Gibbs free energy – when a reaction turns spontaneous
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
Whether a reaction proceeds spontaneously is decided by the sign of ΔG = ΔH − TΔS. Drag the temperature to see where the ΔG(T) line crosses zero for limestone calcination, NH₃ synthesis, dissolving NH₄NO₃ and more, which explains why lime kilns must run above 800 °C. Extra screens count microstates (S = k ln W), calculate ΔS° from standard entropies and link ΔG° = −RT ln K to the equilibrium mixture.
Lesson: Chemical thermodynamics
What it shows
Whether a reaction is spontaneous at constant temperature and pressure depends on the sign of ΔG = ΔH − TΔS, and temperature decides whether the enthalpy or the entropy term wins. The Microstates screen builds entropy from S = k ln W for a gas spreading, a change of state and two gases mixing. The ΔS° from data screen uses standard entropies and enthalpies of formation to find ΔS°, ΔH°, ΔG° and the temperature at which a reaction becomes spontaneous. The ΔG° and K screen links ΔG° = −RT ln K to Q and the equilibrium mixture, and diamond → graphite shows a favorable change that is far too slow to observe.
How to use
On the first screen choose a Reaction and drag the Temperature slider or the dot on the graph; the bars compare ΔH with −TΔS. Use Screen to switch. On Microstates, press Predict ΔS > 0 or Predict ΔS < 0, then Remove the barrier or change the state. On ΔS° from data, tick Students calculate (hide answer). On ΔG° and K, set ΔG° and the Fraction of B, then press Let it react.
Parameters you can change
- Reaction CaCO₃ → CaO + CO₂ (limestone calcination), N₂ + 3H₂ → 2NH₃, NH₄NO₃(s) → NH₄⁺ + NO₃⁻ (dissolution), 2H₂ + O₂ → 2H₂O(l), C(diamond) → C(graphite)
- Temperature 200–1600 K
- Screen ΔG = ΔH − TΔS, Microstates (S = k ln W), ΔS° from data, ΔG° and K
- Microstates: change Gas spreads into a vacuum, Change of state, Two gases mix
- Microstates: number of particles N 2–24
- ΔS° from data: reaction CaCO₃ → CaO + CO₂, N₂ + 3H₂ → 2NH₃, 2H₂ + O₂ → 2H₂O(l), C(diamond) → C(graphite), H₂O(l) → H₂O(g) (boiling), 2Mg + O₂ → 2MgO, CH₄ + 2O₂ → CO₂ + 2H₂O(l), N₂O₄ → 2NO₂, 2SO₂ + O₂ → 2SO₃, NaCl(s) → Na⁺(aq) + Cl⁻(aq) (dissolving)
- Students calculate (hide answer)
- ΔG° (A ⇌ B) -40–40 kJ/mol
- Starting fraction of B 0.01–0.99
Questions to explore
- How can dissolving NH₄NO₃ be spontaneous at room temperature even though it is endothermic?
- What is the sign of ΔS for N₂ + 3H₂ → 2NH₃, and why is the reaction favored at low temperature?
- Above roughly what temperature does the decomposition of CaCO₃ become spontaneous?
