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.
Sample Practice Questions
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For a reaction with Kp > 1 at standard conditions, ΔG° is:
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Answer: Negative
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Born-Haber cycle is used to calculate:
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Answer: Lattice energy of ionic compounds
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For a spontaneous process at constant T and P, ΔG must be:
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Answer: Negative
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Enthalpy of neutralisation of a strong acid with a strong base is approximately:
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Answer: −57 kJ/mol
Practice Questions
Practice 20 randomly selected NEET questions on Thermodynamics. Answers are revealed after each question.
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What are the key concepts in Thermodynamics for NEET?
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.
Which formulae are important for Thermodynamics in NEET?
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).
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