Chapter 5 — Thermodynamics

Chapter 5 of the Class 11 Chemistry NCERT textbook, "Thermodynamics", introduces the study of energy transformations in chemical and physical processes using the first and second laws, internal energy, enthalpy, entropy, and Gibbs energy to predict spontaneity and equilibrium.

• Systems, surroundings, and the first law — The chapter defines the thermodynamic system and its boundary, then states the first law linking internal energy change to heat and work, establishing energy conservation as the foundation for reaction energetics.
• Enthalpy and measuring reaction heat — Enthalpy as heat at constant pressure, calorimetry, and Hess's law together let students calculate reaction heats from formation data and additive steps rather than direct measurement alone.
• Entropy and the direction of change — By introducing entropy as a measure of disorder, the chapter explains the second law and why some processes happen spontaneously while their reverse does not.
• Gibbs energy and spontaneity — Gibbs energy combines enthalpy and entropy to give a single criterion for spontaneity and links free-energy change to equilibrium, predicting reaction direction at different temperatures.

1. 01System classified as open (matter and energy exchange), closed (energy only), or isolated (no exchange) separated by physical or imaginary boundaries from surroundings.
2. 02Internal energy U is a state function; changes via work (w) and heat (q) follow ∆U = q + w (first law of thermodynamics).
3. 03Enthalpy H = U + pV; at constant pressure ∆H = qp (heat of reaction); relates ∆U and ∆H via ∆H = ∆U + ∆ngRT.
4. 04Standard enthalpy of formation (∆fH) for elements in reference states = zero; reaction enthalpy calculated as ∆rH = Σ∆fH(products) − Σ∆fH(reactants).
5. 05Entropy S measures disorder/randomness; spontaneous processes have ∆Stotal = ∆Ssys + ∆Ssurr > 0 (second law).
6. 06Gibbs energy ∆G = ∆H − T∆S determines spontaneity: ∆G < 0 (spontaneous), ∆G > 0 (non-spontaneous), ∆G = 0 (equilibrium); related to equilibrium constant K via ∆rG° = −RT ln K.