Chemistry
Chemical thermodynamics (enthalpy, entropy, Gibbs) — quick study summary
Thermodynamics tells you whether a reaction can happen, not how fast. Three key quantities: enthalpy (ΔH, heat absorbed/released), entropy (ΔS, disorder), and Gibbs free energy (ΔG = ΔH − TΔS). A reaction is spontaneous when ΔG < 0. Exothermic reactions release heat (ΔH < 0); endothermic absorb it (ΔH > 0). Entropy increases when matter spreads out — gases > liquids > solids.
Key points
- ΔH = enthalpy change; exothermic ΔH < 0, endothermic ΔH > 0
- ΔS = entropy change; positive means more disorder
- ΔG = ΔH − TΔS; negative = spontaneous
- Spontaneous doesn't mean fast — kinetics is a separate question
- Hess's law: total ΔH = sum of ΔH for steps, regardless of path
Practice quiz
Click each question to reveal the answer.
1. A reaction has ΔH = −50 kJ and ΔS = +100 J/K at 298 K. Is it spontaneous?
- Yes — ΔG is negative
- No — ΔG is positive
- Only at high temperature
- Only at low temperature
Answer: Yes — ΔG is negative
ΔG = ΔH − TΔS = −50000 − (298 × 100) = −79800 J = −79.8 kJ. Negative → spontaneous at all temperatures.
2. What does a positive ΔS indicate about a reaction?
Answer: The products are more disordered than the reactants
Entropy is disorder. Going from a solid to a gas, or from few molecules to many, increases ΔS.
3. Is a fast reaction always thermodynamically favoured?
Answer: No — kinetics (speed) and thermodynamics (favourability) are independent
Diamond → graphite is thermodynamically favoured but the kinetic barrier is so high that it never happens at room temperature.
Last reviewed: May 2026