Potential Energy Diagram Maker
Chemistry energy diagrams drawn to scale: exothermic and endothermic profiles with Ea and ΔH arrows, catalysed paths, multi-step mechanisms and bond energy curves, for worksheets, lectures and papers.
Energy diagram styles
Four styles, each on a different energy diagram: an SN2 profile for CH₃Br + OH⁻ with its [HO···CH₃···Br]⁻ transition state in textbook colour, an endothermic exam profile on a 20 kJ scale with forward and reverse Ea in black and white, a DFT free energy diagram drawn as levels joined by dashed lines with the rate-determining step marked, as a journal panel, and an enzyme path through ES and EP against the uncatalysed reaction on a dark slide.
Energy diagram examples
Four real renders, each drawn from the prompt on its card: hydrogen peroxide decomposition with and without iodide (Ea 75 and 56 kJ/mol, ΔH = −98 kJ/mol), an endothermic one-step profile labelled with symbols only, a two-step SN1 profile with Ea1 = 95 and Ea2 = 7 kJ/mol and the rate-determining step marked, and the H₂ bond curve with its minimum at 74 pm. Click a card to load its prompt.
What is a potential energy diagram?
A potential energy diagram, also called an energy profile or reaction coordinate diagram, plots the energy of a reacting system against the reaction coordinate: the progress from reactants on the left to products on the right. Every peak is a transition state and every valley between two peaks an intermediate. The climb from the reactants to the top of a barrier is the activation energy, Ea; the vertical gap between the reactant and product plateaus is the enthalpy change, ΔH. Products below the reactants make the reaction exothermic, products above make it endothermic, and a catalyst appears as a lower barrier between the same two plateaus.
About this energy diagram maker
- Type the levels the way your notes give them: reactants 0, TS1 +85, intermediate +20, TS2 +60, products −45 kJ/mol. Or give each step's Ea and ΔH (Ea = 50 kJ/mol, ΔH = −30 kJ/mol) and the levels are added up before anything is drawn.
- The curve is computed through every stationary point: it peaks exactly at each transition state and dips exactly at each intermediate, with no extra bumps, and when every level is a number the heights are drawn to scale.
- Each activation energy arrow starts at the minimum just before its barrier, never at the axis; the ΔH arrow runs from the reactant level to the product level with its sign printed, ΔH = −45 kJ/mol or ΔH = +30 kJ.
- With no numbers at all ("an endothermic reaction with and without a catalyst"), the shape follows your description and the arrows carry symbols only, Ea and ΔH > 0. No value is made up.
- Units stay as you write them (kJ/mol, kcal/mol, eV, kJ). Ask for a free energy diagram and the arrows become ΔG‡ and ΔG°; ask for levels joined by dashed lines and you get the step diagram used in DFT papers.
Energy diagrams you can make
Eight figures that chemistry courses and papers draw on this pair of axes, what sets each apart, and what to type to get it.
Exothermic reaction energy diagram
Products end below the reactants, so ΔH is negative and the ΔH arrow points down. The reverse barrier is always the taller one: with Ea = 50 kJ/mol and ΔH = −120 kJ/mol, the way back from the products climbs 170 kJ/mol. Type the two numbers, or write "exothermic" with none and the arrows are labelled Ea and ΔH < 0.
Endothermic reaction energy diagram
Products end above the reactants, ΔH is positive and the barrier must cover at least that gap. Reactants at 40 kJ, the activated complex at 100 kJ and products at 70 kJ give a forward Ea of 60 kJ, a reverse Ea of 30 kJ and ΔH = +30 kJ, the classic exam set; add "y-axis scale every 20 kJ" for a graded axis.
Activation energy diagram with and without a catalyst
Two paths between the same reactant and product plateaus: the catalysed one lower, dashed, with its own Ea arrow, and one ΔH for both because a catalyst never changes it. Hydrogen peroxide decomposition is the textbook pair, Ea 75 kJ/mol alone and 56 kJ/mol with iodide, ΔH = −98 kJ/mol. An enzyme energy diagram is the same figure with the catalysed path passing through the ES and EP complexes.
Multi-step reaction coordinate diagram
One peak per elementary step and one valley per intermediate, each step with its own Ea measured from the minimum before it. Reactants 0, TS1 +85, intermediate +20, TS2 +60, products −45 kJ/mol gives Ea1 = 85 and Ea2 = 40 kJ/mol. Ask to "mark the rate-determining step" and it is found from the numbers: the largest climb from the lowest point before a peak, which is not always the highest peak.
SN1 and SN2 energy diagrams
SN2 is one step: a single peak at the pentacoordinate transition state, [Nu···C···LG]‡, and no intermediate. SN1 is two: ionisation to a carbocation over the tall first barrier, then a small second barrier as the nucleophile adds. Name the mechanism and the shape follows; give levels for TS1, the carbocation and TS2 to draw them to scale. E2 and E1 profiles have the same two shapes.
Free energy diagram (Gibbs, ΔG‡)
The y axis is G, the arrows become ΔG‡ and ΔG°, and computational papers draw each state as a short horizontal level joined to the next by dashed lines. Type "free energy, kcal/mol, levels joined by dashed lines" with your DFT values; ticks come on in the journal style. At 25 °C a barrier of 18.4 kcal/mol means a rate constant of about 0.2 s⁻¹, a half-life of roughly 3.4 s.
Enthalpy profile diagram
The A-level name for the same figure with enthalpy, H, on the y axis: reactants and products as levels, ΔH between them, Ea over the hump. Give only the reactants and products and you get the two-level enthalpy diagram with a single ΔH arrow and no barrier; add Ea and the curve appears between them.
Potential energy curve of a bond (Morse, Lennard-Jones)
The other figure called a potential energy diagram: the energy of two atoms against the distance between their nuclei, with a steep repulsive wall, one minimum at the bond length and a flat tail at zero once the atoms are apart. H₂ is drawn with its minimum at 74 pm and a well 436 kJ/mol deep; name Lennard-Jones and give ε and σ for a van der Waals pair such as argon (ε = 0.998 kJ/mol, σ = 340 pm), whose minimum sits at 2^(1/6)σ = 382 pm.
How to read an energy diagram
What each feature of a reaction profile is, where to find it on the figure, and what it tells you about the reaction.
| Feature | Where it is on the diagram | What it tells you |
|---|---|---|
| Reactants and products | The flat plateaus at the left and right ends | Their energies; the gap between them is ΔH |
| Transition state (‡) | Each peak | The highest-energy arrangement of one elementary step; it lasts about one bond vibration and cannot be isolated |
| Intermediate | A valley between two peaks | A real species with a finite lifetime, such as the carbocation in SN1; sometimes detectable |
| Activation energy, Ea | From the minimum before a peak up to the peak | How slow that step is: at 25 °C every extra 5.7 kJ/mol (1.36 kcal/mol) makes it about ten times slower |
| Reverse activation energy | From the products up to the last peak | The barrier for the reverse reaction: Ea(reverse) = Ea(forward) − ΔH |
| ΔH (ΔG° on a free energy diagram) | Vertical gap from the reactant level to the product level | Negative: exothermic (exergonic), energy released; positive: endothermic (endergonic) |
| Rate-determining step | The step with the largest climb from the lowest point before it | Which step sets the overall rate; usually, but not always, the step with the highest peak |
| Catalysed path | A lower route between the same two plateaus | Smaller Ea, often extra intermediates; ΔH and the equilibrium position unchanged |
The reaction coordinate has no units and no numbers. It orders the stages of the reaction from left to right; only the vertical axis is a measured quantity, so horizontal spacing on an energy diagram means nothing.
Exothermic vs endothermic energy diagrams
The same three features, the plateaus, the barrier and the two arrows, read differently depending on which plateau is lower.
| Exothermic | Endothermic | |
|---|---|---|
| Products compared with reactants | Lower | Higher |
| Sign of ΔH | Negative, ΔH < 0 | Positive, ΔH > 0 |
| ΔH arrow | Points down | Points up |
| Forward and reverse barriers | Ea(forward) smaller than Ea(reverse) | Ea(forward) larger than Ea(reverse), and at least ΔH |
| Energy and surroundings | Heat released, the surroundings warm up | Heat absorbed, the surroundings cool down |
| Transition state (Hammond postulate) | Early, resembles the reactants | Late, resembles the products |
| Examples | Methane combustion, ΔH = −890 kJ/mol; H₂O₂ decomposition, −98 kJ/mol | CaCO₃ → CaO + CO₂, ΔH = +178 kJ/mol; N₂ + O₂ → 2NO, +181 kJ/mol |
Exothermic does not mean fast. The sign of ΔH comes from the two plateaus and the rate from the barrier: methane and oxygen sit together unreacted until a spark supplies the activation energy, though the reaction releases 890 kJ/mol.
ΔH from the two barriers, and what a catalyst is worth
On any energy diagram ΔH = Ea(forward) − Ea(reverse), and the Arrhenius equation turns a change in barrier into a change in rate. Lowering Ea by a few kJ/mol speeds a reaction many times over, because the barrier sits in an exponent.
ΔH = Ea,fwd − Ea,rev k = A·e^(−Ea / RT) k_cat / k_uncat = e^((Ea,uncat − Ea,cat) / RT)
In words
The gap between the plateaus is the difference of the two climbs; each kJ/mol taken off the barrier multiplies the rate.
Iodide lowers the barrier for hydrogen peroxide decomposition from 75 to 56 kJ/mol. At 25 °C that is e^(19 000 / (8.314 × 298)) ≈ 2,100: the catalysed reaction runs about two thousand times faster (assuming the same A), while ΔH stays −98 kJ/mol. On a free energy diagram the same arithmetic uses ΔG‡ in the Eyring equation, k = (k_B·T / h)·e^(−ΔG‡ / RT).
Energy diagram FAQ
How do I draw a potential energy diagram?
Put energy on the y axis and the reaction coordinate on the x axis, draw the reactants as a short flat line at the left and the products as one at the right, join them with one hump per step, then add the Ea arrow from the reactant level to the top of the hump and the ΔH arrow from reactants to products. Here you type the levels instead, for example "reactants 0, transition state 75, products −98 kJ/mol", or just Ea and ΔH, and the hump, both arrows and their values are placed for you.
Where does the activation energy arrow start on an energy diagram?
At the energy of whatever comes just before the barrier, the reactants for a one-step reaction or the intermediate for a later step, and it ends at the top of that barrier. It does not start at the x axis or at zero. On a two-step profile with reactants 0, TS1 85, intermediate 20 and TS2 60, the second step's Ea is 60 − 20 = 40, not 60.
How can you tell if a reaction is exothermic or endothermic from an energy diagram?
Compare the two plateaus: products lower than reactants is exothermic (ΔH < 0), products higher is endothermic (ΔH > 0). The height of the barrier says nothing about this; an endothermic reaction can have a small Ea and an exothermic one a huge Ea.
What is the difference between a transition state and an intermediate?
A transition state is a peak, an intermediate is a valley. A transition state is the highest point of one elementary step, a structure halfway between bonds breaking and forming that cannot be isolated; an intermediate is a real species formed in one step and used up in the next, such as the carbocation in SN1. A diagram with n steps has n transition states and n − 1 intermediates.
How does a catalyst change the energy diagram?
It adds a lower route between the same reactant and product levels, so Ea drops while ΔH stays exactly the same. The catalysed path often has more steps, each with a smaller barrier, because the catalyst binds the reactants and releases them changed. Enzyme diagrams show this as dips for the enzyme-substrate (ES) and enzyme-product (EP) complexes. A catalyst speeds the forward and reverse reactions alike and does not move the equilibrium.
How do I find the rate-determining step on an energy diagram?
For most textbook profiles it is the step with the highest transition state. Strictly, it is the step with the largest climb from the lowest point that comes before it. The two can differ: with reactants 0, TS1 50, intermediate −40, TS2 30 and products −60, TS1 is the higher peak, but the second step has to climb 70 from the deep intermediate, so step 2 limits the rate. Ask the generator to mark the rate-determining step and it applies that rule to your numbers.
What do SN1 and SN2 energy diagrams look like?
SN2 has one hump: bond making and breaking happen together in one transition state, and rate = k[substrate][nucleophile]. SN1 has two humps with a carbocation intermediate in the valley between them; the first, ionisation, is the higher barrier and the rate-determining step, so rate = k[substrate]. A more stable carbocation (tertiary rather than primary) lowers both that intermediate and the barrier leading to it.
What is the difference between a potential energy, an enthalpy and a free energy diagram?
Only the quantity on the y axis. School chemistry calls it potential energy, UK and IB courses use enthalpy, H, and organic chemistry, biochemistry and computational papers use Gibbs free energy, G, with ΔG‡ for the barrier and ΔG° for the overall change. Say which one you want, or use "free energy" and the axis and arrow labels follow.
What does the potential energy curve of a bond show?
How the energy of two atoms changes with the distance between them. Far apart it is zero; as they approach it falls to a minimum at the bond length, and pushed closer it climbs steeply as the nuclei repel. For H₂ the minimum is at 74 pm. Textbooks label the well 436 kJ/mol, the bond enthalpy; strictly the well depth Dₑ is 458 kJ/mol and the energy needed from the lowest vibrational level, D₀, is 432 kJ/mol. Write the number you want printed and that is the label.
Does the AI change my energies or make up values?
No. Every level, every Ea and ΔH, the y scale and the rate-determining step are worked out from your numbers before the image is generated, and the image model is told only to place them. Values are printed only where you gave them or where they follow from yours (an Ea from two stated levels). A request with no numbers is drawn as a labelled shape with Ea and ΔH symbols and no values.
Sources
- 12.7 Catalysis — OpenStax, Chemistry 2e
- 4.4: Ea and Catalysts — LibreTexts Chemistry, Supplemental Modules (Kinetics)
Keep exploring
Our other diagram tools, and the guides behind them.
Tools
Draw an energy diagram with every level in place
Type the energies of your reactants, transition states, intermediates and products, or simply describe the reaction, and get a reaction profile ready for a worksheet, a lecture or a paper.
Make an energy diagram