Class 11 Chemistry · Chapter 5 NotesThermodynamics

Thermodynamics studies energy changes in systems and their surroundings. A system is the part of the universe under study, while the surroundings include everything else. Systems can be open (exchange matter and energy), closed (exchange only energy), or isolated (no exchange). State functions like internal energy (U), enthalpy (H), and entropy (S) depend only on the current state, not the path taken. Extensive properties (like volume) depend on the amount of substance, while intensive properties (like temperature) do not. The state of a system is defined by measurable properties such as pressure, volume, and temperature.

• SystemPart of the universe under study
• SurroundingsEverything outside the system
• Types of systemsOpen, closed, isolated
• State functionsU, H, S, p, V, T
• Extensive vs. intensive properties

The first law states that energy cannot be created or destroyed, only transferred or converted. It introduces the concept of internal energy (U), which is the total energy of a system. The change in internal energy (∆U) is equal to the heat added (q) minus the work done (w) by the system. For reversible processes, work done by an ideal gas is given by w = -nRT ln(Vf/Vi). The first law provides a foundation for understanding energy transformations in chemical reactions and physical processes.

• Energy conservation∆U = q + w
• Internal energy (U) as a state function
• Heat (q) and work (w) as path-dependent
• Work done by an ideal gasw = -nRT ln(Vf/Vi)
• Sign conventions for q and w

Enthalpy (H) is a state function defined as H = U + pV, where p is pressure and V is volume. The change in enthalpy (∆H) is equal to the heat absorbed or released at constant pressure (∆H = qp). Standard enthalpy of formation (∆Hf°) is the enthalpy change when one mole of a compound is formed from its elements in their standard states. Hess's law states that the enthalpy change for a reaction is independent of the path taken. Thermochemical equations relate enthalpy changes to chemical reactions, with coefficients representing moles of substances.

• Definition of enthalpyH = U + pV
• Standard enthalpy of formation (∆Hf°)
• Hess's law∆H is independent of path
• Thermochemical equations and their interpretation
• Standard states and reference values

Entropy (S) is a measure of the disorder or randomness of a system. The second law of thermodynamics states that the total entropy of an isolated system always increases (∆S > 0). For a spontaneous process, the total entropy change (∆Stotal = ∆Ssystem + ∆Ssurroundings) must be positive. Entropy changes can be calculated using the equation ∆S = qrev/T, where qrev is the reversible heat transfer. The standard entropy (S°) of a substance can be determined from thermal data and is tabulated for many compounds.

• Entropy as a measure of disorder
• Second law of thermodynamics∆Stotal > 0 for spontaneous processes
• Entropy changes in reversible processes∆S = qrev/T
• Standard entropy (S°) and its tabulation
• Third law of thermodynamicsS approaches 0 as T approaches 0 K

Gibbs free energy (G) is a state function defined as G = H - TS, where T is temperature. The change in Gibbs free energy (∆G) is equal to ∆H - T∆S. For a spontaneous process at constant temperature and pressure, ∆G < 0. At equilibrium, ∆G = 0. The standard Gibbs free energy change (∆G°) is related to the equilibrium constant (K) by the equation ∆G° = -RT ln K. This relationship allows the calculation of equilibrium constants from thermodynamic data and vice versa.

• Definition of Gibbs free energyG = H - TS
• Criterion for spontaneity∆G < 0 at constant T and p
• Relationship between ∆G° and equilibrium constant: ∆G° = -RT ln K
• Calculation of ∆G° from ∆H and ∆S
• Temperature dependence of ∆G° and equilibrium constants

The third law of thermodynamics states that the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 K). This law allows the calculation of absolute entropies of substances from thermal data. The standard entropy (S°) of a substance can be used to calculate the standard entropy change (∆S°) for a reaction using Hess's law. The standard Gibbs free energy change (∆G°) can then be calculated using the equation ∆G° = ∆H° - T∆S°, providing a complete thermodynamic description of a reaction.

• Third law of thermodynamicsS approaches 0 as T approaches 0 K
• Calculation of absolute entropies
• Standard entropy change (∆S°) for reactions
• Calculation of ∆G° using ∆H° and ∆S°
• Thermodynamic description of reactions