Thermodynamics
What to know
Thermodynamics predicts whether a reaction occurs spontaneously and how much energy it exchanges. Central quantities: enthalpy (ΔH, heat at constant pressure), entropy (ΔS, disorder), and Gibbs free energy (ΔG = ΔH − TΔS). A reaction is spontaneous when ΔG < 0. Signs matter: exothermic (ΔH < 0) and increasing entropy (ΔS > 0) favor spontaneity; temperature can flip the sign of ΔG when ΔH and ΔS have the same sign.
Know Hess's law (sum of step enthalpies equals overall ΔH), standard heats of formation, and calorimetry (q = mcΔT). Bond breaking absorbs energy; bond forming releases it. Relate to the first law (energy conservation) and second law (entropy of the universe increases).
Traps
- Assuming exothermic always means spontaneous (entropy matters).
- Sign errors in Hess's law when reversing or scaling reactions.
- Forgetting that ΔG = 0 at equilibrium, not always negative.
- Confusing ΔH (enthalpy) with activation energy.
Worked example
A reaction has ΔH = +50 kJ and ΔS = +0.10 kJ/K. At what temperature is it spontaneous? Set ΔG = ΔH − TΔS < 0 → T > ΔH/ΔS = 50/0.10 = 500 K. Rule of thumb: when ΔH and ΔS share a sign, temperature decides spontaneity.