← Thermochemical principles and the properties of particles and substances
✓
For the four cases: “Exo + more mess = always. Endo + more order = never. Otherwise, temperature decides.” (Exothermic AND entropy-increasing → always spontaneous. Endothermic AND entropy-decreasing → never. Mixed signs → T is the tie-breaker.)
Worked example Find the temperature a reaction becomes feasible For a reaction ΔH = +178 kJ mol⁻¹ and ΔS = +161 J K⁻¹ mol⁻¹. Above what temperature is it feasible? Feasible when ΔG ≤ 0, i.e. ΔH − TΔS ≤ 0, so T ≥ ΔH / ΔS. Convert ΔS to kJ: 161 J = 0.161 kJ K⁻¹ mol⁻¹. T = ΔH / ΔS = 178 / 0.161 = 1106 K ✓ Feasible above ≈ 1106 K (about 833 °C). This is the CaCO₃ → CaO + CO₂ type reaction. ✕
7 Spontaneity: system + surroundings (and where Gibbs fits)
Watch: Spontaneity: system + surroundings (and where Gibbs fits) Opens a YouTube search: pick a video that clicks for you ⚠⚠️ Answer this the way the marking schedule does
ΔG = ΔH − TΔS does not appear in any 91390 paper from 2021 to 2025. All five years instead ask you to “justify, in terms of the entropy changes of the system and the surroundings, why the reaction is spontaneous”, and the 2025 schedule words its Excellence criterion exactly that way.
So the argument that earns the marks is:
- System. Count moles of gas each side. Fewer gas moles → entropy of the system decreases (less dispersal of matter and energy).
- Surroundings. Exothermic → heat is released into the surroundings → their particles gain kinetic energy → entropy of the surroundings increases.
- Conclude. Since the reaction is spontaneous, the increase in the surroundings must outweigh the decrease in the system, so the total entropy change is positive.
Gibbs is kept below because it is the same physics written differently and it makes the reasoning click. It is not what this standard asks you to produce.
Written as free energy, the same idea is: Gibbs free energy \Delta G=\Delta H-T\Delta S T in kelvin. Watch units: ΔH in kJ, ΔS usually in J, convert ΔS to kJ (÷1000) first! Temperature of change-over T=\frac{\Delta H}{\Delta S}\quad(\Delta G=0) The temperature at which a reaction just becomes feasible.| ΔG | Meaning |
|---|---|
| < 0 | Spontaneous (thermodynamically feasible) |
| = 0 | At equilibrium |
| > 0 | Not spontaneous in that direction |
Mnemonic: spontaneity
“Good reactions Go when ΔG is neGative.” ΔG < 0 = spontaneous/feasible.For the four cases: “Exo + more mess = always. Endo + more order = never. Otherwise, temperature decides.” (Exothermic AND entropy-increasing → always spontaneous. Endothermic AND entropy-decreasing → never. Mixed signs → T is the tie-breaker.)
| ΔH | ΔS | Spontaneous? |
|---|---|---|
| − (exo) | + (more disorder) | Always (ΔG always −) |
| + (endo) | − (more order) | Never (ΔG always +) |
| − (exo) | − (more order) | Only at low T |
| + (endo) | + (more disorder) | Only at high T |