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Thermodynamics
Enthalpy, entropy, Gibbs energy, Hess's law
Key Concepts
- First law: energy is conserved; ΔU = Q − W
- Enthalpy H = U + PV; at constant pressure, ΔH = Q_p
- Standard enthalpy of formation (ΔH°_f) of elements = 0
- Gibbs free energy predicts spontaneity: ΔG = ΔH − TΔS
- Hess's law: ΔH is independent of pathway
Important Formulae
| First law | ΔU = Q − W |
| Gibbs free energy | ΔG = ΔH − TΔS |
| ΔG and equilibrium constant | ΔG° = −RT ln K |
| Born-Haber cycle (lattice energy) | ΔH_lattice from Hess's law |
| Kirchhoff's equation | ΔH₂ = ΔH₁ + ΔC_p(T₂ − T₁) |
Quick Tips
- ΔG < 0: spontaneous; ΔG > 0: non-spontaneous; ΔG = 0: at equilibrium.
- Exothermic reactions have ΔH < 0; endothermic have ΔH > 0.
- Entropy increases when solids dissolve, gases are produced, or temperature increases.
Practice Questions
Practice 20 randomly selected NEET questions on Thermodynamics. Answers are revealed after each question.
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