Reaction energy diagram – exothermic vs endothermic
ChemistryEnergetics & ThermodynamicsAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Energetics & Thermodynamics
An enthalpy diagram plots the reaction path: reactants climb over the activation-energy barrier Ea, then drop to the product level (exothermic, ΔH < 0) or rise to a higher one (endothermic, ΔH > 0). A beaker with a thermometer shows the heat released or absorbed, q = n·|ΔH|. Students pick a reaction with a real ΔrH°298 value from the textbook, drag the peak to change Ea, and toggle a catalyst to see the path drop lower.
Lesson: Exothermic and endothermic reactions; enthalpy of formation and reaction enthalpy
What it shows
An enthalpy profile compares the enthalpy of reactants and products along the reaction path. When the products lie lower, energy is released to the surroundings: the reaction is exothermic and ΔH is negative, as in the combustion of methane, hydrogen or carbon. When they lie higher, as in the thermal decomposition of CaCO₃, the reaction is endothermic and ΔH is positive. The activation energy Ea is the height of the barrier above the reactants. A catalyst offers a lower barrier but leaves both levels, and therefore ΔH, unchanged. The heat exchanged is q = n·|ΔH|.
How to use
Choose a Reaction with a real standard enthalpy change, or pick Custom ΔH and set your own value. Press Run reaction and watch the thermometer in the beaker rise or fall as the reaction proceeds. Drag the peak or change Activation energy Ea to alter the barrier, then switch the catalyst on and off: the peak drops, but the gap between reactants and products, and the temperature change, stay the same.
Parameters you can change
- Reaction CH₄ + 2O₂ → CO₂ + 2H₂O (ΔH = −890 kJ), 2H₂ + O₂ → 2H₂O(l) (ΔH = −571.6 kJ), C + O₂ → CO₂ (ΔH = −393.5 kJ), CaCO₃ → CaO + CO₂ (ΔH = +178 kJ), N₂ + O₂ → 2NO (ΔH = +180.6 kJ), Custom ΔH
- Custom ΔH (when Custom is selected) -1000–1000 kJ
- Activation energy Ea (illustrative) 20–400 kJ
- Catalyst present (lowers Ea)
Questions to explore
- Does an exothermic reaction still need an input of energy to get started, and how does the diagram show this?
- Does switching on the catalyst change ΔH or the temperature change of the solution?
- Why must limestone (CaCO₃) be heated continuously to keep it decomposing?
Teaching ideas
All guides- Le Chatelier Virtual Lab: Color Shifts, Qc and KcA Le Chatelier virtual lab with two colored equilibria: stress the iron thiocyanate and cobalt chloride systems, read Qc and Kc, and find the sign of ΔH.
- Calorimetry Virtual Lab: Enthalpy of NeutralizationA calorimetry virtual lab for the temperature-change practical: mix HCl and NaOH in a polystyrene cup, find the peak temperature and calculate ΔH.
