Class 11 Chemistry – Chemical Thermodynamics
CBSE 2026–27 | NCERT-aligned free study resource
System and Surroundings
A system is the portion selected for study; everything outside it is surroundings. Systems may be open, closed or isolated. State functions depend only on the state, while path functions depend on the path.
Internal Energy and First Law
Internal energy is the total energy of a system. The first law expresses conservation of energy: ΔU = q + w, with sign conventions applied consistently.
Work and Heat
For pressure-volume work under appropriate conditions, w = −PextΔV. Heat transferred at constant pressure is related to enthalpy change.
Enthalpy
Enthalpy is defined as H = U + PV. For reactions, Hess’s law allows enthalpy changes to be combined algebraically.
Entropy and Gibbs Energy
Entropy measures the dispersal of energy/matter in a thermodynamic description. Gibbs energy is G = H − TS. Under standard conditions, ΔG helps assess spontaneity: negative ΔG indicates thermodynamic favourability in the specified conditions.
MCQs
- The first law is based on conservation of: (A) mass only (B) energy (C) charge only (D) volume. Answer: B.
- For an expansion against external pressure, PV work is generally: (A) positive under the chemistry sign convention (B) negative (C) always zero (D) undefined. Answer: B.
- Hess’s law is useful because enthalpy is a: (A) path function (B) state function (C) catalyst (D) reactant. Answer: B.
Practice
- Distinguish state functions and path functions.
- Apply the first law to expansion/compression situations.
- Use Hess’s law to calculate an unknown enthalpy change.
HOTS
A reaction is exothermic but has negative entropy change. Discuss how temperature can influence ΔG using ΔG = ΔH − TΔS.
Revision
System → q/w → ΔU → ΔH → Hess’s law → entropy → Gibbs energy.