the number of steps, set each barrier, label the intermediate, and let ChemIllusion add a verified ΔH chip from the NIST Chemistry WebBook.

The problem with arbitrary energy diagrams

When a student draws a reaction coordinate diagram from scratch, they are assembling axes, a curve, transition-state peaks, well labels, and molecule chips by hand. Every element is manually placed, and nothing knows what the reaction actually is. The diagram is a picture, not a chemistry object.

ChemIllusion's reaction coordinate tool already changes that: you choose the number of steps, set barrier heights qualitatively (large or small), label the wells, and pin molecule structures to the reactant and product positions. The tool builds the curve from those instructions instead of letting you assemble it piece by piece.

The new thermochemistry overlay goes one step further. When ChemIllusion recognizes the reaction from its SMILES representation or teaching family, it looks up the experimental ΔH and adds it as a compact chip near the product well.

How the overlay works

The feature is on by default. When you draw or generate a supported reaction and open the reaction coordinate tool, ChemIllusion runs a deterministic lookup against a curated seed dataset of Organic Chemistry 1 reactions. If it finds a verified match, a chip appears:

ΔH = −120.9 kJ/mol · gas · NIST

You can turn it off with the toggle Use experimental thermochemistry when available in the diagram settings panel if you want a fully qualitative figure.

The current dataset covers alkene hydrogenation, alkene hydrohalogenation, alkene halogenation, selected alkyne hydrogenation, and selected substitution/elimination net-reaction thermochemistry (SN1/SN2/E1/E2 labeled as net reaction ΔH only, not mechanism-specific barriers).

Data source: NIST Chemistry WebBook

All values in the seed dataset are sourced from the NIST Chemistry WebBook, SRD 69 (National Institute of Standards and Technology). The WebBook is a freely accessible, peer-curated thermochemical reference maintained by a U.S. federal agency. Each entry in ChemIllusion's dataset carries the source ID, the page URL, confidence level, and uncertainty where available.

Each confidence level is one of:

  • direct_experimental — ΔH measured directly for this reaction.

  • reverse_experimental_flipped — experimental value for the reverse reaction, sign-flipped.

  • phase_specific_experimental — real data, but from a specific phase (liquid, aqueous). Shown with a warning chip.

Entries marked verify_before_user_facing or probe_only are excluded from all student and teacher views until they are re-verified by the curation team.

What stays illustrative

The dataset does not include activation barriers. Activation energies (Ea or ΔG‡) come from kinetic measurements and depend strongly on solvent, temperature, catalyst, and mechanism. They are much harder to curate safely for a general teaching tool.

The tooltip on every thermochemistry chip makes this explicit:

Experimental or source-derived ΔH for the net reaction as written. Barrier height remains illustrative.

This is the right pedagogical boundary. Students benefit from knowing that alkene bromination releases roughly 121 kJ/mol. They do not benefit from a made-up activation barrier that looks authoritative because it has a number attached.

Slide deck creator integration

When the deck creator generates a slide that includes a reaction-coordinate diagram, it now defaults to the same thermochemistry lookup. If a match is found, the slide gets the ΔH label on the diagram and a speaker note:

Thermochemistry note: ΔH is experimental or source-derived for the net reaction shown. The activation barrier is illustrative unless a separate barrier value is provided.

Unverified values are never added to exported student-facing decks. The deck planner prompt includes an explicit instruction not to invent ΔH, ΔG, or barrier heights.

Try it

Open the Generator with the reaction coordinate diagram tool pre-loaded. Draw or enter a supported Organic Chemistry 1 reaction, then choose barrier sizes, label the wells, and watch the ΔH chip appear. Every diagram placed on the canvas carries auto-generated alt text for WCAG-compliant export to slides and handouts.

Open Reaction Coordinate Diagram Tool

Related tools on the same canvas: Newman projections for conformational analysis and CineMol 3D space-filling renders.